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"code.doc", documentation for scm4e0. Copyright (C) 1990, 1991, 1992, 1993, 1994 Aubrey Jaffer. This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 1, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. The author can be reached at jaffer@ai.mit.edu or Aubrey Jaffer, 84 Pleasant St., Wakefield MA 01880 Scm is a portable Scheme implementation written in C. Scm provides a machine independent platform for JACAL, a symbolic algebra system. SCM runs under VMS, MS-DOS, MacOS, Unix and similar systems. Scm conforms to Revised^4 Report on the Algorithmic Language Scheme and the IEEE P1178 specification. Scm is interpreted and implements tail recursion for interpreted code. Scm has inexacts, 30 bit immediate integers and large precision integers. Scm uses and garbage collects off the C-stack. This allows routines to be written in C without regard to GC visibility. call-with-current-continuation is fully are supported. ASCII and EBCDIC are supported. PROJECT HISTORY Siod, written by George Carrette, was the starting point for scm. Here is the Siod notice: /* Scheme In One Defun, but in C this time. * COPYRIGHT (c) 1989 BY * * PARADIGM ASSOCIATES INCORPORATED, CAMBRIDGE, MASSACHUSETTS. * * ALL RIGHTS RESERVED * Permission to use, copy, modify, distribute and sell this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the name of Paradigm Associates Inc not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission. PARADIGM DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL PARADIGM BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. gjc@paradigm.com Paradigm Associates Inc Phone: 617-492-6079 29 Putnam Ave, Suite 6 Cambridge, MA 02138 */ The innovation from Siod which scm uses is being able to garbage collect off the c-stack. All the code has been rewritten. See the file "ChangeLog" for a log of recent changes that have been made to scm. SCM DATA TYPES IMMEDIATEs: pointer: pointer to a cell inum: immediate 30 bit integers ichr: immediate characters iflags: #t, #f, (), #<eof>, #<undefined>, and #<unspecified> isym: `and', `begin', `case', `cond', `define', `do', `if', `lambda', `let', `let*', `letrec', `or', `quote', `set!', `#f', `#t', `#<undefined>', `#<eof>', `()', `#<unspecified>' IMCAR IMMEDIATEs: (only in car of evaluated code) ispcsym: special symbols: syntax-checked versions of first 14 isyms iloc: indexes to a variable's location in environment gloc: pointer to a symbol's value cell CELLs: Cells represent all SCM objects besides the immediates. A cell has a car and a cdr. Low-order bits in car identify the type of object. Rest of car and cdr hold object data. SIMPLE: cons: scheme cons-cell returned by (cons arg1 arg2) closure: applicable object returned by (lambda (args) ...) MALLOCs: spare: spare tc7 type code vector: scheme vector ssymbol: static scheme symbol (part of initial system) msymbol: malloced scheme symbol (can be GCed) string: scheme string bvect: uniform vector of booleans (bit-vector) ivect: uniform vector of integers uvect: uniform vector of non-negative integers fvect: uniform vector of short inexact real numbers dvect: uniform vector of double precision inexact real numbers cvect: uniform vector of double precision inexact complex numbers contin: applicable object produced by call-with-current-continuation cclo: environment and SUBR for compiled closure SUBRs: asubr: associative C function of 2 arguments. subr_0: C function of no arguments. subr_1: C function of one argument. cxr: car, cdr, cadr, cddr, ... subr_3: C function of 3 arguments. subr_2: C function of 2 arguments. rpsubr: transitive relational predicate C function of 2 arguments. subr_1o: C function of one optional argument. subr_2o: C function of 1 required and 1 optional argument. LSUBRs: lsubr_2: C function of 2 arguments and a list of arguments. lsubr: C function of list of arguments. PTOBs: inport: input port. outport: output port. ioport: input-output port. inpipe: input pipe. outpipe: output pipe. SMOBs: free_cell: unused cell on the freelist. flo: single-precision float. dblr: double-precision float. dblc: double-precision complex. bigpos: positive bignum. bigneg: negative bignum. promise: made by DELAY. arbiter: synchronization object. macro: macro expanding function. array: multi-dimensional array. DATA TