This file documents the SCM Scheme implementation.

Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Aubrey Jaffer

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

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Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the author.

Copying
*******

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PARADIGM ASSOCIATES INCORPORATED, CAMBRIDGE, MASSACHUSETTS.
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Overview
********

Scm is a portable Scheme implementation written in C.  Scm provides a machine independent platform for [JACAL], a symbolic algebra system.

Features
========

  * Conforms to Revised^4 Report on the Algorithmic Language Scheme [R4RS] and the [IEEE] P1178 specification.

  * Support for [SICP], [R2RS], [R3RS], and (proposed) [R5RS] scheme code.

  * Runs under Amiga, Atari-ST, MacOS, MS-DOS, OS/2, NOS/VE, Unicos, VMS, Unix and similar systems.  Supports ASCII and EBCDIC character sets.

  * Is fully documented in TeXinfo form, allowing documentation to be generated in info, TeX, html, nroff, and troff formats.

  * Supports inexact real and complex numbers, 30 bit immediate integers and large precision integers.

  * Many Common Lisp functions: logand, logor, logxor, lognot, ash, logcount, integer-length, bit-extract, defmacro, macroexpand, macroexpand1, gentemp, defvar, force-output, software-type, get-decoded-time, get-internal-run-time, get-internal-real-time, delete-file, rename-file, copy-tree, acons, and eval.

  * Char-code-limit, most-positive-fixnum, most-negative-fixnum, and internal-time-units-per-second constants.  *Features* and *load-pathname* variables.

  * Arrays and bit-vectors.  String ports and software emulation ports. I/O extensions providing ANSI C and POSIX.1 facilities.

  * Interfaces to standard libraries including REGEX string regular expression matching and the CURSES screen management package.

  * Available add-on packages including an interactive debugger, database, X-window graphics, BGI graphics, Motif, and Open-Windows packages.

  * A compiler (HOBBIT, available separately) and dynamic linking of compiled modules.

  * User definable responses to interrupts and errors, Process-syncronization primitives.  Setable levels of monitoring and timing information printed interactively (the verbose function). Restart, quit, and exec.

Authors
=======

Aubrey Jaffer (jaffer@ai.mit.edu)
     Most of SCM.

Radey Shouman
     Arrays. gsubrs, compiled closures, and records.

Jerry D. Hedden
     Real and Complex functions.  Fast mixed type arithmetics.

Hugh Secker-Walker
     Syntax checking and memoization of special forms by evaluator.  Storage allocation strategy and parameters.

George Carrette
     Siod, written by George Carrette, was the starting point for SCM. The major innovations taken from Siod are the evaluator's use of the C-stack and being able to garbage collect off the C-stack (see Garbage Collection).

There are many other contributors to SCM.  They are acknowledged in the file ChangeLog, a log of changes that have been made to scm.

Bibliography
============

[IEEE]
     IEEE Standard 1178-1990.  IEEE Standard for the Scheme Programming Language.  IEEE, New York, 1991.

[Simply]
     Brian Harvey and Matthew Wright. Simply Scheme: Introducing Computer Science MIT Press, 1994 ISBN 0-262-08226-8

[SICP]
     Harold Abelson and Gerald Jay Sussman with Julie Sussman. Structure and Interpretation of Computer Programs. MIT Press, Cambridge, 1985.

[R4RS]
     William Clinger and Jonathan Rees, Editors. Revised(4) Report on the Algorithmic Language Scheme. In ACM Lisp Pointers IV (July-September 1991).

     Revised(4) Report on the Algorithmic Language Scheme.

[SLIB]
     Todd R. Eigenschink, Dave Love, and Aubrey Jaffer. SLIB, The Portable Scheme Library. Edition 2.01, for SLIB version 2a2, January 1995.

     SLIB.

[JACAL]
     Aubrey Jaffer. JACAL Symbolic Mathematics System. Version 1a5, April 1994.

     JACAL.

scm.texi
scm.info
     Documentation of scm extensions (beyond Scheme standards). Documentation on the internal representation and how to extend or include scm in other programs.

Invoking SCM
============

      scm  [-a kbytes] [-ibvqmu] [-p number]
     [-c expression] [-e expression] [-f filename] [-l filename] [-r feature] [-- | - | -s] [filename] [arguments ...]

Upon startup scm loads the file specified by by the environment variable SCM_INIT_PATH or by the parameter IMPLINIT in the makefile (or scmfig.h) if SCM_INIT_PATH is not defined. The makefiles attempt to set IMPLINIT to Init.scm in the source directory.

Unless the option -no-init-file occurs in the command line, Init.scm checks to see if there is file ScmInit.scm in the path specified by the environment variable HOME (or in the current directory if HOME is undefined). If it finds such a file it is loaded.

Init.scm then looks for command input from one of three sources: From an option on the command line, from a file named on the command line, or from standard input.

This explanation applies to SCMLIT or other builds of SCM.  GUILE is invoked similarly to SCM.  Guile's startup script is Ginit.scm rather than Init.scm.

Scheme-code files can also invoke SCM and its variants.  See #!.

Options
=======

The options are processed in the order specified on the command line.

 -- Command Option: -a kbytes
     specifies that scm should allocate an initial heapsize of KBYTES. This option, if present, must be the first on the command line.

 -- Command Option: -no-init-file
     Inhibits the loading of ScmInit.scm as described above.

 -- Command Option: -e expression
 -- Command Option: -c expression
     specifies that the scheme expression EXPRESSION is to be evaluated. These options are inspired by perl and sh respectively. On Amiga systems the entire option and argument need to be enclosed in quotes. For instance "-e(newline)".

 -- Command Option: -r feature
     requires FEATURE. This will load a file from [SLIB] if that FEATURE is not already supported. If FEATURE is 2, 3, 4, or 5 scm will require the features neccessary to support [R2RS], [R3RS], [R4RS], or proposed [R5RS], respectively.

 -- Command Option: -l filename
 -- Command Option: -f filename
     loads FILENAME. Scm will load the first (unoptioned) file named on the command line if no -c, -e, -f, -l,  or -s option preceeds it.

 -- Command Option: -p level
     sets the prolixity (verboseness) to LEVEL. This is the same as the scm command (verobse LEVEL).

 -- Command Option: -v
     (verbose mode) specifies that scm will print prompts, evaluation times, notice of loading files, and garbage collection statistics. This is the same as -p3.

 -- Command Option: -q
     (quiet mode) specifies that scm will print no extra information. This is the same as -p0.

 -- Command Option: -m
     specifies that subsequent loads, evaluations, and user interactions will be with [R4RS] macro capability. To use a specific [R4RS] macro implementation from [SLIB] (instead of [SLIB]'s default) put -r MACROPACKAGE before -m on the command line.

 -- Command Option: -u
     specifies that subsequent loads, evaluations, and user interactions will be without [R4RS] macro capability. [R4RS] macro capability can be restored by a subsequent -m on the command line or from Scheme code.

 -- Command Option: -i
     specifies that scm should run interactively. That means that scm will not terminate until the (quit) or (exit) command is given, even if there are errors. It also sets the prolixity level to 2 if it is less than 2. This will print prompts, evaluation times, and notice of loading files. The prolixity level can be set by subsequent options. If scm is started from a tty, it will assume that it should be interactive unless given a subsequent -b option.

 -- Command Option: -b
     specifies that scm should run non-interactively. That means that scm will terminate after processing the command line or if there are errors.

 -- Command Option: -s
     specifies, by analogy with sh, that further options are to be treated as program aguments.

 -- Command Option: -
 -- Command Option: --
     specifies that there are no more options on the command line.

Environment Variables
=====================

 -- Environment Variable: SCM_INIT_PATH
     is the pathname where scm will look for its initialization code. The default is the file Init.scm in the source directory.

 -- Environment Variable: SCHEME_LIBRARY_PATH
     is the [SLIB] Scheme library directory.

 -- Environment Variable: HOME
     is the directory where Init.scm will look for the user initialization file ScmInit.scm.

Scheme Variables
================

 -- Variable: *argv*
     contains the list of arguments to the program. *argv* can change during argument processing. This list is suitable for use as an argument to [SLIB] getopt.

 -- Variable: *R4RS-macro*
     controls whether loading and interaction support [R4RS] macros. Define this in ScmInit.scm or files specified on the command line. This can be overridden by subsequent -m and -u options.

 -- Variable: *interactive*
     controls interactivity as explained for the -i and -b options. Define this in ScmInit.scm or files specified on the command line. This can be overridden by subsequent -i and -b options.

Examples
========

% scm foo.scm
     Loads and executes the contents of foo.scm and then enters interactive session.

% scm -f foo.scm arg1 arg2 arg3
     Parameters arg1, arg2, and arg3 are stored in the global list *argv*; Loads and executes the contents of foo.scm and exits.

% scm -s foo.scm arg1 arg2
     Sets *argv* to ("foo.scm" "arg1" "arg2") and enters interactive session.

% scm -e `(write (list-ref *argv* *optind*))' bar
     Prints "bar".

% scm -rpretty-print -r format -i
     Loads pretty-print and format and enters interactive session.

% scm -r5
     Loads dynamic-wind, values, and [R4RS] macros and enters interactive (with macros) session.

% scm -r5 -r4
     Like above but rev4-optional-procedures are also loaded.

SCM Session
===========

  * Options, file loading and features can be specified from the command line.  See System interface (SCM).  See Require (SLIB).

  * Typing the end-of-file character at the top level session (while SCM is not waiting for parenthesis closure) causes SCM to exit.

  * Typing the interrupt character aborts evaluation of the current form and resumes the top level read-eval-print loop.

Installing SCM
**************

SLIB
====

[SLIB] is a portable Scheme library meant to provide compatibility and utility functions for all standard Scheme implementations.

Although SLIB is not neccessary to run SCM, I strongly suggest you obtain and install it.  Bug reports about running SCM without SLIB have very low priority.  SLIB is available from the same sites as SCM:

  * altdorf.ai.mit.edu:archive/scm/slib2a2.tar.gz

  * prep.ai.mit.edu:pub/gnu/jacal/slib2a2.tar.gz

  * ftp.maths.tcd.ie:pub/bosullvn/jacal/slib2a2.tar.gz

  * ftp.cs.indiana.edu:/pub/scheme-repository/imp/slib2a2.tar.gz

Problems Compiling
==================

FILE: PROBLEM
     HOW TO FIX

*.c: include file not found
     Correct the status of STDC_HEADERS in scmfig.h

     fix #include statement or add #define for system type to scmfig.h.

*.c: Function should return a value in ...
*.c: Parameter '...' is never used in ...
*.c: Condition is always false in ...
*.c: Unreachable code in function ...
     Ignore.

scm.c: assignment between incompatible types
     change SIGRETTYPE in scm.c.

time.c: CLK_TCK redefined
     incompatablility between <stdlib.h> and <sys/types.h>.  remove STDC_HEADERS in scmfig.h.

     edit <sys/types.h> to remove incompatability.

subr.c: Possibly incorrect assignment in function lgcd
     Ignore.

sys.c: statement not reached
sys.c: constant in conditional expression
     ignore

sys.c: `???' undeclared, outside of functions
     #undef STDC_HEADERS in scmfig.h.

scl.c: syntax error
     #define SYSTNAME to your system type in scl.c (softtype)

Problems Linking
================

PROBLEM
     HOW TO FIX

_sin etc. missing.
     uncomment LIBS in makefile

Problems Running
================

PROBLEM
     HOW TO FIX

Opening message and then machine crashes.
     Change memory model option to C compiler (or makefile).

     Make sure sizet definition is correct in scmfig.h.

     Reduce size of HEAP_SEG_SIZE in setjump.h.

Input hangs
     #define NOSETBUF

ERROR: heap: need larger initial
     Need to increase the initial heap allocation using -a<kbytes> or INIT_HEAP_SIZE.

ERROR: Could not allocate ...
     Check sizet definition.

     Use 32 bit compiler mode.

     Don't try to run as subproccess

remove ... in scmfig.h and recompile scm
add ... in scmfig.h and recompile scm
     Do it and recompile files.

ERROR: Init.scm not found
     Assign correct IMPLINIT in makefile or scmfig.h or define environment variable SCM_INIT_PATH to be the full pathname of Init.scm (see Installing SCM).

WARNING: require.scm not found
     define environment variable SCHEME_LIBRARY_PATH to be the full pathname of the scheme library [SLIB] or change library-vicinity in Init.scm to point to library or remove. See Installation (SLIB).

     Make sure the value of (library-vicinity) has a trailing file separator (like / or \).

Testing
=======

Loading test.scm in the distribution will run an [R4RS] conformance test on scm.

     > (load "test.scm")
     -|
     ;loading "test.scm"
     SECTION(2 1)
     SECTION(3 4)
      #<primitive-procedure boolean?>
         #<primitive-procedure char?>
            #<primitive-procedure null?>
               #<primitive-procedure number?>
     ...

Loading pi.scm in the distribution will enable you to compute digits of pi.

     > (load "pi")
     ;loading "pi"
     ;done loading "pi.scm"
     ;Evaluation took 20 mSec (0 in gc) 767 cells work, 233 bytes other
     #<unspecified>
     > (pi 100 5)
     00003 14159 26535 89793 23846 26433 83279 50288 41971 69399
     37510 58209 74944 59230 78164 06286 20899 86280 34825 34211
     70679
     ;Evaluation took 550 mSec (60 in gc) 36976 cells work, 1548 bytes other
     #<unspecified>

PROBLEM
     HOW TO FIX

Runs some and then machine crashes.
     See above under machine crashes.

Runs some and then ERROR: ... (after a GC has happened)
     Remove optimization option to C compiler and recompile.

     #define SHORT_ALIGN in scmfig.h.

Some symbol names print incorrectly.
     Change memory model option to C compiler (or makefile).

     Check that HEAP_SEG_SIZE fits within sizet.

     Increase size of HEAP_SEG_SIZE (or INIT_HEAP_SIZE if it is smaller than HEAP_SEG_SIZE).

ERROR: Rogue pointer in Heap.
     See above under machine crashes.

Newlines don't appear correctly in output files.
     Check file mode (define OPEN_... in Init.scm

Spaces or control characters appear in symbol names
     Check character defines in scmfig.h.

Negative numbers turn positive.
     Check SRS in scmfig.h.

VMS: Couldn't unwind stack
VAX: botched longjmp
     #define CHEAP_CONTIUATIONS in scmfig.h.

Sparc(SUN-4) heap is growing out of control
     You are experiencing a GC problem peculiar to the Sparc.  The problem is that SCM doesn't know how to clear register windows.  Every location which is not reused still gets marked at GC time.  This causes lots of stuff which should be collected to not be.  This will be a problem with any conservative GC until we find what instruction will clear the register windows.  This problem is exacerbated by using lots of call-with-current-continuations.

Reporting Problems
==================

Reported problems and solutions are grouped under Compiling, Linking, Running, and Testing.  If you don't find your problem listed there, you can send a bug report to jaffer@ai.mit.edu or bug-scm@scrg.cs.tcd.ie.  The bug report should include:

 1. The version of SCM (printed when SCM is invoked with no arguments).

 2. The type of computer you are using.

 3. The name and version of your computer's operating system.

 4. The values of the environment variables SCM_INIT_PATH and SCHEME_LIBRARY_PATH.

 5. The name and version of your C compiler.

 6. If you are using an executable from a distribution, the name, vendor, and date of that distribution.  In this case, corresponding with the vendor is recommended.

Standard Facilities
*******************

Standards Compliance
====================

Scm conforms to the IEEE Standard 1178-1990.  IEEE Standard for the Scheme Programming Language. (see Bibliography), and Revised(4) Report on the Algorithmic Language Scheme. Revised(4) Report on the Algorithmic Language Scheme. All the required features of these specifications are supported. Many of the optional features are supported as well.

Optionals of [R4RS] Supported by SCM
------------------------------------

two clause if: (if <test> <consequent>)
     See Conditionals (Revised(4) Scheme).

let*
named let
     See Binding constructs (Revised(4) Scheme).

do
     See Iteration (Revised(4) Scheme).

All varieties of define
     See Definitions (Revised(4) Scheme).

list-tail
     See Pairs and lists (Revised(4) Scheme).

string-copy
string-fill!
     See Strings (Revised(4) Scheme).

make-vector of two arguments
vector-fill!
     See Vectors (Revised(4) Scheme).

apply of more than 2 arguments
     See Control features (Revised(4) Scheme).

- and / of more than 2 arguments
exp
log
sin
cos
tan
asin
acos
atan
sqrt
expt
make-rectangular
make-polar
real-part
imag-part
magnitude
angle
exact->inexact
inexact->exact
     See Numerical operations (Revised(4) Scheme).

delay
force
     See Control features (Revised(4) Scheme).
with-input-from-file
with-output-to-file
     See Ports (Revised(4) Scheme).
char-ready?
     See Input (Revised(4) Scheme).
transcript-on
transcript-off
     See System interface (Revised(4) Scheme).

Optionals of [R4RS] not Supported by SCM
----------------------------------------

numerator
denominator
rationalize
     See Numerical operations (Revised(4) Scheme).

[R4RS] appendix Macros
     See Macros (Revised(4) Scheme).

[SLIB] Features of SCM and SCMLIT
---------------------------------

delay
full-continuation
ieee-p1178
object-hash
rev4-report
source
     See SLIB file Template.scm.

current-time
     See Time (SLIB).

defmacro
     See Defmacro (SLIB).

dynamic-wind
     See Dynamic-Wind (SLIB).

eval
     See System (SLIB).

getenv
system
     See System Interface (SLIB).

hash
     See Hashing (SLIB).

logical
     See Bit-Twiddling (SLIB).

multiarg-apply
     See Multi-argument Apply (SLIB).

multiarg/and-
     See Multi-argument / and - (SLIB).

rev4-optional-procedures
     See Rev4 Optional Procedures (SLIB).

string-port
     See String Ports (SLIB).

tmpnam
     See Input/Output (SLIB).

transcript
     See Transcripts (SLIB).

vicinity
     See Vicinity (SLIB).

with-file
     See With-File (SLIB).

[SLIB] Features of SCM
----------------------

array
     See Arrays (SLIB).

array-for-each
     See Array Mapping (SLIB).

bignum
complex
inexact
rational
real
     See Require (SLIB).

System Interface
================

For documentation of the procedures getenv and system see System Interface (SLIB).

 -- Function: quit
 -- Function: quit n
 -- Function: exit
 -- Function: exit n
     Aliases for exit (see exit (SLIB)).  On many systems, SCM can also tail-call another program.  See execp.

 -- Function: vms-debug
     If SCM is compiled under VMS these commands will invoke the editor or debugger respectively.

 -- Function: ed filename
     If SCM is compiled under VMS ed will invoke the editor with a single the single argument FILENAME.

 -- Function: ed arg1 ...
     Otherwise, the value of the environment variable EDITOR (or just ed if it isn't defined) is invoked as a command with arguments ARG1 ....

 -- Function: program-arguments
     Returns a list of strings of the arguments scm was called with.

 -- Function: errno
 -- Function: errno n
     With no argument returns the current value of the system variable errno.  When given an argument, errno sets the system variable errno to N and returns the previous value of errno.  (errno 0) will clear outstanding errors.  This is recommended after try-load returns #f since this occurs when the file could not be opened.

 -- Function: perror string
     Prints on standard error output the argument STRING, a colon, followed by a space, the error message corresponding to the current value of errno and a newline.  The value returned is unspecified.

Internal State
==============

 -- Variable: errobj
     If SCM encounters a non-fatal error it aborts evaluation of the current form, prints a message explaining the error, and resumes the top level read-eval-print loop.  The value of ERROBJ is the offending object if appropriate.  ERROBJ is not set from calls to the procedure error.

 -- Function: error arg1 arg2 arg3 ...
     Alias for error (SLIB).  Outputs an error message containing the arguments, aborts evaluation of the current form and resumes the top level read-eval-print loop.  Error is defined in Init.scm; You change it to suit you.

 -- Function: abort
     Resumes the top level Read-Eval-Print loop.

 -- Function: verbose n
     Controls how much monitoring information is printed. If N is:

     0
          no prompt or information is printed.

     >= 1
          a prompt is printed.

     >= 2
          the CPU time is printed after each top level form evaluated.

     >= 3
          messages about heap growth are printed.

     >= 4
          garbage collection (see Garbage Collection) messages are printed.

     >= 5
          a warning will be printed for each top-level symbol which is defined more than one time.

 -- Function: gc
     Scans all of SCM objects and reclaims for further use those that are no longer accessible.

 -- Function: room
 -- Function: room #t
     Prints out statistics about SCM's current use of storage.  (room #t) also gives the hexadecimal heap segment and stack bounds.

 -- Constant: *scm-version*
     Contains the version string (e.g. 4e2) of SCM.

For other configuration constants and procedures see Configuration (SLIB).

Miscellaneous Procedures
========================

 -- Function: try-load filename
     If the string FILENAME names an existing file, the try-load procedure reads Scheme source code expressions and definitions from the file and evaluates them sequentially and returns #t.  If not, try-load returns #f.  The try-load procedure does not affect the values returned by current-input-port and current-output-port.

 -- Variable: *load-pathname*
     Is set to the pathname given as argument to load, try-load, and dyn:link (see Compiling And Linking). *load-pathname* is used to compute the value of program-vicinity (SLIB).

 -- Function: line-number
     Returns the current line number of the file currently being loaded.

 -- Function: eval obj
     Alias for eval (SLIB).

 -- Function: eval-string str
     Returns the result of reading an expression from STR and evaluating it.  eval-string does not change *load-pathname* or  line-number.

 -- Function: load-string str
     Reads and evaluates all the expressions from STR.  As with load, the value returned is unspecified.  eval-string does not change *load-pathname* or line-number.

 -- Function: vector-set-length! object length
     Change the length of string, vector, bit-vector, or uniform-array OBJECT to LENGTH.  If this shortens OBJECT then the remaining contents are lost.  If it enlarges OBJECT then the contents of the extended part are undefined but the original part is unchanged.  It is an error to change the length of literal datums.  The new object is returned.

 -- Function: copy-tree obj
     See copy-tree (SLIB).  This extends the SLIB version by also copying vectors.

 -- Function: acons obj1 obj2 obj3
     Returns (cons (cons obj1 obj2) obj3).  The expression (set! a-list (acons key datum a-list)) adds a new association to a-list.

 -- Function: terms
     This command displays the GNU General Public License.

 -- Function: list-file filename
     Displays the text contents of FILENAME.

Time
====

 -- Constant: internal-time-units-per-second

     Is the integer number of internal time units in a second.

 -- Function: get-internal-run-time
     Returns the integer run time in internal time units from an unspecified starting time.  The difference of two calls to get-internal-run-time divided by internal-time-units-per-second will give elapsed run time in seconds.

 -- Function: get-internal-real-time
     Returns the integer time in internal time units from an unspecified starting time.  The difference of two calls to get-internal-real-time divided by interal-time-units-per-second will give elapsed real time in seconds.

 -- Function: current-time
     Returns the time since 00:00:00 GMT, January 1, 1970, measured in seconds.  See current-time (SLIB).  current-time is used in Time (SLIB).

Interrupts
==========

 -- Function: ticks n
     Returns the number of ticks remaining till the next tick interrupt. Ticks are an arbitrary unit of evaluation.  Ticks can vary greatly in the amount of time they represent.

     If N is 0, any ticks request is canceled.  Otherwise a ticks-interrupt will be signaled N from the current time. ticks is supported if SCM is compiled with the ticks flag defined.

 -- Callback procedure: ticks-interrupt ...
     Establishes a response for tick interrupts.  Another tick interrupt will not occur unless ticks is called again.  Program execution will resume if the handler returns.  This procedure should (abort) or some other action which does not return if it does not want processing to continue.

 -- Function: alarm secs
     Returns the number of seconds remaining till the next alarm interrupt. If SECS is 0, any alarm request is canceled.  Otherwise an alarm-interrupt will be signaled SECS from the current time.  ALARM is not supported on all systems.

 -- Callback procedure: user-interrupt ...
 -- Callback procedure: alarm-interrupt ...
     Establishes a response for SIGINT (control-C interrupt) and SIGALRM interrupts.  Program execution will resume if the handler returns.  This procedure should (abort) or some other action which does not return if it does not want processing to continue after it returns.

     Interrupt handlers are disabled during execution system and ed procedures.

     To unestablish a response for an interrupt set the handler symbol to #f.  For instance, (set! user-interrupt #f).

 -- Callback procedure: out-of-storage ...
 -- Callback procedure: could-not-open ...
 -- Callback procedure: end-of-program ...
 -- Callback procedure: hang-up ...
 -- Callback procedure: arithmetic-error ...
     Establishes a response for storage allocation error, file opening error, end of program, SIGHUP (hang up interrupt) and arithmetic errors respectively.  This procedure should (abort) or some other action which does not return if it does not want the default error message to also be displayed.  If no procedure is defined for HANG-UP then END-OF-PROGRAM (if defined) will be called.

     To unestablish a response for an error set the handler symbol to #f.  For instance, (set! could-not-open #f).

Process Synchronization
=======================

 -- Function: make-arbiter name

     Returns an object of type arbiter and name NAME.  Its state is initially unlocked.

 -- Function: try-arbiter arbiter

     Returns #t and locks ARBITER if ARBITER was unlocked. Otherwise, returns #f.

 -- Function: release-arbiter arbiter

     Returns #t and unlocks ARBITER if ARBITER was locked. Otherwise, returns #f.

Files and Ports
===============

These procedures generalize and extend the standard capabilities in Ports (Revised(4) Scheme).

 -- Function: open-file string modes
     Returns a port capable of receiving or delivering characters as specified by the MODES string.  If a file cannot be opened #f is returned.

 -- Constant: open_read
 -- Constant: open_write
 -- Constant: open_both
     Contain modes strings specifying that a file is to be opened for reading, writing, and both reading and writing respectively.

 -- Function: _ionbf modestr
     Returns a version of modestr which when open-file is called with it as the second argument will return an unbuffered port.  A non-file input-port must be unbuffered in order for char-ready? to work correctly on it.  The initial value of (current-input-port) is unbuffered if the platform supports it.

 -- Function: close-port port
     Closes PORT.  The same as close-input-port and close-output-port.

 -- Function: open-io-file filename
 -- Function: close-io-port port
     These functions are analogous to the standard scheme file functions. The ports are open to FILENAME in read/write mode.  Both input and output functions can be used with io-ports.  An end of file must be read or a file-set-position done on the port between a read operation and a write operation or vice-versa.

 -- Function: current-error-port
     Returns the current port to which diagnostic output is directed.

 -- Function: with-error-to-file string thunk
     THUNK must be a procedure of no arguments, and string must be a string naming a file.  The file is opened for output, an output port connected to it is made the default value returned by current-error-port, and the THUNK is called with no arguments.  When the thunk returns, the port is closed and the previous default is restored.  With-error-to-file returns the value yielded by THUNK.

 -- Function: with-input-from-port port thunk
 -- Function: with-output-to-port port thunk
 -- Function: with-error-to-port port thunk
     These routines differ from with-input-from-file, with-output-to-file, and with-error-to-file in that the first argument is a port, rather than a string naming a file.

Soft Ports
==========

A soft-port is a port based on a vector of procedures capable of accepting or delivering characters.  It allows emulation of I/O ports.

 -- Function: make-soft-port vector modes
     Returns a port capable of receiving or delivering characters as specified by the MODES string (see open-file).  VECTOR must be a vector of length 6.  Its components are as follows:

      0. procedure accepting one character for output

      1. procedure accepting a string for output

      2. thunk for flushing output

      3. thunk for getting one character

      4. thunk for closing port (not by garbage collection)

     For an output-only port only elements 0, 1, 2, and 4 need be procedures.  For an input-only port only elements 3 and 4 need be procedures.  Thunks 2 and 4 can instead be #f if there is no useful operation for them to perform.

     If thunk 3 returns #f or an eof-object (see eof-object? (Revised(4) Scheme)) it indicates that the port has reached end-of-file.  For example:

          (define stdout (current-output-port))
          (define p (make-soft-port
                     (vector
                      (lambda (c) (write c stdout))
                      (lambda (s) (display s stdout))
                      (lambda () (display "." stdout))
                      (lambda () (char-upcase (read-char)))
                      (lambda () (display "@" stdout)))
                     "rw"))

          (write p p) => #<input-output-soft#\space45d10#\>

Syntax Extensions
=================

 -- Read syntax: #. expression
     Is read as the object resulting from the evaluation of EXPRESSION. This substitution occurs even inside quoted structure.

     In order to allow compiled code to work with #. it is good practice to define those symbols used inside of EXPRESSION with #.(define ...).  For example:

          #.(define foo 9)                        => #<unspecified>
          '(#.foo #.(+ foo foo))                  => (9 18)

 -- Read syntax: #+ feature form
     If feature is provided? (by *features*) then FORM is read as a scheme expression.  If not, then FORM is treated as whitespace.

     Feature is a boolean expression composed of symbols and and, or, and not of boolean expressions.

     For more information on provided? and *features*, see Require (SLIB).

 -- Read syntax: #- feature form
     is equivalent to #+(not feature) expression.

 -- Read syntax: #' form
     is equivalent to FORM (for compatibility with common-lisp).

 -- Read syntax: #| any thing |#
     Is a balanced comment.  Everything up to the matching |# is ignored by the read.  Nested #|...|# can occur inside ANY THING.

 -- Read syntax: #! any thing
     On the first line of a file will be ignored when loaded by SCM.  This makes SCM files usable as POSIX shell scripts if the first line is:

          #!/usr/local/bin/scm

     When such a file is invoked it executes /usr/local/bin/scm with the name of this file as the first argument.  The following shell script will print factorial of its argument:

          #!/usr/local/bin/scm
          ;;; -*-scheme-*- tells emacs this is a scheme file.
          (define (fact n) (if (< n 2) 1 (* n (fact (+ -1 n)))))
          (display (fact (string->number (caddr (program-arguments)))))
          (newline)
          (quit)

     This technique has some drawbacks:

       * Some Unixes limit the length of the #! interpreter line to the size of an object file header, which can be as small as 32 bytes.

       * A full, explicit pathname must be specified, perhaps requiring more than 32 bytes and making scripts vulnerable to programs being moved.

     The following approach solves both problems -- at the expense of slightly slower startup.  type; should appear on every line to be executed by the shell.  These lines do not have the length restriction mentioned above.  Also, /bin/sh searches the directories listed in the `PATH' environment variable for scm, eliminating the need to know absolute locations in order to invoke a program.

          #!/bin/sh
          type;exec scm $0 $*
          ;;; -*-scheme-*- tells emacs this is a scheme file.
          (define (fact n) (if (< n 2) 1 (* n (fact (+ -1 n)))))
          (display (fact (string->number (caddr (program-arguments)))))
          (newline)
          (quit)

 -- Special Form: defined? symbol
     Equivalent to #t if SYMBOL is a syntactic keyword (such as if) or a symbol with a value in the top level environment (see Variables and regions (Revised(4) Scheme)).  Otherwise equivalent to #f.

 -- Special Form: defvar identifier initial-value
     If IDENTIFIER is unbound in the top level environment, then IDENTIFIER is defined to the result of evaluating the form INITIAL-VALUE as if the defvar form were instead the form (define identifier initial-value) .  If IDENTIFIER already has a value, then INITIAL-VALUE is not evaluated and IDENTIFIER's value is not changed.

SCM also supports the following constructs from Common Lisp: defmacro, macroexpand, macroexpand-1, and gentemp.  See Defmacro (SLIB).

Low Level Syntactic Hooks
=========================

 -- Callback procedure: read:sharp c port
     If a # followed by a character (for a non-standard syntax) is encountered by read, read will call the value of the symbol read:sharp with arguments the character and the port being read from.  The value returned by this function will be the value of read for this expression unless the function returns #<unspecified> in which case the expression will be treated as whitespace.  #<unspecified> is the value returned by the expression (if #f #f).

     Note: When adding new # syntaxes, have your code save the previous value of read:sharp when defining it.  Call this saved value if an invocation's syntax is not recognized.  This will allow #+, #-, #!, and Uniform Arrays to still be supported (as they use read:sharp).

 -- Function: procedure->syntax proc
     Returns a macro which, when a symbol defined to this value appears as the first symbol in an expression, returns the result of applying PROC to the expression and the environment.

 -- Function: procedure->macro proc
 -- Function: procedure->memoizing-macro proc
     Returns a macro which, when a symbol defined to this value appears as the first symbol in an expression, evaluates the result of applying PROC to the expression and the environment.  The value returned from PROC which has been passed to PROCEDURE->MEMOIZING-MACRO replaces the form passed to PROC.  For example:

          (define trace
            (procedure->macro
             (lambda (x env) `(set! ,(cadr x) (tracef ,(cadr x) ',(cadr x))))))

          (trace foo) == (set! foo (tracef foo 'foo)).

     An environment is a list of environment frames.  There are 2 types of environment frames:

     ((lambda (variable1 ...) ...) value1 ...)
     (let ((variable1 value1) (variable2 value2) ...) ...)
     (letrec ((variable1 value1)  ...) ...)
          result in a single enviroment frame:

               ((variable1 ...) value1 ...)

     (let ((variable1 value1)) ...)
     (let* ((variable1 value1) ...) ...)
          result in an environment frame for each variable:

               (variable1 . value1) (variable2 .  value2) ...

 -- Special Form: @apply procedure argument-list
     Returns the result of applying procedure to argument-list.  (apply procedure argument-list) will produce the same result.

 -- Special Form: @call-with-current-continuation procedure)
     Returns the result of applying procedure to the current continuation. (call-with-current-continuation PROCEDURE) will have the same effect.

Packages
********

Compiling And Linking
=====================

 -- Function: compile-file name1 name2 ...
     If the HOBBIT compiler is installed in the (implementation-vicinity), compiles the files NAME1 NAME2 ... to an object file name NAME1<object-suffix>, where <object-suffix> is the object file suffix for your computer (for instance, .o).  NAME1 must be in the current directory. NAME2 ... can be in other directories.

 -- Function: link-named-scm name module1 ...
     Creates a new SCM executable with name NAME.  NAME will include the object modules MODULE1 ... which can be produced with compile-file.

If SCM has been compiled with dynl.c then the additional properties of load and require (from [SLIB]) specified below are supported.  The require forms are preferred.  The variable *catalog* can be extended to define other requirable packages. See Link.scm for details.

 -- Function: load filename lib1 ...
     In addition to the [R4RS] requirement of loading Scheme expressions if FILENAME is a Scheme source file, load will also dynamically load/link object files (produced by compile-file, for instance).  The object-suffix need not be given to load.  For example,

          (load (in-vicinity (implementation-vicinity) "sc2"))
          or (load (in-vicinity (implementation-vicinity) "sc2.o"))
          or (require 'rev2-procedures)
          or (require 'rev3-procedures)

     will load/link sc2.o if it exists.

     The LIB1 ... pathnames are for additional libraries which may be needed for object files not produced by the Hobbit compiler.  For instance, crs is linked on Linux by

          (load (in-vicinity (implementation-vicinity) "crs.o")
                (usr:lib "ncurses") (usr:lib "c"))
          or (require 'curses)

     Turtlegr graphics library is linked by:

          (load (in-vicinity (implementation-vicinity) "turtlegr")
                (usr:lib "X11") (usr:lib "c") (usr:lib "m"))
          or (require 'turtle-graphics)

     And the string regular expression (see Regular Expression Pattern Matching) package is linked by:

          (load (in-vicinity (implementation-vicinity) "rgx") (usr:lib "c"))

     or

          (require 'regex)

 -- Function: require 'db
 -- Function: require 'wb
     Either form will dynamically load the WB database system from the wb:vicinity (../wb/) specified in Link.scm.  See scm/ANNOUNCE for ftp sites where WB is available.

The following functions comprise the low-level Scheme interface to dynamic linking.  See the file Link.scm in the SCM distribution for an example of their use.

 -- Function: dyn:link filename
     FILENAME should be a string naming an object or archive file, the result of C-compiling.  The dyn:link procedure links and loads FILENAME into the current SCM session. If successfull, dyn:link returns a link-token suitable for passing as the second argument to dyn:call.  If not successful, #f is returned.

 -- Function: dyn:call name link-token
     LINK-TOKEN should be the value returned by a call to dyn:link.  NAME should be the name of C function of no arguments defined in the file named FILENAME which was succesfully dyn:linked in the current SCM session.  The dyn:call procedure calls the C function corresponding to NAME.  If successful, dyn:call returns #t; If not successful, #f is returned.

     dyn:call is used to call the init_... function after loading SCM object files.  The init_... function then makes the identifiers defined in the file accessible as Scheme procedures.

 -- Function: dyn:unlink link-token
     LINK-TOKEN should be the value returned by a call to dyn:link.  The dyn:unlink procedure removes the previously loaded file from the current SCM session.  If successful, dyn:unlink returns #t; If not successful, #f is returned.

 -- Function: usr:lib lib
     Returns the pathname of the C library named lib.  For example: (usr:lib "m") could return "/usr/lib/libm.a", the path of the C math library.

Numeric
=======

 -- Constant: most-positive-fixnum
     The immediate integer closest to positive infinity. See Configuration (SLIB).

 -- Constant: most-negative-fixnum
     The immediate integer closest to negative infinity.

These procedures augment the standard capabilities in Numerical operations (Revised(4) Scheme).

 -- Function: sinh z
 -- Function: cosh z
 -- Function: tanh z
     Return the hyperbolic sine, cosine, and tangent of Z

 -- Function: asinh z
 -- Function: acosh z
 -- Function: atanh z
     Return the inverse hyperbolic sine, cosine, and tangent of Z

 -- Function: $sqrt x
 -- Function: $abs x
 -- Function: $exp x
 -- Function: $log x
 -- Function: $sin x
 -- Function: $cos x
 -- Function: $tan x
 -- Function: $asin x
 -- Function: $acos x
 -- Function: $atan x

 -- Function: $sinh x
 -- Function: $cosh x
 -- Function: $tanh x
 -- Function: $asinh x
 -- Function: $acosh x
 -- Function: $atanh x
     Real-only versions of these popular functions.  The argument X must be a real number.  It is an error if the value which should be returned by a call to these procedures is not real.

 -- Function: $atan2 y x
     Computes (angle (make-rectangular x y)) for real numbers Y and X.

 -- Function: $expt x1 x2
     Returns real number X1 raised to the real power X2.  It is an error if the value which should be returned by a call to $expt is not real.

Arrays
======

Conventional Arrays
-------------------

Arrays read and write as a # followed by the rank (number of dimensions) followed by what appear as lists (of lists) of elements.  The lists must be nested to the depth of the rank.  For each depth, all lists must be the same length.

     (make-array 'ho 3 3) =>
     #2((ho ho ho) (ho ho ho) (ho ho ho))

Unshared conventional (not uniform) 0-based arrays of rank 1 (dimension) are equivalent to (and can't be distinguished from) vectors.

     (make-array 'ho 3) => (ho ho ho)

When constructing an array, BOUND is either an inclusive range of indices expressed as a two element list, or an upper bound expressed as a single integer.  So

     (make-array 'foo 3 3) == (make-array 'foo '(0 2) '(0 2))

 -- Function: array? obj
     Returns #t if the OBJ is an array, and #f if not.

 -- Function: make-array initial-value bound1 bound2 ...
     Creates and returns an array that has as many dimensions as there are BOUNDs and fills it with INITIAL-VALUE.

 -- Function: array-ref array index1 index2 ...
     Returns the element at the (index1, index2) element in ARRAY.

 -- Function: array-in-bounds? array index1 index2 ...
     Returns #t if its arguments would be acceptable to array-ref.

 -- Function: array-set! array new-value index1 index2 ...
     Sets the element at the (index1, index2) element in ARRAY to NEW-VALUE.  The value returned by array-set! is unspecified.

 -- Function: make-shared-array array mapper bound1 bound2 ...
     make-shared-array can be used to create shared subarrays of other arrays.  The MAPPER is a function that translates coordinates in the new array into coordinates in the old array.  A MAPPER must be linear, and its range must stay within the bounds of the old array, but it can be otherwise arbitrary.  A simple example:

          (define fred (make-array #f 8 8))
          (define freds-diagonal
            (make-shared-array fred (lambda (i) (list i i)) 8))
          (array-set! freds-diagonal 'foo 3)
          (array-ref fred 3 3) => foo
          (define freds-center
            (make-shared-array fred (lambda (i j) (list (+ 3 i) (+ 3 j))) 2 2))
          (array-ref freds-center 0 0) => foo

 -- Function: transpose-array array dim0 dim1 ...
     Returns an array sharing contents with ARRAY, but with dimensions arranged in a different order.  There must be one DIM argument for each dimension of ARRAY.  DIM0, DIM1, ... should be integers between 0 and the rank of the array to be returned.  Each integer in that range must appear at least once in the argument list.

     The values of DIM0, DIM1, ... correspond to dimensions in the array to be returned, their positions in the argument list to dimensions of ARRAY.  Several DIMs may have the same value, in which case the returned array will have smaller rank than ARRAY.

     examples:

          (transpose-array '#2((a b) (c d)) 1 0) => #2((a c) (b d))
          (transpose-array '#2((a b) (c d)) 0 0) => #1(a d)
          (transpose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 1 0) =>
                          #2((a 4) (b 5) (c 6))

 -- Function: enclose-array array dim0 dim1 ...
     DIM0, DIM1 ... should be nonnegative integers less than the rank of ARRAY.  ENCLOSE-ARRAY returns an array resembling an array of shared arrays.  The dimensions of each shared array are the same as the DIMth dimensions of the original array, the dimensions of the outer array are the same as those of the original array that did not match a DIM.

     An enclosed array is not a general Scheme array.  Its elements may not be set using array-set!.  Two references to the same element of an enclosed array will be equal? but will not in general be eq?.  The value returned by ARRAY-PROTOTYPE when given an enclosed array is unspecified.

     examples:

          (enclose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1) =>
             #<enclosed-array (#1(a d) #1(b e) #1(c f)) (#1(1 4) #1(2 5) #1(3 6))>

          (enclose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 0) =>
             #<enclosed-array #2((a 1) (d 4)) #2((b 2) (e 5)) #2((c 3) (f 6))>

 -- Function: array-shape array
     Returns a list of inclusive bounds of integers.

          (array-shape (make-array 'foo '(-1 3) 5)) => ((-1 3) (0 4))

 -- Function: array-dimensions array
     Array-dimensions is similar to array-shape but replaces elements with a 0 minimum with one greater than the maximum. So:

          (array-dimensions (make-array 'foo '(-1 3) 5)) => ((-1 3) 5)

 -- Function: array-rank obj
     Returns the number of dimensions of OBJ.  If OBJ is not an array, 0 is returned.

 -- Function: array->list array
     Returns a list consisting of all the elements, in order, of ARRAY.

 -- Function: array-copy! source destination
     Copies every element from vector or array SOURCE to the corresponding element of DESTINATION.  DESTINATION must have the same rank as SOURCE, and be at least as large in each dimension.  The order is unspecified.

 -- Function: serial-array-copy! source destination
     Same as array-copy! but guaranteed to copy in row-major order.

 -- Function: array-fill! array fill
     Stores FILL in every element of ARRAY.  The value returned is unspecified.

 -- Function: array-equal? array0 array1 ...
     Returns #t iff all arguments are arrays with the same shape, the same type, and have corresponding elements which are either equal?  or array-equal?.  This function differs from equal? in that a one dimensional shared array may be ARRAY-EQUAL? but not EQUAL? to a vector or uniform vector.

 -- Function: array-contents array
 -- Function: array-contents array strict
     If ARRAY may be unrolled into a one dimensional shared array without changing their order (last subscript changing fastest), then array-contents returns that shared array, otherwise it returns #f.  All arrays made by MAKE-ARRAY and MAKE-UNIFORM-ARRAY may be unrolled, some arrays made by MAKE-SHARED-ARRAY may not be.

     If the optional argument STRICT is provided, a shared array will be returned only if its elements are stored internally contiguous in memory.

Array Mapping
-------------

 -- Function: array-map! array0 proc array1 ...
     ARRAY1, ... must have the same number of dimensions as ARRAY0 and have a range for each index which includes the range for the corresponding index in ARRAY0.  PROC is applied to each tuple of elements of ARRAY1 ... and the result is stored as the corresponding element in ARRAY0.  The value returned is unspecified.  The order of application is unspecified.

 -- Function: serial-array-map! array0 proc array1 ...
     Same as ARRAY-MAP!, but guaranteed to apply PROC in row-major order.

 -- Function: array-for-each proc array0 ...
     PROC is applied to each tuple of elements of ARRAY0 ... in row-major order.  The value returned is unspecified.

 -- Function: array-index-map! array proc
     applies PROC to the indices of each element of ARRAY in turn, storing the result in the corresponding element.  The value returned and the order of application are unspecified.

     One can implement ARRAY-INDEXES as

          (define (array-indexes array)
              (let ((ra (apply make-array #f (array-shape array))))
                (array-index-map! ra (lambda x x))
                ra))

     Another example:

          (define (apl:index-generator n)
              (let ((v (make-uniform-vector n 1)))
                (array-index-map! v (lambda (i) i))
                v))

Uniform Array
-------------

Uniform Arrays and vectors are arrays whose elements are all of the same type.  Uniform vectors occupy less storage than conventional vectors.  Uniform Array procedures also work on vectors, uniform-vectors, bit-vectors, and strings.

PROTOTYPE arguments in the following procedures are interpreted according to the table:

     prototype       type                            printing character

     #t              boolean (bit-vector)                    b
     #\a             char (string)                           a
     integer >0      unsigned integer                        u
     integer <0      signed integer                          e
     1.0             float (single precision)                s
     1/3             double (double precision float)         i
     +i              complex (double precision)              c
     ()              conventional vector

Unshared uniform character 0-based arrays of rank 1 (dimension) are equivalent to (and can't be distinguished from) strings.

     (make-uniform-array #\a 3) => "$q2"

Unshared uniform boolean 0-based arrays of rank 1 (dimension) are equivalent to (and can't be distinguished from) bit-vectors.

     (make-uniform-array #t 3) => #*000
     ==
     #b(#f #f #f) => #*000
     ==
     #1b(#f #f #f) => #*000

Other uniform vectors are written in a form similar to that of vectors, except that a single character from the above table is put between # and (.  For example, '#e(3 5 9) returns a uniform vector of signed integers.

 -- Function: array? obj prototype
     Returns #t if the OBJ is an array of type corresponding to PROTOTYPE, and #f if not.

 -- Function: make-uniform-array prototype bound1 bound2 ...
     Creates and returns a uniform array of type corresponding to PROTOTYPE that has as many dimensions as there are BOUNDs and fills it with PROTOTYPE.

 -- Function: array-prototype array
     Returns an object that would produce an array of the same type as ARRAY, if used as the PROTOTYPE for make-uniform-array.

 -- Function: list->uniform-array rank prot lst
 -- Function: list->uniform-vector prot lst
     Returns a uniform array of the type indicated by prototype PROT with elements the same as those of LST.  Elements must be of the appropriate type, no coercions are done.

 -- Function: uniform-vector-fill! uve fill
     Stores FILL in every element of UVE.  The value returned is unspecified.

 -- Function: uniform-vector-length uve
     Returns the number of elements in UVE.

 -- Function: dimensions->uniform-array dims prototype fill
 -- Function: dimensions->uniform-array dims prototype
 -- Function: make-uniform-vector length prototype fill
 -- Function: make-uniform-vector length prototype
     Creates and returns a uniform array or vector of type corresponding to PROTOTYPE with dimensions DIMS or length LENGTH.  If the FILL argument is supplied, the returned array is filled with this value.

 -- Function: uniform-array-read! ura
 -- Function: uniform-array-read! ura port
 -- Function: uniform-vector-read! uve
 -- Function: uniform-vector-read! uve port
     Attempts to read all elements of URA, in lexicographic order, as binary objects from PORT.  If an end of file is encountered during uniform-array-read! the objects up to that point only are put into URA (starting at the beginning) and the remainder of the array is unchanged.

     uniform-array-read! returns the number of objects read. PORT may be omitted, in which case it defaults to the value returned by (current-input-port).

 -- Function: uniform-array-write ura
 -- Function: uniform-array-write ura port
 -- Function: uniform-vector-write uve
 -- Function: uniform-vector-write uve port
     Writes all elements of URA as binary objects to PORT. The number of objects actually written is returned. PORT may be omitted, in which case it defaults to the value returned by (current-output-port).

Bit Vectors
-----------

Bit vectors can be written and read as a sequence of 0s and 1s prefixed by #*.

     #b(#f #f #f #t #f #t #f) => #*0001010

Some of these operations will eventually be generalized to other uniform-arrays.

 -- Function: bit-count bool bv
     Returns the number occurrences of BOOL in BV.

 -- Function: bit-position bool bv k
     Returns the minimum index of an occurrence of BOOL in BV which is at least K.  If no BOOL occurs within the specified range #f is returned.

 -- Function: bit-invert! bv
     Modifies BV by replacing each element with its negation.

 -- Function: bit-set*! bv uve bool
     If uve is a bit-vector BV and uve must be of the same length.  If BOOL is #t, uve is OR'ed into BV; If BOOL is #f, the inversion of uve is AND'ed into BV.

     If uve is a unsigned integer vector all the elements of uve must be between 0 and the LENGTH of BV.  The bits of BV corresponding to the indexes in uve are set to BOOL.

     The return value is unspecified.

 -- Function: bit-count* bv uve bool
     Returns

          (bit-count (bit-set*! (if bool bv (bit-invert! bv)) uve #t) #t).

     BV is not modified.

I/O-Extensions
==============

If 'i/o-extensions is provided (by linking in ioext.o), Line I/O (SLIB), and the following functions are defined:

 -- Function: isatty? port
     Returns #t if PORT is input or output to a serial non-file device.

 -- Function: stat <port-or-string>
     Returns a vector of integers describing the argument.  The argument can be either a string or an open input port. If the argument is an open port then the returned vector describes the file to which the port is opened; If the argument is a string then the returned vector describes the file named by that string.  If there exists no file with the name string, or if the file cannot be accessed #f is returned. The elements of the returned vector are as follows:

     0  st_dev
          ID of device containing a directory entry for this file

     1  st_ino
          Inode number

     2  st_mode
          File type, attributes, and access control summary

     3  st_nlink
          Number of links

     4  st_uid
          User ID of file owner

     5  st_gid
          Group ID of file group

     6  st_rdev
          Device ID; this entry defined only for char or blk spec files

     7  st_size
          File size (bytes)

     8  st_atime
          Time of last access

     9  st_mtime
          Last modification time

     10 st_ctime
          Last file status change time

 -- Function: getpid
     Returns the process ID of the current process.

 -- Function: file-position port
     Returns the current position of the character in PORT which will next be read or written.  If PORT is not open to a file the result is unspecified.

 -- Function: file-set-position port integer
     Sets the current position in PORT which will next be read or written.  If PORT is not open to a file the action of file-set-position is unspecified.  The result of file-set-position is unspecified.

 -- Function: reopen-file filename modes port
     Closes port PORT and reopens it with FILENAME and MODES.  reopen-file returns #t if successful, #f if not.

 -- Function: duplicate-port port modes
     Creates and returns a duplicate port from PORT.  Duplicate unbuffered ports share one file position.  MODES are as for open-file.

 -- Function: redirect-port! from-port to-port
     Closes TO-PORT and makes TO-PORT be a duplicate of FROM-PORT.  redirect-port! returns TO-PORT if successful, #f if not.  If unsuccessful, TO-PORT is not closed.

 -- Function: opendir dirname
     Returns a directory object corresponding to the file system directory named DIRNAME.  If unsuccessful, returns #f.

 -- Function: readdir dir
     Returns the string name of the next entry from the directory DIR. If there are no more entries in the directory, readdir returns a #f.

 -- Function: rewinddir dir
     Reinitializes DIR so that the next call to readdir with DIR will return the first entry in the directory again.

 -- Function: closedir dir
     Closes DIR and returns #t.  If DIR is already closed,, closedir returns a #f.

 -- Function: mkdir path mode
     The mkdir function creates a new, empty directory whose name is PATH.  The integer argument MODE specifies the file permissions for the new directory.  See The Mode Bits for Access Permission (Gnu C Library), for more information about this.

     mkdir returns if successful, #f if not.

 -- Function: rmdir path
     The rmdir function deletes the directory PATH.  The directory must be empty before it can be removed.  rmdir returns if successful, #f if not.

 -- Function: chdir filename
     Changes the current directory to FILENAME.  If FILENAME does not exist or is not a directory, #f is returned.  Otherwise, #t is returned.

 -- Function: getcwd
     The function getcwd returns a string containing the absolute file name representing the current working directory.  If this string cannot be obtained, #f is returned.

 -- Function: rename-file oldfilename newfilename
     Renames the file specified by OLDFILENAME to NEWFILENAME. If the renaming is successful, #t is returned.  Otherwise, #f is returned.

 -- Function: chmod file mode
     The function chmod sets the access permission bits for the file named by FILE to MODE.  The FILE argument may be a string containing the filename or a port open to the file.

     chmod returns if successful, #f if not.

 -- Function: utime pathname acctime modtime
     Sets the file times associated with the file named PATHNAME to have access time ACCTIME and modification time MODTIME. utime returns if successful, #f if not.

 -- Function: umask mode
     The function umask sets the file creation mask of the current process to MASK, and returns the previous value of the file creation mask.

 -- Function: fileno port
     Returns the integer file descriptor associated with the port PORT. If an error is detected, #f is returned.

 -- Function: access pathname how
     Returns #t if the file named by PATHNAME can be accessed in the way specified by the HOW argument.  The HOW argument can be the logior of the flags:

      0. File exists.

      1. File is executable.

      2. File is writable

      4. File is readable

 -- Function: execl command arg0 ...
 -- Function: execlp command arg0 ...
     Transfers control to program COMMAND called with arguments ARG0 ....  For execl, COMMAND must be an exact pathname of an executable file.  execlp searches for COMMAND in the list of directories specified by the environment variable PATH.  The convention is that ARG0 is the same name as COMMAND.

     If successful, this procedure does not return.  Otherwise an error message is printed and the integer errno is returned.

 -- Function: execv command arglist
 -- Function: execvp command arglist
     Like execl and execlp except that the set of arguments to COMMAND is ARGLIST.

 -- Function: putenv string
     adds or removes definitions from the environment. If the STRING is of the form NAME=VALUE, the definition is added to the environment.  Otherwise, the STRING is interpreted as the name of an environment variable, and any definition for this variable in the environment is removed.

     Names of environment variables are case-sensitive and must not contain the character =.  System-defined environment variables are invariably uppercase.

     Putenv is used to set up the environment before calls to execl, execlp, execv, execvp, system, or open-pipe (see open-pipe).

     To access environment variables, use getenv (see getenv (SLIB)).

Posix Extensions
================

If 'posix is provided (by linking in posix.o), the following functions are defined:

 -- Function: open-pipe string modes
     If the string MODES contains an r, returns an input port capable of delivering characters from the standard output of the system command STRING.  Otherwise, returns an output port capable of receiving characters which become the standard input of the system command STRING.  If a pipe cannot be created #f is returned.

 -- Function: open-input-pipe string
     Returns an input port capable of delivering characters from the standard output of the system command STRING.  If a pipe cannot be created #f is returned.

 -- Function: open-output-pipe string
     Returns an output port capable of receiving characters which become the standard input of the system command STRING.  If a pipe cannot be created #f is returned.

 -- Function: close-port pipe
     Closes the PIPE, rendering it incapable of delivering or accepting characters.  This routine has no effect if the pipe has already been closed.  The value returned is unspecified.

 -- Function: pipe
     Returns (cons RD WD) where RD and WD are the read and write (port) ends of a pipe respectively.

 -- Function: fork
     Creates a copy of the process calling fork.  Both processes return from fork, but the calling (parent) process's fork returns the child process's ID whereas the child process's fork returns 0.

For a discussion of IDs see Process Persona (libc).

 -- Function: getppid
     Returns the process ID of the parent of the current process. For a process's own ID see getpid.

 -- Function: getuid
     Returns the real user ID of this process.

 -- Function: getgid
     Returns the real group ID of this process.

 -- Function: getegid
     Returns the effective group ID of this process.

 -- Function: geteuid
     Returns the effective user ID of this process.

 -- Function: setuid id
     Sets the real user ID of this process to ID. Returns #t if successful, #f if not.

 -- Function: setgid id
     Sets the real group ID of this process to ID. Returns #t if successful, #f if not.

 -- Function: setegid id
     Sets the effective group ID of this process to ID. Returns #t if successful, #f if not.

 -- Function: seteuid id
     Sets the effective user ID of this process to ID. Returns #t if successful, #f if not.

 -- Function: kill pid sig
     The kill function sends the signal SIGNUM to the process or process group specified by PID.  Besides the signals listed in Standard Signals (GNU C Library), SIGNUM can also have a value of zero to check the validity of the PID.

     The PID specifies the process or process group to receive the signal:

     > 0
          The process whose identifier is PID.

     0
          All processes in the same process group as the sender.  The sender itself does not receive the signal.

     -1
          If the process is privileged, send the signal to all processes except for some special system processes. Otherwise, send the signal to all processes with the same effective user ID.

     < -1
          The process group whose identifier is (abs PID).

     A process can send a signal to itself with (kill (getpid) SIGNUM).  If kill is used by a process to send a signal to itself, and the signal is not blocked, then kill delivers at least one signal (which might be some other pending unblocked signal instead of the signal SIGNUM) to that process before it returns.

     The return value from kill is zero if the signal can be sent successfully.  Otherwise, no signal is sent, and a value of -1 is returned.  If PID specifies sending a signal to several processes, kill succeeds if it can send the signal to at least one of them. There's no way you can tell which of the processes got the signal or whether all of them did.

 -- Function: waitpid pid options

     The waitpid function suspends execution of the current process until a child as specified by the PID argument has exited, or until a signal is deliverd whose action is to terminate the current process or to call a signal handling function.  If a child as requested by PID has already exited by the time of the call (a so-called zombie process), the function returns immediately.  Any system resources used by the child are freed.

     The value of PID can be one of:

     < -1
          which means to wait for any child process whose process group ID is equal to the absolute value of

     -1
          which means to wait for any child process whose process group ID is equal to the (abs PID).

     -1
          which means to wait for any child process; this is the same behaviour which wait exhibits.

     0
          which means to wait for any child process whose process group ID is equal to that of the calling process.

     > 0
          which means to wait for the child whose process ID is equal to the value of PID.

     The value of OPTIONS is one of the following:

      0. Nothing special.

      1. (WNOHANG) which means to return immediately if no child is there to be waited for.

      2. (WUNTRACED) which means to also return for children which are stopped, and whose status has not been reported.

      3. Which means both of the above.

     The return value is normally the process ID of the child process whose status is reported.  If the WNOHANG option was specified and no child process is waiting to be noticed, the value is zero.  A value of #f is returned in case of error and errno is set.  For information about the errno codes see Process Completion (libc).

 -- Function: uname
     You can use the uname procedure to find out some information about the type of computer your program is running on.

     Returns a vector of strings.  These strings are:

      0. The name of the operating system in use.

      1. The network name of this particular computer.

      2. The current release level of the operating system implementation.

      3. The current version level within the release of the operating system.

      4. Description of the type of hardware that is in use.

          Some examples are "i386-ANYTHING", "m68k-hp", "sparc-sun", "m68k-sun", "m68k-sony" and "mips-dec".

 -- Function: getpw name
 -- Function: getpw uid
 -- Function: getpw
     Returns a vector of information for the entry for NAME, UID, or the next entry if no argument is given.  The information is:

      0. The user's login name.

      1. The encrypted password string.

      2. The user ID number.

      3. The user's default group ID number.

      4. A string typically containing the user's real name, and possibly other information such as a phone number.

      5. The user's home directory, initial working directory, or #f, in which case the interpretation is system-dependent.

      6. The user's default shell, the initial program run when the user logs in, or #f, indicating that the system default should be used.

 -- Function: setpwent #t
     Rewinds the pw entry table back to the begining.

 -- Function: setpwent #f
 -- Function: setpwent
     Closes the pw table.

 -- Function: getgr name
 -- Function: getgr uid
 -- Function: getgr
     Returns a vector of information for the entry for NAME, UID, or the next entry if no argument is given.  The information is:

      0. The name of the group.

      1. The encrypted password string.

      2. The group ID number.

      3. A list of (string) names of users in the group.

 -- Function: setgrent #t
     Rewinds the group entry table back to the begining.

 -- Function: setgrent #f
 -- Function: setgrent
     Closes the group table.

 -- Function: getgroups
     Returns a vector of all the supplementary group IDs of the process.

 -- Function: link oldname newname
     The link function makes a new link to the existing file named by OLDNAME, under the new name NEWNAME.

     link returns a value of #t if it is successful and #f on failure.

 -- Function: chown filename owner group
     The chown function changes the owner of the file FILENAME to OWNER, and its group owner to GROUP.

     chown returns a value of #t if it is successful and #f on failure.

 -- Function: ttyname port
     If port PORT is associated with a terminal device, returns a string containing the file name of termainal device; otherwise #f.

Unix Extensions
===============

If 'unix is provided (by linking in unix.o), the following functions are defined:

These priveledged and symbolic link functions are not in Posix:

 -- Function: symlink oldname newname
     The symlink function makes a symbolic link to OLDNAME named NEWNAME.

     symlink returns a value of #t if it is successful and #f on failure.

 -- Function: readlink filename
     Returns the value of the symbolic link FILENAME or #f for failure.

 -- Function: lstat filename
     The lstat function is like stat, except that it does not follow symbolic links.  If FILENAME is the name of a symbolic link, lstat returns information about the link itself; otherwise, lstat works like stat.  See I/O-Extensions.

 -- Function: nice increment
     Increment the priority of the current process by INCREMENT. chown returns a value of #t if it is successful and #f on failure.

 -- Function: acct filename
     When called with the name of an exisitng file as argument, accounting is turned on, records for each terminating pro-cess are appended to FILENAME as it terminates.  An argument of #f causes accounting to be turned off.

     acct returns a value of #t if it is successful and #f on failure.

 -- Function: mknod filename mode dev
     The mknod function makes a special file with name FILENAME and modes MODE for device number DEV.

     mknod returns a value of #t if it is successful and #f on failure.

 -- Function: sync
     sync first commits inodes to buffers, and then buffers to disk. sync() only schedules the writes, so it may return before the actual writing is done.  The value returned is unspecified.

Regular Expression Pattern Matching
===================================

These functions are defined in rgx.c using a POSIX or GNU regex library.  If your computer does not support regex, a package is available via ftp from prep.ai.mit.edu:/pub/gnu/regex-0.12.tar.gz.  For a description of regular expressions, see syntax ("regex" regular expression matching library).

 -- Function: regcomp PATTERN [FLAGS]
     Compile a regular expression.  Return a compiled regular expression, or an integer error code suitable as an argument to regerror.

     FLAGS in regcomp is a string of option letters used to control the compilation of the regular expression. The letters may consist of:

     n
          newlines won't be matched by . or hat lists; ( [^...] )

     i
          ignore case.
          only when compiled with _GNU_SOURCE:

     0
          allows dot to match a null character.

     f
          enable GNU fastmaps.

 -- Function: regerror ERRNO
     Returns a string describing the integer ERRNO returned when regcomp fails.

 -- Function: regexec RE STRING
     Returns #f or a vector of integers.  These integers are in doublets.  The first of each doublet is the index of STRING of the start of the matching expression or sub-expression (delimited by parentheses in the pattern).  The last of each doublet is index of STRING of the end of that expression.  #f is returned if the string does not match.

 -- Function: regmatch? RE STRING
     Returns #t if the PATTERN such that REGEXP = (regcomp PATTERN) matches STRING as a POSIX extended regular expressions.  Returns #f otherwise.

 -- Function: regsearch RE STRING [START [LEN]]
 -- Function: regsearchv RE STRING [START [LEN]]
 -- Function: regmatch RE STRING [START [LEN]]
 -- Function: regmatchv RE STRING [START [LEN]]
     Regsearch searches for the pattern within the string.

     Regmatch anchors the pattern and begins matching it against string.

     Regsearch returns the character position where RE starts, or #f if not found.

     Regmatch returns the number of characters matched, #f if not matched.

     Regsearchv and regmatchv return the match vector is returned if RE is found, #f otherwise.

     re
          may be either:

           1. a compiled regular expression returned by regcomp;

           2. a string representing a regular expression;

           3. a list of a string and a set of option letters.

     string
          The string to be operated upon.

     start
          The character position at which to begin the search or match. If absent, the default is zero.

          Compiled _GNU_SOURCE and using GNU libregex only:

          When searching, if START is negative, the absolute value of START will be used as the start location and reverse searching will be performed.

     len
          The search is allowed to examine only the first LEN characters of STRING. If absent, the entire string may be examined.

 -- Function: string-split RE STRING
 -- Function: string-splitv RE STRING
     String-split splits a string into substrings that are separated by RE, returning a vector of substrings.

     String-splitv returns a vector of string positions that indicate where the substrings are located.

 -- Function: string-edit RE EDIT-SPEC STRING [COUNT]
     Returns the edited string.

     edit-spec
          Is a string used to replace occurances of RE.  Backquoted integers in the range of 1-9 may be used to insert subexpressions in RE, as in sed.

     count
          The number of substitutions for string-edit to perform.  If #t, all occurances of RE will be replaced.  The default is to perform one substitution.

Line Editing
============

These procedures provide input line editing and recall.

These functions are defined in edline.c and Iedline.scm using the editline or GNU readline libraries available from:

  * ftp.sys.toronto.edu:/pub/rc/editline.shar

  * prep.ai.mit.edu:pub/gnu/readline-2.0.tar.gz

When Iedline.scm is loaded, if the current input port is the default input port and the environment variable EMACS is not defined, line-editing mode will be entered.

 -- Function: default-input-port
     Returns the initial current-input-port SCM was invoked with (stdin).

 -- Function: default-output-port
     Returns the initial current-output-port SCM was invoked with (stdout).

 -- Function: make-edited-line-port
     Returns an input/output port that allows command line editing and retrieval of history.

 -- Function: line-editing
     Returns the current edited line port or #f.

 -- Function: line-editing bool
     If BOOL is false, exits line-editing mode and returns the previous value of (line-editing).  If BOOL is true, sets the current input and output ports to an edited line port and returns the previous value of (line-editing).

Curses
======

These functions are defined in crs.c using the curses library.  Unless otherwise noted these routines return #t for successful completion and #f for failure.

 -- Function: initscr
     Returns a port for a full screen window.  This routine must be called to initialize curses.

 -- Function: endwin
     A program should call endwin before exiting or escaping from curses mode temporarily, to do a system call, for example.  This routine will restore termio modes, move the cursor to the lower left corner of the screen and reset the terminal into the proper non-visual mode.  To resume after a temporary escape, call refresh.

Output Options Setting
----------------------

These routines set options within curses that deal with output.  All options are initially #f, unless otherwise stated.  It is not necessary to turn these options off before calling endwin.

 -- Function: clearok win bf
     If enabled (BF is #t), the next call to force-output or refresh with WIN will clear the screen completely and redraw the entire screen from scratch.  This is useful when the contents of the screen are uncertain, or in some cases for a more pleasing visual effect.

 -- Function: idlok win bf
     If enabled (BF is #t), curses will consider using the hardware ``insert/delete-line'' feature of terminals so equipped.  If disabled (BF is #f), curses will very seldom use this feature.  The ``insert/delete-character'' feature is always considered. This option should be enabled only if your application needs ``insert/delete-line'', for example, for a screen editor.  It is disabled by default because

     ``insert/delete-line'' tends to be visually annoying when used in applications where it is not really needed.  If ``insert/delete-line'' cannot be used, curses will redraw the changed portions of all lines.

 -- Function: leaveok win bf
     Normally, the hardware cursor is left at the location of the window cursor being refreshed.  This option allows the cursor to be left wherever the update happens to leave it.  It is useful for applications where the cursor is not used, since it reduces the need for cursor motions.  If possible, the cursor is made invisible when this option is enabled.

 -- Function: scrollok win bf
     This option controls what happens when the cursor of window WIN is moved off the edge of the window or scrolling region, either from a newline on the bottom line, or typing the last character of the last line.  If disabled (BF is #f), the cursor is left on the bottom line at the location where the offending character was entered. If enabled (BF is #t), force-output is called on the window WIN, and then the physical terminal and window WIN are scrolled up one line.

     Note: in order to get the physical scrolling effect on the terminal, it is also necessary to call idlok.

 -- Function: nodelay win bf
     This option causes wgetch to be a non-blocking call.  If no input is ready, wgetch will return an eof-object. If disabled, wgetch will hang until a key is pressed.

Terminal Mode Setting
---------------------

These routines set options within curses that deal with input.  The options involve using ioctl(2) and therefore interact with curses routines.  It is not necessary to turn these options off before calling endwin.  The routines in this section all return an unspecified value.

 -- Function: cbreak
 -- Function: nocbreak
     These two routines put the terminal into and out of CBREAK mode, respectively.  In CBREAK mode, characters typed by the user are immediately available to the program and erase/kill character processing is not performed.  When in NOCBREAK mode, the tty driver will buffer characters typed until a LFD or RET is typed. Interrupt and flowcontrol characters are unaffected by this mode. Initially the terminal may or may not be in CBREAK mode, as it is inherited, therefore, a program should call cbreak or nocbreak explicitly.  Most interactive programs using curses will set CBREAK mode.

     Note: cbreak overrides raw.  For a discussion of how these routines interact with echo and noecho see read-char.

 -- Function: raw
 -- Function: noraw
     The terminal is placed into or out of RAW mode.  RAW mode is similar to CBREAK mode, in that characters typed are immediately passed through to the user program.  The differences are that in RAW mode, the interrupt, quit, suspend, and flow control characters are passed through uninterpreted, instead of generating a signal.  RAW mode also causes 8-bit input and output.  The behavior of the BREAK key depends on other bits in the terminal driver that are not set by curses.

 -- Function: echo
 -- Function: noecho
     These routines control whether characters typed by the user are echoed by read-char as they are typed.  Echoing by the tty driver is always disabled, but initially read-char is in ECHO mode, so characters typed are echoed.  Authors of most interactive programs prefer to do their own echoing in a controlled area of the screen, or not to echo at all, so they disable echoing by calling noecho. For a discussion of how these routines interact with echo and noecho see read-char.

 -- Function: nl
 -- Function: nonl
     These routines control whether LFD is translated into RET and LFD on output, and whether RET is translated into LFD on input.  Initially, the translations do occur.  By disabling these translations using nonl, curses is able to make better use of the linefeed capability, resulting in faster cursor motion.

 -- Function: resetty
 -- Function: savetty
     These routines save and restore the state of the terminal modes. savetty saves the current state of the terminal in a buffer and resetty restores the state to what it was at the last call to savetty.

Window Manipulation
-------------------

 -- Function: newwin nlines ncols begy begx
     Create and return a new window with the given number of lines (or rows), NLINES, and columns, NCOLS.  The upper left corner of the window is at line BEGY, column BEGX.  If either NLINES or NCOLS is 0, they will be set to the value of LINES-BEGY and COLS-BEGX.  A new full-screen window is created by calling newwin(0,0,0,0).

 -- Function: subwin orig nlines ncols begy begx
     Create and return a pointer to a new window with the given number of lines (or rows), NLINES, and columns, NCOLS.  The window is at position (BEGY, BEGX) on the screen.  This position is relative to the screen, and not to the window ORIG.  The window is made in the middle of the window ORIG, so that changes made to one window will affect both windows.  When using this routine, often it will be necessary to call touchwin or touchline on ORIG before calling force-output.

 -- Function: close-port win
     Deletes the window WIN, freeing up all memory associated with it. In the case of sub-windows, they should be deleted before the main window WIN.

 -- Function: refresh
 -- Function: force-output win
     These routines are called to write output to the terminal, as most other routines merely manipulate data structures.  force-output copies the window WIN to the physical terminal screen, taking into account what is already there in order to minimize the amount of information that's sent to the terminal (called optimization).  Unless leaveok has been enabled, the physical cursor of the terminal is left at the location of window WIN's cursor.  With refresh, the number of characters output to the terminal is returned.

 -- Function: mvwin win y x
     Move the window WIN so that the upper left corner will be at position (Y, X).  If the move would cause the window WIN to be off the screen, it is an error and the window WIN is not moved.

 -- Function: overlay srcwin dstwin
 -- Function: overwrite srcwin dstwin

     These routines overlay SRCWIN on top of DSTWIN; that is, all text in SRCWIN is copied into DSTWIN.  SRCWIN and DSTWIN need not be the same size; only text where the two windows overlap is copied.  The difference is that overlay is non-destructive (blanks are not copied), while overwrite is destructive.

 -- Function: touchwin win
 -- Function: touchline win start count
     Throw away all optimization information about which parts of the window WIN have been touched, by pretending that the entire window WIN has been drawn on.  This is sometimes necessary when using overlapping windows, since a change to one window will affect the other window, but the records of which lines have been changed in the other window will not reflect the change.  touchline only pretends that COUNT lines have been changed, beginning with line START.

 -- Function: wmove win y x
     The cursor associated with the window WIN is moved to line (row) Y, column X.  This does not move the physical cursor of the terminal until refresh (or force-output) is called.  The position specified is relative to the upper left corner of the window WIN, which is (0, 0).

Output
------

These routines are used to draw text on windows

 -- Function: display ch win
 -- Function: display str win
 -- Function: wadd win ch
 -- Function: wadd win str
     The character CH or characters in STR are put into the window WIN at the current cursor position of the window and the position of WIN's cursor is advanced.  At the right margin, an automatic newline is performed.  At the bottom of the scrolling region, if scrollok is enabled, the scrolling region will be scrolled up one line.

     If CH is a TAB, LFD, or backspace, the cursor will be moved appropriately within the window WIN.  A LFD also does a wclrtoeol before moving.  TAB characters are considered to be at every eighth column.  If CH is another control character, it will be drawn in the C-x notation.  (Calling winch after adding a control character will not return the control character, but instead will return the representation of the control character.)

     Video attributes can be combined with a character by or-ing them into the parameter.  This will result in these attributes also being set. The intent here is that text, including attributes, can be copied from one place to another using inch and display.  See standout, below.

     Note: For wadd CH can be an integer and will insert the character of the corresponding value.

 -- Function: werase win
     This routine copies blanks to every position in the window WIN.

 -- Function: wclear win
     This routine is like werase, but it also calls clearok, arranging that the screen will be cleared completely on the next call to refresh or force-output for window WIN, and repainted from scratch.

 -- Function: wclrtobot win
     All lines below the cursor in window WIN are erased.  Also, the current line to the right of the cursor, inclusive, is erased.

 -- Function: wclrtoeol win
     The current line to the right of the cursor, inclusive, is erased.

 -- Function: wdelch win
     The character under the cursor in the window WIN is deleted.  All characters to the right on the same line are moved to the left one position and the last character on the line is filled with a blank.  The cursor position does not change.  This does not imply use of the hardware ``delete-character'' feature.

 -- Function: wdeleteln win
     The line under the cursor in the window WIN is deleted.  All lines below the current line are moved up one line.  The bottom line WIN is cleared.  The cursor position does not change.  This does not imply use of the hardware ``deleteline'' feature.

 -- Function: winsch win ch
     The character CH is inserted before the character under the cursor.  All characters to the right are moved one SPC to the right, possibly losing the rightmost character of the line.  The cursor position does not change .  This does not imply use of the hardware ``insertcharacter'' feature.

 -- Function: winsertln win
     A blank line is inserted above the current line and the bottom line is lost.  This does not imply use of the hardware ``insert-line'' feature.

 -- Function: scroll win
     The window WIN is scrolled up one line.  This involves moving the lines in WIN's data structure.  As an optimization, if WIN is stdscr and the scrolling region is the entire window, the physical screen will be scrolled at the same time.

Input
-----

 -- Function: read-char win
     A character is read from the terminal associated with the window WIN.  Depending on the setting of cbreak, this will be after one character (CBREAK mode), or after the first newline (NOCBREAK mode).  Unless noecho has been set, the character will also be echoed into WIN.

     When using read-char, do not set both NOCBREAK mode (nocbreak) and ECHO mode (echo) at the same time. Depending on the state of the terminal driver when each character is typed, the program may produce undesirable results.

 -- Function: winch win
     The character, of type chtype, at the current position in window WIN is returned.  If any attributes are set for that position, their values will be OR'ed into the value returned.

 -- Function: getyx win
     A list of the y and x coordinates of the cursor position of the window WIN is returned

Curses Miscellany
-----------------

 -- Function: wstandout win
 -- Function: wstandend win

     These functions set the current attributes of the window WIN.  The current attributes of WIN are applied to all characters that are written into it.  Attributes are a property of the character, and move with the character through any scrolling and insert/delete line/character operations.  To the extent possible on the particular terminal, they will be displayed as the graphic rendition of characters put on the screen.

     wstandout sets the current attributes of the window WIN to be visibly different from other text.  wstandend turns off the attributes.

 -- Function: box win vertch horch
     A box is drawn around the edge of the window WIN.  VERTCH and HORCH are the characters the box is to be drawn with.  If VERTCH and HORCH are 0, then appropriate default characters, ACS_VLINE and ACS_HLINE, will be used.

     Note: VERTCH and HORCH can be an integers and will insert the character (with attributes) of the corresponding values.

 -- Function: unctrl c
     This macro expands to a character string which is a printable representation of the character C.  Control characters are displayed in the C-x notation.  Printing characters are displayed as is.

Sockets
=======

These procedures (defined in socket.c) provide a Scheme interface to most of the C socket library.  For more information on sockets, see Sockets (The GNU C Library Reference Manual).

Host Data, Network, Protocol, and Service Inquiries
---------------------------------------------------

 -- Constant: af_inet
 -- Constant: af_unix
     Integer family codes for Internet and Unix sockets, respectively.

 -- Function: gethost host-spec
 -- Function: gethost
     Returns a vector of information for the entry for HOST-SPEC or the next entry if HOST-SPEC isn't given.  The information is:

      0. host name string

      1. list of host aliases strings

      2. integer address type (AF_INET)

      3. integer size of address entries (in bytes)

      4. list of integer addresses

 -- Function: sethostent stay-open
 -- Function: sethostent
     Rewinds the host entry table back to the begining if given an argument. If the argument STAY-OPEN is #f queries will be be done using UDP datagrams.  Otherwise, a connected TCP socket will be used.  When called without an argument, the host table is closed.

 -- Function: getnet name-or-number
 -- Function: getnet
     Returns a vector of information for the entry for NAME-OR-NUMBER or the next entry if an argument isn't given.  The information is:

      0. official network name string

      1. list of network aliases strings

      2. integer network address type (AF_INET)

      3. integer network number

 -- Function: setnetent stay-open
 -- Function: setnetent
     Rewinds the network entry table back to the begining if given an argument.  If the argument STAY-OPEN is #f the table will be closed between calls to getnet.  Otherwise, the table stays open.  When called without an argument, the network table is closed.

 -- Function: getproto name-or-number
 -- Function: getproto
     Returns a vector of information for the entry for NAME-OR-NUMBER or the next entry if an argument isn't given.  The information is:

      1. official protocol name string

      2. list of protocol aliases strings

      3. integer protocol number

 -- Function: setprotoent stay-open
 -- Function: setprotoent
     Rewinds the protocol entry table back to the begining if given an argument.  If the argument STAY-OPEN is #f the table will be closed between calls to getproto.  Otherwise, the table stays open.  When called without an argument, the protocol table is closed.

 -- Function: getserv name-or-port-number protocol
 -- Function: getserv
     Returns a vector of information for the entry for NAME-OR-PORT-NUMBER and PROTOCOL or the next entry if arguments aren't given.  The information is:

      0. official service name string

      1. list of service aliases strings

      2. integer port number

      3. protocol

 -- Function: setservent stay-open
 -- Function: setservent
     Rewinds the service entry table back to the begining if given an argument.  If the argument STAY-OPEN is #f the table will be closed between calls to getserv.  Otherwise, the table stays open.  When called without an argument, the service table is closed.

Internet Addresses
------------------

 -- Function: inet:string->address string
     Returns the host address number (integer) for host STRING or #f if not found.

 -- Function: inet:address->string address
     Converts an internet (integer) address to a string in numbers and dots notation.  This is an inverse function to inet:address.

 -- Function: inet:network address
     Returns the network number (integer) specified from ADDRESS or #f if not found.

 -- Function: inet:local-network-address address
     Returns the integer for the address of ADDRESS within its local network or #f if not found.

 -- Function: inet:make-address network local-address
     Returns the Internet address of LOCAL-ADDRESS in NETWORK.

Socket
------

When a port is returned from one of these calls it is unbuffered. This allows both reading and writing to the same port to work.  If you want buffered ports you can (assuming sock-port is a socket i/o port):

     (require 'i/o-extensions)
     (define i-port (duplicate-port sock-port "r"))
     (define o-port (duplicate-port sock-port "w"))

 -- Function: make-stream-socket family
 -- Function: make-stream-socket family protocol

     Returns a SOCK_STREAM socket of type FAMILY using PROTOCOL.  If FAMILY has the value AF_INET, SO_REUSEADDR will be set.  The integer argument PROTOCOL corresponds to the integer protocol numbers returned (as vector elements) from (getproto).  If the PROTOCOL argument is not supplied, the default (0) for the specified FAMILY is used.  SCM sockets look like ports opened for neither reading nor writing.

 -- Function: make-stream-socketpair family
 -- Function: make-stream-socketpair family protocol

     Returns a pair (cons) of connected SOCK_STREAM (socket) ports of type FAMILY using PROTOCOL.  Many systems support only socketpairs of the af-unix FAMILY.  The integer argument PROTOCOL corresponds to the integer protocol numbers returned (as vector elements) from (getproto).  If the PROTOCOL argument is not supplied, the default (0) for the specified FAMILY is used.

 -- Function: socket:shutdown socket how
     Makes SOCKET no longer respond to some or all operations depending on the integer variable HOW:

      0. Further input is disallowed.

      1. Further output is disallowed.

      2. Further input or output is disallowed.

     Socket:shutdown returns SOCKET if successful, #f if not.

 -- Function: socket:connect inet-socket host-number port-number
 -- Function: socket:connect unix-socket pathname
     Returns SOCKET (changed to a read/write port) connected to the Internet socket on host HOST-NUMBER, port PORT-NUMBER or the Unix socket specified by PATHNAME.  Returns #f if not successful.

 -- Function: socket:bind inet-socket port-number
 -- Function: socket:bind unix-socket pathname
     Returns INET-SOCKET bound to the integer PORT-NUMBER or the UNIX-SOCKET bound to new socket in the file system at location PATHNAME.  Returns #f if not successful. Binding a UNIX-SOCKET creates a socket in the file system that must be deleted by the caller when it is no longer needed (using delete-file).

 -- Function: socket:listen socket backlog
     The bound (see socket-bind) SOCKET is readied to accept connections.  The positive integer BACKLOG specifies how many pending connections will be allowed before further connection requests are refused.  Returns SOCKET if successful, #f if not.

 -- Function: socket:accept socket
     Accepts a connection on a bound, listening SOCKET.  Returns an input/output port for the connection.

     For example:

          (let ((sock (socket:bind (make-stream-socket af_inet) 8001)))
            (socket:listen sock 5)
            (do ((connection (socket:accept sock) (socket:accept sock)))
                (#f)
              (handle-client-connection connection))))

     A type socket-name is used for inquiries about open sockets in the following procedures:

 -- Function: getsockname socket
     Returns the socket-name of SOCKET.  Returns #f if unsuccessful or SOCKET is closed.

 -- Function: getpeername socket
     Returns the socket-name of the socket connected to SOCKET. Returns #f if unsuccessful or SOCKET is closed.

 -- Function: socket-name:family socket-name
     Returns the integer code for the family of SOCKET-NAME.

 -- Function: socket-name:port-number socket-name
     Returns the integer port number of SOCKET-NAME.

 -- Function: socket-name:address socket-name
     Returns the integer Internet address for SOCKET-NAME.

Guile Facilities
****************

Using SCM as a base, GUILE is a being developed to be a GNU scripting language.

One design goal of GUILE is to fully support a large body of Emacs lisp code and a large population of Emacs lisp programmers.  GUILE accomplishes this goal by means of a translator that rewrites elisp programs as equivalent programs in extended GUILE Scheme.  We anticipate a port of the GNU Emacs built-ins to the GUILE environment.

Tom Lord (Lord@cygnus.com) is coordinating GUILE's development.

Locked Vectors
==============

Locked vectors lvectors are a low level means by which Scheme code can define the representation of new types.  They are not themselves especially easy to use, but useful system can be built on top of them (for example, see Records (SLIB)).

User code should be selective about using lvector functions.  Lvectors are intended as the representation for user-defined types.  Types which otherwise have nothing to do with one another may have in common an lvector representation.  Using lvector functions in a context other than defining a new type or family of types is likely to be an unsupported violation of abstractions -- something you generally only want to do when you are debugging or are willing to write hacked up (in the pejorative sense) code.

Locked vectors have an internal representation similar to vectors. They are represented by a contiguous array in memory.  Access and modifications to fields can be done in constant time.  Like vectors, fields of a locked vector are addressed by integer addresses, based at 0.

The 0 element of a locked vector is special.  It is called the key vector.  It's use is comparable to that of type objects in other systems.

The key vector must always be present (there is no such thing as a 0 length lvector).  It must be a (normal) vector with at least as many elements as the lvector.  The elements of the key vector are used as keys which control access to the corresponding elements of the lvector.

Keys are used this way: Procedures like lvector-ref and lvector-set! take a key argument in addition to the lvector and field index. The key argument is compared to the indexed element of the key vector. If they are the same (eq?), access to the lvector is granted. If an access or modification fails because of an incorrect key, it may be retried.

If element 0 of the key vector is itself an lvector, the first several elements of that lvector may be hook functions.  These hook functions are used to extend the behavior of built-in procedures. Hook functions are explained in more detail at the end of this section.

Lvector Procedures
------------------

 -- Function: lock-vector! vec

     Modify VEC so that it is a locked vector instead of a vector.  Once it is locked, ordinary vector operations no longer apply to VEC, and vector? returns #f for VEC.

     In order to be locked, VEC must have at least one element.  The 0 element of VEC must be a key vector (see above).

     lock-vector! is currently the only way to construct a locked vector.  However, the decision to create locked vectors by mutating vectors is a dubious one.  Therefore, you should only use lock-vector! by wrapping it around a vector constructor, as in:

          (lock-vector! (apply vector (cons key-vector field-inits)))

     Following this convention will make it easier to replace lock-vector! with a more reasonable constructor later.

 -- Function: unlock-vector! lvec
     Modify LVEC so that it is once again the vector passed to lock-vector!.

     You should only use unlock-vector! for debugging.  It is not guaranteed to be present in future versions of Guile.

 -- Function: lvector-ref lvec key index
     Return the INDEXth element of LVEC.

     If KEY matches the INDEXth element of the key vector of LVEC, then the INDEXth element of LVEC is returned. If the key does not match, then the ref-fn hook is tried (hooks are documented below).  If the INDEXth element can not be unlocked, an error is signaled.

 -- Function: lvector-set! lvec key index val
     Set the INDEXth element of LVEC.

     If KEY matches the INDEXth element of the key vector of LVEC, then the INDEXth element of LVEC is set.  If the key does not match, then the set-fn hook is tried (see below). If the INDEXth element can not be unlocked, an error is signaled.

 -- Function: lvector? obj
     True if OBJ is indeed a locked vector.

 -- Function: lvector-keys lvec
     Return the key vector of LVEC, a locked vector.

 -- Function: lvector-accessor key-vec index
     Return an accessor for KEY-VEC and INDEX.  This function is defined as:

          (define (lvector-accessor keyvec index)
             (lambda (lvec)
                (lvector-ref lvec (vector-ref keyvec index) index)))

     However, the built-in version returns an accessor function that is faster than what the above code would return.

 -- Function: lvector-modifier key-vec index
     Return an modifier for KEY-VEC and INDEX.

     This function is defined as:

          (define (lvector-modifier keyvec index)
             (lambda (lvec val)
                (lvector-set!  lvec (vector-ref keyvec index) index val)))

     However, the built-in version returns an modifier function that is faster than what the above code would return.

 -- Function: lvector-isa? lvector key-vector
     Return #t if LVECTOR's 0th element is eq? to KEY-VECTOR.

     If KEY-VECTOR is not equal to element 0, lvector-isa? may yet return a true value -- it returns the (arbitrary) return value of the isa-fn?, if that hook is provided.

Key-Vector 0 Elements
---------------------

The first element of the key vector is special.  It may refer to a number of hook functions used by the lvector procedures and other parts of the run time system.

The 0 element of a key vector may (optionally) be an lvector called the type lvector.  A type lvector is characterized by having as a key the procedure lvector-ref for elements at offsets where hook functions are expected.

The hook function offsets are bound to the variables LVECTOR-HOOK-<FN>; so the REF-FN is at the offset bound to LVECTOR-REF-FN, the SET-FN to LVECTOR-SET-FN and so on.

The order and position of the hooks is explicitly specified:

 1. ref-fn

 2. set-fn

 3. print-fn

 4. equal-fn

 5. isa-fn

Here is how they are used:

 -- Hook: ref-fn lvec key index
     This function is called by lvector-ref for illegal keys.  A KEY is illegal if it is not eq? to the INDEXth key of the key vector of LVEC.  The ref-fn hook may return a value for lvector-ref or can signal an error.  The default action is to signal an error.

 -- Hook: set-fn lvec key index val
     This function is called by lvector-set! for illegal keys.  This hook can interpret the assignment arbitrarily, including signal an error.  It can return a value for lvector-set!.  The default action is to signal an error.

 -- Hook: print-fn lvec port writing?
     This function, if present, is called whenever the object is printed.  It should print the object on PORT.  WRITING?, if #t, indicates that the value is being writen; if #f -- displayed.  The print function should return a true value if it succeeds.  It should return #f for objects which it can not print.  In that case, a generic print routine for unknown objects is used. The default action (if no PRINT-FN is supplied) is to print something like: #<locked-vector 0x7856af8>.

 -- Predicate: equal-fn? lvec lvec2
     If present, called to evaluate (equal? lvec lvec2). The default action is a member-wise comparison of the two lvecs. Note that the first argument to equal? chooses the hook function.

 -- Predicate: isa-fn? lvec key-vector
     If present, this may be called by lvector-isa?.

     If KEY-VECTOR is eq to lvecs key-vector, then lvector-isa? simply returns #t.  If not, if the isa-fn is present, it is called.  The default action is to return #f.

Any of the hooks may be #f specifying that the default action should be used.

 -- Variable: lvector-hook-slots
     The variable lvector-hook-slots is bound to the number of hook slots looked for by the run time system.  A type lvector should have at least that many fields.

Here is an illustration of the construction of lvectors.  Note that two lvectors are built -- one to hold the hook functions, and then the one for user code.

     ;; Create a key vector compatible with the run-time system's
     ;; way of finding hook functions.
     ;;
     (define a-type-keyvec
       ;; The run-time system uses procedure lvector-ref as the key
       ;; to access hook functions.  You could define type keyvecs
       ;; with more elements if you had a use for them.
       ;;
       (make-vector lvector-hook-slots lvector-ref))

     ;; Lock up user-provided hook functions for the run-time system.
     ;; A var-args implementation would make more sense -- it would
     ;; make it easier to add hooks later.  The example is written
     ;; this way just to provide a reference for the proper order
     ;; of hook functions within the lvector.
     ;;
     (define (make-type-lvector ref-hook set-hook print-hook
                                equal-hook isa-hook)
       (lock-vector! (vector a-type-keyvec
                             ref-hook
                             set-hook
                             print-hook
                             equal-hook
                             isa-hook)))

     ;; Turn a list of field keys into a key vector of a particular
     ;; type:
     ;;
     (define (make-key-vector type-lvector . field-keys)
       (apply vector (cons type-lvector field-keys)))

     ;; Turn a key-vector and list of field initializers into
     ;; a locked vector.  Note that lock-vector! does some error
     ;; checking on the lengths of the input arguments.
     ;;
     (define (make-object key-vector . field-inits)
       (lock-vector! (apply (vector (cons key-vector field-inits)))))

Exceptions
==========

These are the low level entry points to exceptions.  Individual subsystems may define their own uses for exceptions.

 -- Function: catch key thunk handler
     Invoke THUNK in the dynamic context of HANDLER for excpetions matching KEY.  If thunk throws to the symbol KEY, then HANDLER is invoked this way:

          (handler key args ...)

     KEY may be a symbol.  In that case, THUNK takes no arguments.  If THUNK returns normally, that is the return value of catch.

     Handler is invoked outside the scope of its own catch.  If HANDLER against throws to the same key, a new handler from further up the call chain is invoked.

     If the key is #t, then a throw to any symbol will match this call to catch.

     Key may also be the value #f.  In that case, THUNK takes one argument which will be passed a jump buffer object.  A jump buffer object may be used as the key argument to throw to throw to a specific catch without an intervening search for a symbolic key.

 -- Function: throw key args ...
 -- Function: throw key
     Invoke the catch form matching KEY, passing ARGS to the HANDLER.

     If the key is a symbol it will match catches of the same symbol or of #t.  If no catch matches, the throw-default-handler property of the key is checked.  If it is bound to a procedure, that procedure is called:

          (handler key args ...)

     That procedure is called the default handler.

     If there is no handler at all, or if the default handler returns to its caller, an error is signaled.

     It is traditional in Scheme to implement exception systems using call-with-current-continuation, but his has not been done, for performance reaons.  The implementation of call-with-current-continuation is a stack copying implementation. This allows it to interact well with ordinary C code.  Unfortunately, a stack-copying implementation can be slow -- creating a new continuation involves a block copy of the stack.

     Instead of using call-with-current-continuation, the exception primitives are implemented as built-ins that take advantage of the upward only nature of exceptions.

System Exceptions
=================

These two expressions are equivalent:

     (error arg ...)

     (throw 'error arg ...)

The following are the names of excpetions that may be thrown by the interpreter itself.

     ARGn		"Wrong type argument"
     ARG1		"Wrong type argument in position 1"
     ARG2		"Wrong type argument in position 2"
     ARG3		"Wrong type argument in position 3"
     ARG4	        "Wrong type argument in position 4"
     ARG5		"Wrong type argument in position 5"
     WNA		"Wrong number of arguments"
     OVFLOW		"Numerical overflow"
     OUTOFRANGE	"Argument out of range"
     NALLOC		"Could not allocate"
     HUP_SIGNAL	"hang-up"
     INT_SIGNAL	"user interrupt"
     FPE_SIGNAL	"arithmetic error"
     BUS_SIGNAL	"bus error"
     SEGV_SIGNAL	"segmentation violation"
     ALRM_SIGNAL	"alarm"

     read-sharp-error

Variables
=========

All top level bindings are stored in locations called variables. Variables are first class objects.

 -- Function: make-variable init
 -- Function: make-variable init name-hint
     Return a new variable, bound to INIT.

     The optional NAME-HINT is a symbolic name for the variable.  It will be used in some error messages relating to the variable.

 -- Function: variable-ref variable
     Return the current binding of VARIABLE.

     It is an error to call variable-ref on a variable which is unbound.

 -- Function: variable-set! variable value
     Modify the binding of VARIABLE.

 -- Function: builtin-variable name
     Return the built-in variable for named NAME.  NAME must be a symbol.

     Even symbols with no top level bindings have built-in variables. There is an unbounded supply of undefined variables.

 -- Predicate: variable? obj
     Return #t if OBJ is a variable;  #f otherwise.

 -- Predicate: variable-bound? var
     Return #t if VAR is a bound variable; #f otherwise.

 -- Function: make-undefined
 -- Function: make-undefined name-hint
     Return a new variable, initially unbound.

     The optional NAME-HINT a symbolic name for the variable.  It will be used in some error messages relating to the variable.

User Defined Top Levels
=======================

Low level support is provided for multiple top levels.  This procedure should only be used advisadly.  Most people should stick to the user level module system interface.

 -- Function: eval2 form env-fn
     Evaluate FORM in the top level environment described by ENV-FN.

     ENV-FN is called this way:

          (env-fn name defining)

     Return a variable object for the symbol NAME.  Returning #f indicates that NAME has no binding.

     The boolean DEFINING is #t, then the lookup is for a define form or the equivalent -- a new variable may be created if none already exists.

     If #f, then the request should not create a new binding.

Obarrays
========

An obarray is an ordinary Scheme vector, used in a particular way. It represents two mapping: a mapping of strings to symbols, and a mapping of symbols to arbitrary values.

To initialize an obarray, create a vector of any non-0 size, filled with the empty list.  Thereafter, referring to the contents of the vector yields unspecified values.  (You might be able guess the detailed representation of obarrays by examining the array, but if your code depends on that representation, it might easily break in future versions of Guile.)

 -- Function: intern-string obarray name
     Return the SYMBOL bound to the stringNAME in OBARRAY.

     If there is no such symbol, a new SYMBOL is allocated. OBARRAY may be #f which guarantees that a new SYMBOL is returned.

 -- Function: intern-symbol obarray symbol
     Add SYMBOL to OBARRAY.  If SYMBOL is already present, this has no effect.

     After this addition, SYMBOL is the symbol binding of its name. That is, even if another symbol of the same name was previously interned in OBARRAY, SYMBOL becomes the binding of that name after intern-symbol.  (This does not mean that the previous symbol is removed from the OBARRAY).

 -- Function: symbol-interned? obarray symbol
     Return #t if SYMBOL is in OBARRAY.

 -- Function: symbol-bound? obarray symbol
 -- Function: symbol-binding obarray symbol
 -- Function: symbol-set! obarray symbol value
     A SYMBOL in an OBARRAY may be bound arbitrarily.  The binding may be changed.  When first interned, a symbol is unbound.

Keywords
========

A keyword is a self-evaluating symbol-like object with a convenient read syntax.

             (keyword? :keyword) => #t

 -- Function: symbol->keyword symbol
     Construct a keyword from a symbol.

                  (symbol->keyword 'fnord) => :fnord

      -- Function: make-symbol symbol
          A low-level way to construct a keyword from a symbol, convenient when dealing with Tcl or Unix. The first character of the symbol's name must be '-'.

                       (make-keyword '-command) => :command

      -- Function: keyword? obj
          Return #f unless OBJ is a keyword object.

      -- Function: keyword->symbol keyword
          Return the symbol that corresponds to a keyword.

                       (keyword->symbol :text) => -text

Procedure Properties
====================

Procedures created using "define" or "lambda" have a binding slot which you can use arbitrarily.  Many parts of the system will presume that this slot will contain a (possible empty) assoc list, keyed by symbols (`procedure properties').  If your procedure will be manipulated by a part of the system which is documented as using property lists, you should follow the convention, and use a unique prefix for all property names that you define.  Procedures which do not escape to such parts of the system can hve any value at all in their binding slot.

It is an arbitrary restriction that (some) built-in procedures lack a binding slot.  This ought to be fixed.  The reason it hasn't been fixed already is that the representation of the binding slot has to be chosen carefully for built-in procedure types.

 -- Function: procedure-properties procedure
 -- Function: set-procedure-properties! procedure value
     Return or set the value held in a procedures binding slow.

 -- Function: procedure-property proc key
 -- Function: procedure-assoc proc key
 -- Function: procedure-putprop! proc key value
     Retrieve or get the binding of a particular procedure property.

     These functions assume that the binding slot of the procedure holds an assoc list.

     procedure-property returns the binding of the property while procedure-assoc returns the key-value pair that holds the binding.

Dynamic Roots
=============

A dynamic root is a root frame of Scheme evaluation. The top-level repl, for example, is an instance of a dynamic root.

Each dynamic root has its own chain of dynamic-wind information.  Each has its own set of continuations, jump-buffers, and pending CATCH statements which are inaccessible from the dynamic scope of any other dynamic root.

In a thread-based system, each thread has its own dynamic root.  Therefore, continuations created by one thread may not be invoked by another. [Thread support is not yet fully implemented.]

Even in a single-threaded system, it is sometimes useful to create a new dynamic root.  For example, if you want to apply a procedure, but to not allow that procedure to capture the current continuation, calling the procedure under a new dynamic root will do the job.

 -- Function: call-with-dynamic-root thunk error-thunk
     Evaluate (THUNK) in a new dynamic context, returning its value.

     If an error occurs during evaluation, call error-thunk, passing it an error code describing the condition.  [Error codes are currently meaningless integers.  In the future, real values will be specified.] If this happens, the error-thunk is called outside the scope of the new root -- it is called in the same dynamic context in which call-with-dynamic-root was evaluated.

     If THUNK captures a continuation, the continuation is rooted at the call to THUNK.  In particular, the call to call-with-dynamic-root is not captured.  Therefore, call-with-dynamic-root always returns at most one time.

     Before calling THUNK, the dynamic-wind chain is un-wound back to the root and a new chain started for THUNK.  Therefore, this call may not do what you expect:

          ;; Almost certainly a bug:
          (call-with-output-to-port
           some-port

           (lambda ()
             (call-with-dynamic-root
              (lambda ()
                (display 'fnord)
                (newline))
              (lambda (errcode) errcode))))

     The problem is, on what port will fnord\n be displayed?  You might expect that because of the call-with-input-from-port that it will be displayed on the port bound to some-port.  But it probably won't -- before evaluating the thunk, dynamic winds are unwound, including those created by call-with-input-from-port. So, the standard output port will have been re-set to its default value before display is evaluated.

     (This function was added to Guile mostly to help calls to functions in C libraries that can not tolerate non-local exits or calls that return multiple times.  If such functions call back to the interpreter, it should be under a new dynamic root.)

 -- Function: dynamic-root
     Return an object representing the current dynamic root.

     These objects are only useful for comparison using eq?. They are currently represented as numbers, but your code should in no way depend on this.

Tcl Facilities
==============

If you have Tcl extension modules written in C, these can be accessed from Guile programs.  If you have an application that uses the Tcl interpreter, you can use Guile to define new modules for it -- modules that can be loaded without having to re-compile.

Documented here are the low-level facilities linking Tcl to Guile.  In the future, a friendlier Scheme interface will be provided in preference to these entry points.

 -- Function: tcl-create-interp
     Return a new Tcl interpreter object.

     An interpreter object is a namespace of functions and variables managed by modules that use the Tcl calling conventions.

 -- Function: tcl-global-eval interpreter string
     Evaluate a string according to Tcl's evaluation rules.

     INTERPRETER must be an object returned by tcl-create-interp.

     The return value is an integer/string pair.  The integer is the Tcl status code, the string is the Tcl result.

     This is not the way to call a Tcl command.  See the functions tcl-command and tcl-apply-command.  Using those functions is faster.

     N.B.: by default, most built-in Tcl commands are absent in Guile. Consequently, most Tcl programs will not run.  However, facilities exist so that you can link all of libtcl with Guile programs -- in which case all the usual built-in commands will be present.  [add reference!]

 -- Function: tcl-get-int interpreter string
 -- Function: tcl-get-double interpreter string
 -- Function: tcl-get-boolean interpreter string
     Convert strings to Scheme types according to the conventions of Tcl.

 -- Function: tcl-split-list interpreter string
     Split a Tcl-style "list" (a string following Tcl's list syntax) into a list.

 -- Function: tcl-merge interpreter list
     Combine a list of strings into a string following Tcl's list syntax.

 -- Function: tcl-create-command interpreter name procedure
     Define a new Tcl command which is recognized by the Tcl evaluator.

     INTERPRETER must be an object returned by tcl-create-command.

     NAME must be a string which is a valid Tcl identifier.

     PROCEDURE may be any Scheme procedure object.

     When the Tcl evaluator invokes the new command, PROCEDURE is called with as many arguments as were passed to the Tcl command. The arguments are all passed as strings.

     The return value of PROCEDURE is used as the Tcl result.  The return value of PROCEDURE determines the Tcl return value according to these rules:

          if PROCEDURE returns:      the Tcl result is:   and Tcl status is:
          __________________________________________________________________

          string (e.g. "foo")           that string ("foo")       TCL_OK

          integer (e.g. 1)              the empty string          the int
                                                                      (1)

          an int/string pair            the string                the int
          (e.g. (1 . "bogus frob"))        ("bogus frob")             (1)

 -- Function: tcl-delete-command interpreter name
     Remove the named command from a Tcl interpreter.

 -- Function: tcl-trace-var2 interpreter name index flags procedure
 -- Function: tcl-untrace-var2 interpreter name index flags procedure
     Add a callback to a Tcl variable.

     NAME is the name of the variable.

     INDEX is the subscript of the variable, or #f for a scalar.

     After tcl-trace-var2, modifications to the named variable or array position cause procedure to be called with four arguments: a tcl interpreter, the variable being modified, an index into the named variable (possible the empty string), and an integer of flags.

 -- Function: tcl-set-var2 interpreter name index value flags
 -- Function: tcl-get-var2 interpreter name index flags
     Set or return the current value of a Tcl variable.

Symbolic names for Tcl flags are provided in the source file Gtcl.scm and are the same as their C counterparts.  The procedure flags is used to combine flags.  For example:

(flags TCL_TRACE_READS TCL_TRACE_WRITES TCL_TRACE_UNSETS)

Tcl commands can be invoked from Scheme directly, without having to use the Tcl evaluator:

 -- Function: tcl-command interpreter name
     Return an object representing the named command (or #f if none is defined).

 -- Function: tcl-apply-command command args
     Apply Tcl command COMMAND to the list of strings ARGS.

     COMMAND should be an object returned by tcl-command.

     ARGS should be a list of arguments.

     The return value is an integer/string pair.  The integer is a Tcl return code, the string the Tcl result.

     The arguments can be of several types but must ultimately be converted to strings because of the way Tcl and Tk work internally.  Types are converted this way:

            ;;----------------------------------------
            ;; Argument             Converted argument
            ;;----------------------------------------
              123                     "123"
              123.34                  "123.34"
              "a-string"              "a-string"
              'a-symbol               "a-symbol"
              :keyword                "-keyword"
              #t                      "1"
              #f                      "0"
              (lambda (...) ...)      "*__guile#234"

     The automatic conversion of a procedure works this way.  First, a new Tcl name is generated for the procedure and a Tcl command with that name is defined.  When evaluated, the Tcl command calls the Scheme procedure. Second, if the procedure has a property defined called 'tcl-calling-convention, and if that is bound to a string, then that string is appended to the name with an intervening space.  For example, if a procedure's properties bind 'tcl-calling-convention to "%x %y", then the name is appended and winds up like "__guile#234 %x %y". Finally, an asterix is prepended to the name (e.g. "*__guile#234 %x %y").

     In the GNU modified version of Tcl/Tk, an asterix prepended to a command name has special meaning if the name is passed as a "-command" or similar configuration parameter to a widget, or if passed as the command argument to a "bind" operation.  In that case, the command is renamed to a canonical name that is unique to the widget or binding sequence (overwriting any previous definition).  If the widget or binding sequence is later deleted, so is the canonicalized command.  This is an extremely twisted way to trick Tcl into managing anonymous Scheme procedures with acceptable semantics (e.g., without core leaks and without requiring Scheme programmers to invent liveness for anonymous procedures they'd otherwise drop).

Tk Facilities
=============

So that Tk widgets can be used form Scheme, Guile provides the following functions:

 -- Function: tk-init-main-window interpreter display name class
     Initialize a Tcl interpreter as a Tk main window.

     DISPLAY, NAME, and CLASS must all be strings.

 -- Function: tk-do-one-event flags
     Process one window system event.

     Symbolic definitions for FLAGS are provided in Gtk.scm and have the same name as their C counterparts.

 -- Function: tk-main-loop
     Process window events until no windows remain.

Gwish
=====

Gwish is a Wish-like application of Guile, based on STk by Erick Gallesio.

Running Gwish
-------------

Mapping Tcl/Tk Constructs onto Gwish
------------------------------------

In Gwish, there is a single Tcl/Tk interpreter which is an implicit default.  Commands defined in that interpreter are automaticly defined as Scheme commands as well (unless a Scheme-specific binding overwrites them).  This means that you can write Scheme code that closely matches the corresponding Tcl code.  For example:

     (button '.b)
     (.b 'configure :text "hello world" :command (tcl-lambda () (beep)))

Arguments passed to a procedure implemented by a Tcl command must be converted to a string for the sake of Tcl's calling conventions and data representations.  This table illustrates how argument types are converted:

       ;;----------------------------------------
       ;; Argument             Converted argument
       ;;----------------------------------------
         123                     "123"
         123.34                  "123.34"
         "a-string"              "a-string"
         'a-symbol               "a-symbol"
         :keyword                "-keyword"
         #t                      "1"
         #f                      "0"
         (lambda (...) ...)      "*__guile#234"

Most of these conversions are simple and obvious but the conversion of procedure objects is subtle and non-obvious.  It is not necessary to understand the details, though they are documented elsewhere.  It is necessary to know that procedure arguments `do the right thing' when they are passed as command parameters to widget configuration commands, and as event bindings.  You might ask: what is the right thing?

A procedure passed as either an event binding or widget command is protected by the binding or widget involved.  Effectively, the widget or binding keeps a reference to the procedure object.  For this reason, it is possible to use an anonymous Scheme procedure as an event binding or widget command. For example:

     (let ((n 0))
        (.b configure :command (lambda ()
                                  (write n)
                                  (newline)
                                  (set! n + 1)
                                  "")))

     ;; Incidently, note that the command procedure returns an empty string.
     ;; All procedures called by Tcl have to return a proper Tcl result.
     ;; Return values are explained further in the next subsection.
     ;;

When a command or binding is overwritten and that command or binding is to a Scheme procedure, the reference to the procedure is dropped.  The procedure is also dropped if the binding or widget is deleted.  For this reason, there are no core leaks associated with passing anonymous procedures as these kinds of arguments.

At this time, it isn't generally useful to pass Scheme procedures to Tcl commands other than as an argument to a binding command or widget configuration command.

The details of how procedures are translated are described in the documentation for `tcl-apply-command'.  See Tcl Facilities.

Defining Tcl Command in Scheme
------------------------------

Gwish provides a convenient syntax for defining new Tcl commands.

 -- Syntax: proc name formals . body
     Define a new Tcl command and Scheme procedure simultaneously.

          ;; A two argument string-append callable as a Tcl command.
          ;;
          (proc string-smash (s1 s2)  (string-append s1 s2))

     Proc expands to a tcl-lambda form, which is explained below. The example above is equivalent to:

          (begin
            (define string-smash (tcl-lambda (s1 s2) (string-append s1 s2)))
            (tcl-create-command the-interpreter 'string-smash string-smash))

 -- Syntax: tcl-lambda (?calling-convention? ?.? formals) . body
     Similar to lambda, but imposes Tcl-specific calling conventions. Type declarations are supported.  This is best explained via examples.

     A simple use of tcl-lambda:

          ;; By default, when called from Tcl, all of the arguments
          ;; to a tcl-lambda are strings.
          ;;
          (tcl-lambda (a b) (string-append a b))

     The return value of a tcl-lambda has special signficance when the function is invoked by Tcl.

     An integer return value is converted to a string.  A string or symbol is used used directly as a Tcl result.  A cons pair with an integer car and string cdr is a Tcl status/message combination.

     Type declarations can be used to specify that parameters passed as strings should be coerced to some other type.  For example:

          (define tcl-plus
             (tcl-lambda ((number a) (number b))
                (+ a b)))

          (tcl-plus "23" 19)  => 42

     Tcl-lambda is very useful for using anonymous Scheme functions as Tk event bindings.  For this purpose, you can can specify a Tk "calling convention" by writing a string in the first of the argument specification.  For example:

          (.c 'bind 'node "<Button-2>"
              (tcl-lambda ("%x %y" (number x) (number y))
                (set! cur-x x)
                (set! cur-y y)
                #t))                    ; Tcl-compatible return value.

     In that example, "%x %y" is the calling convention.  The full command that Tcl calls when invoking the binding will be similar to:

          __binding#0x123123 %x %y

     and therefore the Scheme procedure will be passed (in this case) the x and y position of the mouse.

An Example: Ousterhout's Graph Editor
-------------------------------------

This program implements the toy graph editor from the Tcl/Tk <fixme>.

System Calls
============

A volunteer is need to write this section.  Most of the system call documentation already exists in other sections. See also I/O-Extensions, see Posix Extensions.

All of the unix system calls are available as Scheme procedures.

Any call that fails with errno EINTR is automaticly retried.

By default, any other error during a system call will cause the call to signal an error and not return.  There are two functions that control this behavior and which you can redefine.  These are syscall and syserror.  The default definitions are:

     (define (syserror key fn err . args)
       (errno err)
       (apply error (cons fn args)))

     (define (syscall thunk)
       (errno 0)
       (catch 'syserror thunk syserror))

A typical system call is written this way:

     (define (connect port address . args)
       (syscall
        (lambda ()
          (let ((rv (apply %connect (cons port (cons address args)))))
            (if (eq? rv #f)
     	   (throw 'syserror 'connect (errno) port address args))))))

Gscsh
=====

 -- Function: fork
 -- Function: fork thunk
     Fork a child process: returns child pid to parent, #f to child.

     With an argument, fork a child process executing the Scheme procedure thunk.  Returns child pid to parent.

 -- Function: wait
 -- Function: wait pid
 -- Function: wait pid options
     Wait for any child to terminate.

     Pid argument means for that child process to terminate.  Special values for pid are: -1 wait for any child process; 0 wait for any child process in the process group of the calling process.

     Options is an integer with the following OR'd values:

          WNOHANG: do not wait.
          WUNTRACED: also return status information from stopped processes.

 -- Function: exec-pipe pipe
 -- Function: fg-pipe pipe
 -- Function: bg-pipe pipe
     These functions run external programs in a pipeline.  exec-pipe causes the pipeline to overlay the current process.  fg-pipe waits for the first command in the pipeline to exit and returns its exit status code.  bg-pipe runs the pipeline in the background, returning a list with the process id of the first program and ports which can be used for communicating with the programs in the pipeline.

     The pipe argument consists of a series of commands and optionally connection lists.  A command can be either an external Unix program or a scheme procedure.  An external program is written as a list of strings with the name and arguments.  An example of a pipeline connecting three programs and using the default connection lists is:

          (exec-pipe
            '("cat" "/etc/passwd") '("sort" "-t:" "+2") '("grep" "^u"))

     Alternatively a Scheme procedure (taking no arguments) may be specified and will be run in its own subprocess.  The standard ports (current-input-port, current-output-port and current-error-port) are created automatically from file descriptors 0, 1 and 2.  A Scheme procedure used in a pipeline would typically be a filter, such as:

          (define filter
            (lambda ()
              (let loop ((ch (read-char)))
                (cond ((not (eof-object? ch))
          	     (display ch)
          	     (loop (read-char)))))))

     and the term in the pipeline would be written:

          `(,filter)

     A connection list specifies how open Scheme ports are connected to Unix file descriptors in the programs in the pipeline and how file descriptors are wired together between programs. For example, the default connection list between the sort and grep commands in the pipeline above is:

          `((1 0) (2 ,(current-error-port)))

     which says to connect file descriptor 1 (standard output) from the sort command to file descriptor 0 (standard input) in the grep command and to connect file descriptor 2 (standard error) from the sort command to the Scheme current error port.

     Connection lists may be placed between programs in the pipeline and if omitted the default above will be used.  Connection lists may also be placed before the first program and after the last to specify connections at the beginning and end of the pipeline.  The default connection list then depends on which pipeline function is used.

     The defaults for exec-pipe and fg-pipe are:

          `((,(current-input-port) 0))
          `((1 ,(current-output-port)) (2 ,(current-error-port)))

     for the input and output ends of the pipeline respectively.

     The defaults for bg-pipe are:

          '((return-port 0))
          `((1 return-port) (2 ,(current-error-port)))

     The symbol 'return-port in the connection list adds a port to the list returned by bg-pipe.  In the default case two ports are returned: the first is an output port connected to the standard input of the first program in the pipeline and the second is an input port connected to the standard output of the last process. [need to beware of deadlocks if multiple output ports are returned].

     The pipeline example above can be rewritten with all the connection lists given explicitly:

          (exec-pipe `((,(current-input-port) 0))
            '("cat" "/etc/passwd") `((1 0) (2 ,(current-error-port)))
            '("sort" "-t:" "+2")   `((1 0) (2 ,(current-error-port)))
            '("grep" "^u") `((1 ,(current-output-port)) (2 ,(current-error-port)))

     Multiple file descriptors can be connected to a single target if a connection list such as '((1 0) (2 0)) or equivalently '((1 2 0)) is specified.

     A final construction which can be used in a connection list is a string to be interpreted as the name of a file. For example, the connection list between the sort and grep commands could be written:

          '((1 0) (2 "sort-errors"))

     which would capture the standard error from sort to a newly created file. [Maybe this should be extended to allow appending to a file, e.g., with ">sort-error-log", although it could also be done with ports].

Internals
*********

Data Types
==========

In the descriptions below it is assumed that long ints are 32 bits in length.  Acutally, SCM is written to work with any long int size larger than 31 bits.  With some modification, SCM could work with word sizes as small as 24 bits.

All SCM objects are represented by type SCM.  Type SCM come in 2 basic flavors, Immediates and Cells:

Immediates
----------

An immediate is a data type contained in type SCM (long int).  The type codes distinguishing immediate types from each other vary in length, but reside in the low order bits.

 -- Macro: IMP x
 -- Macro: NIMP x
     Return non-zero if the SCM object X is an immediate or non-immediate type, respectively.

 -- Immediate: inum
     immediate 30 bit signed integer.  An INUM is flagged by a 1 in the second to low order bit position.  The high order 30 bits are used for the integer's value.

      -- Macro: INUMP x
      -- Macro: NINUMP x
          Return non-zero if the SCM X is an immediate integer or not an immediate integer, respectively.

      -- Macro: INUM x
          Returns the C long integer corresponding to SCM X.

      -- Macro: MAKINUM x
          Returns the SCM inum corresponding to C long integer x.

      -- Immediate Constant: INUM0
          is equivalent to MAKINUM(0).

     Computations on INUMs are performed by converting the arguments to C integers (by a shift), operating on the integers, and converting the result to an inum.  The result is checked for overflow by converting back to integer and checking the reverse operation.

     The shifts used for conversion need to be signed shifts.  If the C implementation does not support signed right shift this fact is detected in a #if statement in scmfig.h and a signed right shift, SRS, is constructed in terms of unsigned right shift.

 -- Immediate: ichr
     characters.

      -- Macro: ICHRP x
          Return non-zero if the SCM object X is a character.

      -- Macro: ICHR x
          Returns corresponding unsigned char.

      -- Macro: MAKICHR x
          Given char X, returns SCM character.

 -- Immediate: iflags
     These are frequently used immediate constants.
      -- Immediate Constant: SCM BOOL_T
          #t

      -- Immediate Constant: SCM BOOL_F
          #f

      -- Immediate Constant: SCM EOL
          ().  If SICP is #defined, EOL is #defined to be identical with BOOL_F.  In this case, both print as #f.

      -- Immediate Constant: SCM EOF_VAL
          end of file token, #<eof>.

      -- Immediate Constant: SCM UNDEFINED
          #<undefined> used for variables which have not been defined and absent optional arguments.

      -- Immediate Constant: SCM UNSPECIFIED
          #<unspecified> is returned for those procedures whose return values are not specified.

 -- Macro: IFLAGP n
     Returns non-zero if N is an ispcsym, isym or iflag.

 -- Macro: ISYMP n
     Returns non-zero if N is an ispcsym or isym.

 -- Macro: ISYMNUM n
     Given ispcsym, isym, or iflag N, returns its index in the C array isymnames[].

 -- Macro: ISYMCHARS n
     Given ispcsym, isym, or iflag N, returns its char * representation (from isymnames[]).

 -- Macro: MAKSPCSYM n
     Returns SCM ispcsym N.

 -- Macro: MAKISYM n
     Returns SCM iisym N.

 -- Macro: MAKIFLAG n
     Returns SCM iflag N.

 -- Variable: isymnames
     An array of strings containing the external representations of all the ispcsym, isym, and iflag immediates.  Defined in repl.c.

 -- Constant: NUM_ISPCSYM
 -- Constant: NUM_ISYMS
     The number of ispcsyms and ispcsyms+isyms, respectively.  Defined in scm.h.

 -- Immediate: isym
     and, begin, case, cond, define, do, if, lambda, let, let*, letrec, or, quote, set!, #f, #t, #<undefined>, #<eof>, (), and #<unspecified>.

 -- CAR Immediate: ispcsym
     special symbols: syntax-checked versions of first 14 isyms

 -- CAR Immediate: iloc
     indexes to a variable's location in environment

 -- CAR Immediate: gloc
     pointer to a symbol's value cell

 -- Immediate: CELLPTR
     pointer to a cell (not really an immediate type, but here for completeness).  Since cells are always 8 byte aligned, a pointer to a cell has the low order 3 bits 0.

     There is one exception to this rule, CAR Immediates, described next.

A CAR Immediate is an Immediate point which can only occur in the CARs of evaluated code (as a result of ceval's memoization process).

Cells
-----

Cells represent all SCM objects other than immediates.  A cell has a CAR and a CDR.  Low-order bits in CAR identify the type of object.  The rest of CAR and CDR hold object data.  The number after tc specifies how many bits are in the type code.  For instance, tc7 indicates that the type code is 7 bits.

 -- Macro: NEWCELL x
     Allocates a new cell and stores a pointer to it in SCM local variable X.

     Care needs to be taken that stores into the new cell pointed to by X do not create an inconsistent object.  See Signals.

All of the C macros decribed in this section assume that their argument is of type SCM and points to a cell (CELLPTR).

 -- Macro: CAR x
 -- Macro: CDR x
     Returns the car and cdr of cell X, respectively.

 -- Macro: TYP3 x
 -- Macro: TYP7 x
 -- Macro: TYP16 x
     Returns the 3, 7, and 16 bit type code of a cell.

 -- Cell: tc3_cons
     scheme cons-cell returned by (cons arg1 arg2).

      -- Macro: CONSP x
      -- Macro: NCONSP x
          Returns non-zero if X is a tc3_cons or isn't, respectively.

 -- Cell: tc3_closure
     applicable object returned by (lambda (args) ...). tc3_closures have a pointer to other the body of the procedure in the CAR and a pointer to the environment in the CDR.

      -- Macro: CLOSUREP x
          Returns non-zero if X is a tc3_closure.

      -- Macro: CODE x
      -- Macro: ENV x
          Returns the code body or environment of closure X, respectively.

Headers are Cells whose CDRs point elsewhere in memory, such as to memory allocated by malloc.

 -- Header: spare
     spare tc7 type code

 -- Header: tc7_vector
     scheme vector.

      -- Macro: VECTORP x
      -- Macro: NVECTORP x
          Returns non-zero if X is a tc7_vector or if not, respectively.

      -- Macro: VELTS x
      -- Macro: LENGTH x
          Returns the C array of SCMs holding the elements of vector X or its length, respectively.

 -- Header: tc7_ssymbol
     static scheme symbol (part of initial system)

 -- Header: tc7_msymbol
     malloced scheme symbol (can be GCed)

      -- Macro: SYMBOLP x
          Returns non-zero if X is a tc7_ssymbol or tc7_msymbol.

      -- Macro: CHARS x
      -- Macro: UCHARS x
      -- Macro: LENGTH x
          Returns the C array of chars or as unsigned chars holding the elements of symbol X or its length, respectively.

 -- Header: tc7_string
     scheme string

      -- Macro: STRINGP x
      -- Macro: NSTRINGP x
          Returns non-zero if X is a tc7_string or isn't, respectively.

      -- Macro: CHARS x
      -- Macro: UCHARS x
      -- Macro: LENGTH x
          Returns the C array of chars or as unsigned chars holding the elements of string X or its length, respectively.

 -- Header: tc7_bvect
     uniform vector of booleans (bit-vector)

 -- Header: tc7_ivect
     uniform vector of integers

 -- Header: tc7_uvect
     uniform vector of non-negative integers

 -- Header: tc7_fvect
     uniform vector of short inexact real numbers

 -- Header: tc7_dvect
     uniform vector of double precision inexact real numbers

 -- Header: tc7_cvect
     uniform vector of double precision inexact complex numbers

 -- Header: tc7_contin
     applicable object produced by call-with-current-continuation

 -- Header: tc7_cclo
     Subr and environment for compiled closure

     A cclo is similar to a vector (and is GCed like one), but can be applied as a function:

      1. the cclo itself is consed onto the head of the argument list

      2. the first element of the cclo is applied to that list.  Cclo invocation is currently not tail recursive when given 2 or more arguments.

      -- Function: makcclo proc len
          makes a closure from the subr PROC with LEN-1 extra locations for SCM data.  Elements of a CCLO are referenced using VELTS(cclo)[n] just as for vectors.

A Subr is a header whose CDR points to a C code procedure. Scheme primitive procedures are subrs.  Except for the arithmetic tc7_cxrs, the C code procedures will be passed arguments (and return results) of type SCM.

 -- Subr: tc7_asubr
     associative C function of 2 arguments.  Examples are +, -, *, /, max, and min.

 -- Subr: tc7_subr_0
     C function of no arguments.

 -- Subr: tc7_subr_1
     C function of one argument.

 -- Subr: tc7_cxr
     These subrs are handled specially.  If inexact numbers are enabled, the CDR should be a function which takes and returns type double.  Conversions are handled in the interpreter.

     floor, ceiling, truncate, round, $sqrt, $abs, $exp, $log, $sin, $cos, $tan, $asin, $acos, $atan, $sinh, $cosh, $tanh, $asinh, $acosh, $atanh, and exact->inexact are defined this way.

     If the CDR is 0 (NULL), the name string of the procedure is used to control traversal of its list structure argument.

     car, cdr, caar, cadr, cdar, cddr, caaar, caadr, cadar, caddr, cdaar, cdadr, cddar, cdddr, caaaar, caaadr, caadar, caaddr, cadaar, cadadr, caddar, cadddr, cdaaar, cdaadr, cdadar, cdaddr, cddaar, cddadr, cdddar, and cddddr are defined this way.

 -- Subr: tc7_subr_3
     C function of 3 arguments.

 -- Subr: tc7_subr_2
     C function of 2 arguments.

 -- Subr: tc7_rpsubr
     transitive relational predicate C function of 2 arguments.  The C function should return either BOOL_T or BOOL_F.

 -- Subr: tc7_subr_1o
     C function of one optional argument.  If the optional argument is not present, UNDEFINED is passed in its place.

 -- Subr: tc7_subr_2o
     C function of 1 required and 1 optional argument.  If the optional argument is not present, UNDEFINED is passed in its place.

 -- Subr: tc7_lsubr_2
     C function of 2 arguments and a list of (rest of) SCM arguments.

 -- Subr: tc7_lsubr
     C function of list of SCM arguments.

A ptob is a port object, capable of delivering or accepting characters.  See Ports (Revised(4) Report on the Algorithmic Language Scheme).  Unlike the types described so far, new varieties of ptobs can be defined dynamically (see Defining Ptobs).  These are the initial ptobs:

 -- ptob: tc16_inport
     input port.

 -- ptob: tc16_outport
     output port.

 -- ptob: tc16_ioport
     input-output port.

 -- ptob: tc16_inpipe
     input pipe created by popen().

 -- ptob: tc16_outpipe
     output pipe created by popen().

 -- ptob: tc16_strport
     String port created by cwos() or cwis().

 -- ptob: tc16_sfport
     Software (virtual) port created by mksfpt() (see Soft Ports).

 -- Macro: PORTP x
 -- Macro: OPPORTP x
 -- Macro: OPINPORTP x
 -- Macro: OPOUTPORTP x
 -- Macro: INPORTP x
 -- Macro: OUTPORTP x
     Returns non-zero if X is a port, open port, open input-port, open output-port, input-port, or output-port, respectively.

 -- Macro: OPENP x
 -- Macro: CLOSEDP x
     Returns non-zero if port X is open or closed, respectively.

 -- Macro: STREAM x
     Returns the FILE * stream for port X.

Ports which are particularly well behaved are called fports. Advanced operations like file-position and reopen-file only work for fports.

 -- Macro: FPORTP x
 -- Macro: OPFPORTP x
 -- Macro: OPINFPORTP x
 -- Macro: OPOUTFPORTP x
     Returns non-zero if X is a port, open port, open input-port, or open output-port, respectively.

A smob is a miscellaneous datatype.  The type code and GCMARK bit occupy the lower order 16 bits of the CAR half of the cell.  The rest of the CAR can be used for sub-type or other information. The CDR contains data of size long and is often a pointer to allocated memory.

Like ptobs, new varieties of smobs can be defined dynamically (see Defining Smobs).  These are the initial smobs:

 -- smob: tc_free_cell
     unused cell on the freelist.

 -- smob: tc16_flo
     single-precision float.

     Inexact number data types are subtypes of type tc16_flo.  If the sub-type is:

      0. a single precision float is contained in the CDR.

      1. CDR is a pointer to a malloced double.

      3. CDR is a pointer to a malloced pair of doubles.

      -- smob: tc_dblr
          double-precision float.

      -- smob: tc_dblc
          double-precision complex.

 -- smob: tc16_bigpos
 -- smob: tc16_bigneg
     positive and negative bignums, respectively.

     Scm has large precision integers called bignums.  They are stored in sign-magnitude form with the sign occuring in the type code of the SMOBs bigpos and bigneg.  The magnitude is stored as a malloced array of type BIGDIG which must be an unsigned integral type with size smaller than long.  BIGRAD is the radix associated with BIGDIG.

 -- smob: tc16_promise
     made by DELAY.  See Control features (Revised(4) Scheme).

 -- smob: tc16_arbiter
     synchronization object.  See Process Synchronization.

 -- smob: tc16_macro
     macro expanding function.  See Low Level Syntactic Hooks.

 -- smob: tc16_array
     multi-dimensional array.  See Arrays.

     This type implements both conventional arrays (those with arbitrary data as elements see Conventional Arrays) and uniform arrays (those with elements of a uniform type see Uniform Array).

     Conventional Arrays have a pointer to a vector for their CDR. Uniform Arrays have a pointer to a Uniform Vector type (string, bvect, ivect, uvect, fvect, dvect, or cvect) in their CDR.

Data Type Representations
-------------------------

     IMMEDIATE:      B,D,E,F=data bit, C=flag code, P=pointer address bit
             ................................
     inum    BBBBBBBBBBBBBBBBBBBBBBBBBBBBBB10
     ichr    BBBBBBBBBBBBBBBBBBBBBBBB11110100
     iflag                   CCCCCCC101110100
     isym                    CCCCCCC001110100
             IMCAR:  only in car of evaluated code, cdr has cell's GC bit
     ispcsym                 000CCCC00CCCC100
     iloc    0DDDDDDDDDDDDDDDEFFFFFFF11111100
     pointer PPPPPPPPPPPPPPPPPPPPPPPPPPPPP000
     gloc    PPPPPPPPPPPPPPPPPPPPPPPPPPPPP001

        HEAP CELL:   G=gc_mark; 1 during mark, 0 other times.
             1s and 0s here indicate type.     G missing means sys (not GC'd)
             SIMPLE:
     cons    ..........SCM car..............0  ...........SCM cdr.............G
     closure ..........SCM code...........011  ...........SCM env.............G
             HEADERs:
     ssymbol .........long length....G0000101  ..........char *chars...........
     msymbol .........long length....G0000111  ..........char *chars...........
     string  .........long length....G0001101  ..........char *chars...........
     vector  .........long length....G0001111  ...........SCM **elts...........
     bvect   .........long length....G0010101  ..........long *words...........
      spare                          G0010111
     ivect   .........long length....G0011101  ..........long *words...........
     uvect   .........long length....G0011111  ......unsigned long *words......
      spare                          G0100101
      spare                          G0100111
     fvect   .........long length....G0101101  .........float *words...........
     dvect   .........long length....G0101111  ........double *words...........
     cvect   .........long length....G0110101  ........double *words...........

     contin  .........long length....G0111101  .............*regs..............
     cclo    .........long length....G0111111  ...........SCM **elts...........
             SUBRs:
      spare                          010001x1
      spare                          010011x1
     subr_0  ..........int hpoff.....01010101  ...........SCM (*f)()...........
     subr_1  ..........int hpoff.....01010111  ...........SCM (*f)()...........
     cxr     ..........int hpoff.....01011101  .........double (*f)()..........
     subr_3  ..........int hpoff.....01011111  ...........SCM (*f)()...........
     subr_2  ..........int hpoff.....01100101  ...........SCM (*f)()...........
     asubr   ..........int hpoff.....01100111  ...........SCM (*f)()...........
     subr_1o ..........int hpoff.....01101101  ...........SCM (*f)()...........
     subr_2o ..........int hpoff.....01101111  ...........SCM (*f)()...........
     lsubr_2 ..........int hpoff.....01110101  ...........SCM (*f)()...........
     lsubr_2n..........int hpoff.....01110111  ...........SCM (*f)()...........
     rpsubr  ..........int hpoff.....01111101  ...........SCM (*f)()...........
                             PTOBs:
        port            0bwroxxxxxxxxG1110111  ..........FILE *stream..........
      socket ttttttt    00001xxxxxxxxG1110111  ..........FILE *stream..........
      inport uuuuuuuuuuU00011xxxxxxxxG1110111  ..........FILE *stream..........
     outport 0000000000000101xxxxxxxxG1110111  ..........FILE *stream..........
      ioport uuuuuuuuuuU00111xxxxxxxxG1110111  ..........FILE *stream..........
     fport              00   00000000G1110111  ..........FILE *stream..........
     pipe               00   00000001G1110111  ..........FILE *stream..........
     strport            00   00000010G1110111  ..........FILE *stream..........
     sfport             00   00000011G1110111  ..........FILE *stream..........
                             SMOBs:
     free_cell
             000000000000000000000000G1111111  ...........*free_cell........000
     flo     000000000000000000000001G1111111  ...........float num............
     dblr    000000000000000100000001G1111111  ..........double *real..........
     dblc    000000000000001100000001G1111111  .........complex *cmpx..........
     bignum  ...int length...0000001 G1111111  .........short *digits..........
     bigpos  ...int length...00000010G1111111  .........short *digits..........
     bigneg  ...int length...00000011G1111111  .........short *digits..........
                             xxxxxxxx = code assigned by newsmob();
     promise 000000000000000fxxxxxxxxG1111111  ...........SCM val..............
     arbiter 000000000000000lxxxxxxxxG1111111  ...........SCM name.............
     macro   000000000000000mxxxxxxxxG1111111  ...........SCM name.............
     array   ...short rank..cxxxxxxxxG1111111  ............*array..............

Operations
==========

Garbage Collection
------------------

The garbage collector is in the latter half of sys.c.  Immediates always appear as parts of other objects, so they are not subject to explicit garbage collection.  There is a heap (composed of heap segments) in which all cells reside.  The storage for strings, vectors, continuations, doubles, complexes, and bignums is managed by malloc. There is only one pointer to each malloc object from its type-header cell in the heap.  This allows malloc objects to be freed when the associated heap object is garbage collected.

To garbage collect, first certain protected objects are marked (such as symhash).  Then the stack (and marked continuations) are traversed. Each longword in the stack is tried to see if it is a valid cell pointer into the heap.  If it is, the object itself and any objects it points to are marked.  If the stack is word rather than longword aligned (#define WORD_ALIGN), both alignments are tried.  This arrangement will occasionally mark an object which is no longer used. This has not been a problem in practice and the advantage of using the c-stack far outweighs it.

The heap is then swept.  If a type-header cell pointing to malloc space is collected the malloc object is then freed.  If the type header of smob is collected, the smob's free procedure is called to free its storage.

Signals
-------

 -- Function: init_signals
     (in scm.c) initializes handlers for SIGINT and SIGALRM if they are supported by the C implementation.  All of the signal handlers immediately reestablish themselves by a call to signal().

 -- Function: int_signal sig
 -- Function: alrm_signal sig
     The low level handlers for SIGINT and SIGALRM.

If an interrupt handler is defined when the interrupt is received, the code is interpreted.  If the code returns, execution resumes from where the interrupt happened.  Call-with-current-continuation allows the stack to be saved and restored.

SCM does not use any signal masking system calls.  These are not a portable feature.  However, code can run uninterrupted by use of the C macros DEFER_INTS and ALLOW_INTS.

 -- Macro: DEFER_INTS
     sets the global variable ints_disabled to 1.  If an interrupt occurs during a time when ints_disabled is 1 one of the global variables sig_deferred or alrm_deferred is set to 1 and the handler returns.

 -- Macro: ALLOW_INTS
     Checks the deferred variables and if set the appropriate handler is called.

     Calls to DEFER_INTS can not be nested.  An ALLOW_INTS must happen before another DEFER_INTS can be done.  In order to check that this constraint is satisfied #define CAREFUL_INTS in scmfig.h.

C Macros
--------

 -- Macro: ASSERT cond arg pos subr
     signals an error if the expression (COND) is 0.  ARG is the offending object, SUBR is the string naming the subr, and POS indicates the position or type of error.  POS can be one of

       * ARGn (> 5 or unknown ARG number)

       * ARG1

       * ARG2

       * ARG3

       * ARG4

       * ARG5

       * WNA (wrong number of args)

       * OVFLOW

       * OUTOFRANGE

       * NALLOC

       * EXIT

       * HUP_SIGNAL

       * INT_SIGNAL

       * FPE_SIGNAL

       * BUS_SIGNAL

       * SEGV_SIGNAL

       * ALRM_SIGNAL

       * a C string (char *)

     Error checking is not done by ASSERT if the flag RECKLESS is defined.  An error condition can still be signaled in this case with a call to wta(arg, pos, subr).

 -- Macro: ASRTGO cond label
     goto LABEL if the expression (COND) is 0.  Like ASSERT, ASRTGO does is not active if the flag RECKLESS is defined.

Changing Scm
------------

When writing C-code for SCM, a precaution is recommended.  If your routine allocates a non-cons cell which will not be incorporated into a SCM object which is returned, you need to make sure that a SCM variable in your routine points to that cell as long as part of it might be referenced by your code.

In order to make sure this SCM variable does not get optimized out you can put this assignment after its last possible use:

     SCM_dummy1 = foo;

or put this assignment somewhere in your routine:

     SCM_dummy1 = (SCM) &foo;

SCM_dummy variables are not currently defined.  Passing the address of the local SCM variable to any procedure also protects it.

Also, if you maintain a static pointer to some (non-immediate) SCM object, you must either make your pointer be the value cell of a symbol (see errobj for an example) or make your pointer be one of the sys_protects (see symhash for an example).  The former method is prefered since it does not require any changes to the SCM distribution.

To add a C routine to scm:

 1. choose the appropriate subr type from the type list.

 2. write the code and put into scm.c.

 3. add a make_subr or make_gsubr call to init_scm.  Or put an entry into the appropriate iproc structure.

To add a package of new procedures to scm (see crs.c for example):

 1. create a new C file (foo.c).

 2. at the front of foo.c put declarations for strings for your procedure names.

          static char s_twiddle_bits[]="twiddle-bits!";
          static char s_bitsp[]="bits?";

 3. choose the appropriate subr types from the type list in code.doc.

 4. write the code for the procedures and put into foo.c

 5. create one iproc structure for each subr type used in foo.c

          static iproc subr3s[]= {
                  {s_twiddle-bits,twiddle-bits},
                  {s_bitsp,bitsp},
                  {0,0} };

 6. create an init_<name of file> routine at the end of the file which calls init_iprocs with the correct type for each of the iprocs created in step 5.

          void init_foo()
          {
            init_iprocs(subr1s, tc7_subr_1);
            init_iprocs(subr3s, tc7_subr_3);
          }

     If your package needs to have a finalization routine called to free up storage, close files, etc, then also have a line in init_foo like:

          add_final(final_foo);

     final_foo should be a (void) procedure of no arguments.  The finals will be called in opposite order from their definition.

     The line:

          add_feature("foo");

     will append a symbol 'foo to the (list) value of *features*.

 7. put any scheme code which needs to be run as part of your package into Ifoo.scm.

 8. put an if into Init.scm which loads Ifoo.scm if your package is included:

          (if (defined? twiddle-bits!)
              (load (in-vicinity (implementation-vicinity)
                                 "Ifoo"
                                 (scheme-file-suffix))))

     or use (provided? 'foo) instead of (defined? twiddle-bits!) if you have added the feature.

 9. put documentation of the new procedures into foo.doc

 10. add lines to your Makefile to compile and link SCM with your object file.  Add a init_foo\(\)\; to the INITS=... line at the beginning of the makefile.

These steps should allow your package to be linked into SCM with a minimum of difficulty.  Your package should also work with dynamic linking if your SCM has this capability.

Special forms (new syntax) can be added to scm.

 1. define a new MAKISYM in scm.h and increment NUM_ISYMS.

 2. add a string with the new name in the corresponding place in isymnames in repl.c.

 3. add case: clause to ceval() near i_quasiquote (in eval.c).

New syntax can now be added without recompiling SCM by the use of the procedure->syntax, procedure->macro, procedure->memoizing-macro, and defmacro.  For details, see Syntax Extensions.

Defining Subrs
--------------

If CCLO is #defined when compiling, the compiled closure feature will be enabled.  It is automatically enabled if dynamic linking is enabled.

The SCM interpreter directly recognizes subrs taking small numbers of arguments.  In order to create subrs taking larger numbers of arguments use:

 -- Function: make_gsubr name req opt rest fcn
     returns a cclo (compiled closure) object of name char * NAME which takes int REQ required arguments, int OPT optional arguments, and a list of rest arguments if int REST is 1 (0 for not).

     SCM (*fcn)() is a pointer to a C function to do the work.

     The C function will always be called with REQ + OPT + REST arguments, optional arguments not supplied will be passed UNDEFINED.  An error will be signaled if the subr is called with too many or too few arguments.  Currently a total of 10 arguments may be specified, but increasing this limit should not be difficult.

          /* A silly example, taking 2 required args,
             1 optional, and a list of rest args */

          SCM gsubr_21l(req1,req2,opt,rst)
               SCM req1,req2,opt,rst;
          {
            lputs("gsubr-2-1-l:\n req1: ", cur_outp);
            display(req1,cur_outp);
            lputs("\n req2: ", cur_outp);
            display(req2,cur_outp);
            lputs("\n opt: ", cur_outp);
            display(opt,cur_outp);
            lputs("\n rest: ", cur_outp);
            display(rst,cur_outp);
            newline(cur_outp);
            return UNSPECIFIED;
          }

          void init_gsubr211()
          {
            make_gsubr("gsubr-2-1-l", 2, 1, 1, gsubr_21l);
          }

Defining Smobs
--------------

Here is an example of how to add a new type named foo to SCM. The following lines need to be added to your code:

long tc16_foo;
     The type code which will be used to identify the new type.

static smobfuns foosmob = {markfoo,freefoo,printfoo,equalpfoo};
     smobfuns is a structure composed of 4 functions:

          typedef struct {
            SCM   (*mark)P((SCM));
            sizet (*free)P((CELLPTR));
            int   (*print)P((SCM exp, SCM port, int writing));
            SCM   (*equalp)P((SCM, SCM));
          } smobfuns;

     smob.mark
          is a function of one argument of type SCM (the cell to mark) and returns type SCM which will then be marked.  If no further objects need to be marked then return an immediate object such as BOOL_F.  2 functions are provided:

          markcdr(ptr)
               which marks the current cell and returns CDR(ptr).

          mark0(ptr)
               which marks the current cell and returns BOOL_F.

     smob.free
          is a function of one argument of type CELLPTR (the cell to collected) and returns type sizet which is the number of malloced bytes which were freed.  Smob.free should free any malloced storage associated with this object.  The function free0(ptr) is provided which does not free any storage and returns 0.

     smob.print
          is 0 or a function of 3 arguments.  The first, of type SCM, is the smob object.  The second, of type SCM, is the stream on which to write the result.  The third, of type int, is 1 if the object should be writen, 0 if it should be displayed.  This function should return non-zero if it printed, and zero otherwise (in which case a hexadecimal number will be printed).

     smob.equalp
          is 0 or a function of 2 SCM arguments.  Both of these arguments will be of type tc16foo.  This function should return BOOL_T if the smobs are equal, BOOL_F if they are not.  If smob.equalp is 0, equal? will return BOOL_F if they are not eq?.

tc16_foo = newsmob(&foosmob);
     Allocates the new type with the functions from foosmob.  This line goes in an init_ routine.

Promises and macros in eval.c and arbiters in repl.c provide examples of SMOBs.  There are a maximum of 256 SMOBs.

Defining Ptobs
--------------

ptobs are similar to smobs but define new types of port to which SCM procedures can read or write.  The following functions are defined in the ptobfuns:

     typedef struct {
       SCM   (*mark)P((SCM ptr));
       int   (*free)P((FILE *p));
       int   (*print)P((SCM exp, SCM port, int writing));
       SCM   (*equalp)P((SCM, SCM));
       int   (*fputc)P((int c, FILE *p));
       int   (*fputs)P((char *s, FILE *p));
       sizet (*fwrite)P((char *s, sizet siz, sizet num, FILE *p));
       int   (*fflush)P((FILE *stream));
       int   (*fgetc)P((FILE *p));
       int   (*fclose)P((FILE *p));
     } ptobfuns;

The .free component to the structure takes a FILE * or other C construct as its argument, unlike .free in a smob, which takes the whole smob cell.  Often, .free and .fclose can be the same function.  See fptob and pipob in sys.c for examples of how to define ptobs.

Calling Scheme From C
---------------------

To use SCM as a whole from another program call init_scm or run_scm as is done in main() in scm.c.

In order to call indivdual Scheme procedures from C code more is required; SCM's storage system needs to be initialized.  The simplest way to do this for a statically linked single-thread program is to:

 1. make a SCM procedure which calls your code's startup routine.

 2. use the #define RTL flag when compiling scm.c to elide SCM's main().

 3. In your main(), call run_scm with arguments (argc and argv) to invoke your code's startup routine.

 4. link your code with SCM at compile time.

For a dynamically linked single-thread program:

 1. make an init_ procedure for your code which will set up any Scheme definitions you need and then call your startup routine (see Changing Scm).

 2. Start SCM with command line arguments to dynamically link your code. After your module is linked, the init_ procedure will be called, and hence your startup routine.

Now use apply (and perhaps intern) to call Scheme procedures from your C code.  For example:

     /* If this apply fails, SCM will catch the error */
     apply(CDR(intern("srv:startup",sizeof("srv:startup")-1)),
           mksproc(srvproc),
           listofnull);

     func = CDR(intern(rpcname,strlen(rpcname)));
     retval = apply(func, cons(mksproc(srvproc), args), EOL);

SCM now has routines to make calling back to Scheme procedures easier:

 -- Function: int scm_ldfile (char *FILE)
     Loads the Scheme source file FILE.  Returns 0 if successful, non-0 if not.  This function is used to load SCM's initialization file Init.scm.

 -- Function: int scm_ldprog (char *FILE)
     Loads the Scheme source file (in-vicinity (program-vicinity) FILE).  Returns 0 if successful, non-0 if not.

     This function is useful for compiled code init_ functions to load non-compiled Scheme (source) files.  program-vicinity is the directory from which the calling code was loaded (see Vicinity (SLIB)).

 -- Function: SCM scm_evstr (char *STR)
     Returns the result of reading an expression from STR and evaluating it.

 -- Function: void scm_ldstr (char *STR)
     Reads and evaluates all the expressions from STR.

If you wish to catch errors during execution of Scheme code, then you can use a wrapper like this for your Scheme procedures:

     (define (srv:protect proc)
       (lambda args
         (define result #f)                  ; put default value here
         (call-with-current-continuation
          (lambda (cont)
            (dynamic-wind (lambda () #t)
                          (lambda ()
                            (set! result (apply proc args))
                            (set! cont #f))
                          (lambda ()
                            (if cont (cont #f))))))
         result))

Calls to procedures so wrapped will return even if an error occurs.

Continuations
-------------

The scm procedure call-with-current-continuation calls it's argument with an object of type contin.

If CHEAP_CONTINUATIONS is #defined (in scmfig.h) the contin just contains a jmp_buf.  When the contin is applied, a longjmp of the jmp_buf is done.

If CHEAP_CONTINUATIONS is not #defined the contin contains the jmp_buf and a copy of the C stack between the call_cc stack frame and BASE(rootcont).  When the contin is applied:

  * the stack is grown larger than the saved stack, if neccessary.

  * the saved stack is copied back into it's original position.

  * longjmp of the jmp_buf is called.

On systems with nonlinear stack disciplines (multiple stacks or non-contiguous stack frames) copying the stack will not work properly. These systems need to #define CHEAP_CONTINUATIONS in scmfig.h.

Evaluation
----------

 -- Variable: symhash
     Top level symbol values are stored in the symhash table. symhash is an array of lists of ISYMs and pairs of symbols and values.

 -- Immediate: ILOC
     Whenever a symbol's value is found in the local environment the pointer to the symbol in the code is replaced with an immediate object (ILOC) which specifies how many environment frames down and how far in to go for the value.  When this immediate object is subsequently encountered, the value can be retrieved quickly.

 -- Immediate: GLOC
     Pointers to symbols not defined in local environments are changed to one plus the value cell address in symhash.  This incremented pointer is called a GLOC.  The low order bit is normally reserved for GCmark; But, since references to variables in the code always occur in the CAR position and the GCmark is in the CDR, there is no conflict.

If the compile FLAG CAUTIOUS is #defined then the number of arguments is always checked for application of closures.  If the compile FLAG RECKLESS is #defined then they are not checked. Otherwise, number of argument checks for closures are made only when the function position (whose value is the closure) of a combination is not an ILOC or GLOC.  When the function position of a combination is a symbol it will be checked only the first time it is evaluated because it will then be replaced with an ILOC or GLOC.

 -- Macro: EVAL expression env
 -- Macro: SIDEVAL expression env
     EVAL Returns the result of evaluating EXPRESSION in ENV.  SIDEVAL evaluates EXPRESSION in ENV when the value of the expression is not used.

     Both of these macros alter the list structure of EXPRESSION as it is memoized and hence should be used only when it is known that EXPRESSION will not be referenced again.  The C function eval is safe from this problem.

 -- Function: SCM eval (SCM EXPRESSION)
     Returns the result of evaluating EXPRESSION in the top-level environment.  eval copies expression so that memoization does not modify expression.

Improvements To Make
====================

  * Prefix and make more uniform all C function, variable, and constant names.  Provide a file full of #define's to provide backward compatability.

  * lgcd() needs to generate at most one bignum, but currently generates more.

  * divide() could use shifts instead of multiply and divide when scaling.

  * If an open fails because there are no unused file handles, GC should be done so that file handles which are no longer used can be collected.

  * If the symhash array is specially marked in garbage collection, msymbols with value UNDEFINED which have no pointers to them can be collected.  In Maclisp this was called gctwa.

  * Compaction could be done to malloced objects by freeing and reallocing all the malloc objects encountered in a scan of the heap. Whether compactions would actually occur is system depenedent.

  * Copying all of the stack is wasteful of storage.  Any time a call-with-current-continuation is called the stack could be re-rooted with a frame which calls the contin just created.  This in combination with checking stack depth could also be used to allow stacks deeper than 64K on the IBM PC.

  * lookupcar in eval.c should not memoize (to ILOCs) when it retrieves environments deeper or longer than 4095.  The values can still be retrieved (albeit slowly), but an ILOC should not be made.  The MEMOIZE_LOCALS flag could then be flushed.

  * The must- or make- routines need some sort of C macros or conditionalization so that they check:

       * that the LENGTH field fits into a size_t (as is checked now) for platforms with (sizeof(size_t) < sizeof(SCM)).

       * that the LENGTH field fits into 24 (or 56) bits on machines where size_t is 32 bits or more.

     This is trickier than it first looks because the must_malloc() routine is also used for allocating heap segments, which do not have the LENGTH field restriction.  Putting the 24 bit test into must_malloc() should be tested for speed impact.

Finishing Dynamic Linking
=========================

Scott Schwartz <schwartz@galapagos.cse.psu.edu> suggests: One way to tidy up the dynamic loading stuff would be to grab the code from perl5.

VMS
...

George Carrette (gjc@mitech.com) outlines how to dynamically link on VMS.  There is already some code in dynl.c to do this, but someone with a VMS system needs to finish and debug it.

 1. Say you have this main.c program:

          main()
          {init_lisp();
           lisp_repl();}

 2. and you have your lisp in files repl.c, gc.c, eval.c and there are some toplevel non-static variables in use called the_heap, the_environment, and some read-only toplevel structures, such as the_subr_table.

          $ LINK/SHARE=LISPRTL.EXE/DEBUG REPL.OBJ,GC.OBJ,EVAL.OBJ,LISPRTL.OPT/OPT

 3. where LISPRTL.OPT must contain at least this:

          SYS$LIBRARY:VAXCRTL/SHARE
          UNIVERSAL=init_lisp
          UNIVERSAL=lisp_repl
          PSECT_ATTR=the_subr_table,SHR,NOWRT,LCL
          PSECT_ATTR=the_heap,NOSHR,LCL
          PSECT_ATTR=the_environment,NOSHR,LCL

     Notice: The psect (Program Section) attributes.

     LCL
          means to keep the name local to the shared library.  You almost always want to do that for a good clean library.

     SHR,NOWRT
          means shared-read-only.  Which is the default for code, and is also good for efficiency of some data structures.

     NOSHR,LCL
          is what you want for everything else.

     Note: If you do not have a handy list of all these toplevel variables, do not dispair.  Just do your link with the /MAP=LISPRTL.MAP/FULL and then search the map file,

          $SEARCH/OUT=LISPRTL.LOSERS LISPRTL.MAP  ",  SHR,NOEXE,  RD,  WRT"

     And use an emacs keyboard macro to muck the result into the proper form. Of course only the programmer can tell if things can be made read-only. I have a DCL command procedure to do this if you want it.

 4. Now MAIN.EXE would be linked thusly:

          $ DEFINE LISPRTL USER$DISK:[JAFFER]LISPRTL.EXE

          $LINK MAIN.OBJ,SYS$INPUT:/OPT
           SYS$LIBRARY:VAXCRTL/SHARE
           LISPRTL/SHARE

     Note the definition of the LISPRTL logical name.  Without such a definition you will need to copy LISPRTL.EXE over to SYS$SHARE: (aka SYS$LIBRARY:) in order to invoke the main program once it is linked.

 5. Now say you have a file of optional subrs, MYSUBRS.C.  And there is a routine INIT_MYSUBRS that must be called before using it.

          $ CC MYSUBRS.C
          $ LINK/SHARE=MYSUBRS.EXE MYSUBRS.OBJ,SYS$INPUT:/OPT
            SYS$LIBRARY:VAXCRTL/SHARE
            LISPRTL/SHARE
            UNIVERSAL=INIT_MYSUBRS

     Ok.  Another hint is that you can avoid having to add the PSECT declaration of NOSHR,LCL by declaring variables status in the C language source.  That works great for most things.

 6. Then the dynamic loader would have to do this:

          {void (*init_fcn)();
           long retval;
           retval = lib$find_image_symbol("MYSUBRS","INIT_MYSUBRS",&init_fcn,
                                          "SYS$DISK:[].EXE");
           if (retval != SS$_NORMAL) error(...);
           (*init_fcn)();}

     But of course all string arguments must be (struct dsc$descriptor *) and the last argument is optional if MYSUBRS is defined as a logical name or if MYSUBRS.EXE has been copied over to SYS$SHARE.  The other consideration is that you will want to turn off C-c or other interrupt handling while you are inside most lib$ calls.

     As far as the generation of all the UNIVERSAL=... declarations.  Well, you could do well to have that automatically generated from the public LISPRTL.H file, of course.

     VMS has a good manual called the Guide to Writing Modular Procedures or something like that, which covers this whole area rather well, and also talks about advanced techniques, such as a way to declare a program section with a pointer to a procedure that will be automatically invoked whenever any shared image is dynamically activated.  Also, how to set up a handler for normal or abnormal program exit so that you can clean up side effects (such as opening a database). But for use with LISPRTL you probably don't need that hair.

     One fancier option that is useful under VMS for LISPLIB.EXE is to define all your exported procedures through an call vector instead of having them just be pointers into random places in the image, which is what you get by using UNIVERSAL.

     If you set up the call vector thing correctly it will allow you to modify and relink LISPLIB.EXE without having to relink programs that have been linked against it.

Windows NT
..........

George Carrette (gjc@mitech.com) outlines how to dynamically link on Windows NT:

  * The Software Developers Kit has a sample called SIMPLDLL. Here is the gist of it, following along the lines of the VMS description above (contents of a makefile for the SDK NMAKE)

          LISPLIB.exp:
          LISPLIB.lib: LISPLIB.def
              $(implib) -machine:$(CPU) -def:LISPLIB.def -out:LISPLIB.lib

          LISPLIB.DLL : $(LISPLIB_OBJS) LISPLIB.EXP
              $(link) $(linkdebug)              \
              -dll                 \
              -out:LISPLIB.DLL     \
              LISPLIB.EXP $(LISPLIB_OBJS) $(conlibsdll)

  * The LISPDEF.DEF file has this:

          LIBRARY lisplib
          EXPORT
           init_lisp
           init_repl

  * And MAIN.EXE using:

          CLINK = $(link) $(ldebug) $(conflags) -out:$*.exe $** $(conlibsdll)

          MAIN.EXE : MAIN.OBJ LISPLIB.LIB
           $(CLINK)

  * And MYSUBRS.DLL is produced using:

          mysubrs.exp:
          mysubrs.lib: mysubrs.def
              $(implib) -machine:$(CPU) -def:MYSUBRS.def -out:MYSUBRS.lib

          mysubrs.dll : mysubrs.obj mysubrs.exp mysubrs.lib
              $(link) $(linkdebug) \
              -dll                 \
              -out:mysubrs.dll     \
              MYSUBRS.OBJ MYSUBRS.EXP LISPLIB.LIB $(conlibsdll)

  * Where MYSUBRS.DEF has

          LIBRARY mysubrs
          EXPORT
           INIT_MYSUBRS

  * And the dynamic loader looks something like this, calling the two procedures LoadLibrary and GetProcAddress.

          LISP share_image_load(LISP fname)
          {long iflag;
           LISP retval,(*fcn)(void);
           HANDLE hLib;
           DWORD err;
           char *libname,fcnname[64];
           iflag = nointerrupt(1);
           libname = c_string(fname);
           _snprintf(fcnname,sizeof(fcnname),"INIT_%s",libname);
           if (!(hLib = LoadLibrary(libname)))
             {err = GetLastError();
              retval = list2(fname,LSPNUM(err));
              serror1("library failed to load",retval);}
           if (!(fcn = (LISP (*)(void)) GetProcAddress(hLib,fcnname)))
             {err = GetLastError();
              retval = list2(fname,LSPNUM(err));
              serror1("could not find library init procedure",retval);}
           retval = (*fcn)();
           nointerrupt(iflag);
           return(retval);}

  * Note: in VMS the linker and dynamic loader is case sensitive, but all the language compilers, including C, will by default upper-case external symbols for use by the linker, although the debugger gets its own symbols and case sensitivity is language mode dependant.  In Windows NT things are case sensitive generally except for file and device names, which are case canonicalizing like in the Symbolics filesystem.

  * Also: All this WINDOWS NT stuff will work in MS-DOS MS-Windows 3.1 too, by a method of compiling and linking under Windows NT, and then copying various files over to MS-DOS/WINDOWS.

Procedure and Macro Index
*************************

This is an alphabetical list of all the procedures and macros in SCM.

Variable Index
**************

This is an alphabetical list of all the global variables in SCM.

Type Index
**********

This is an alphabetical list of all the data types in SCM.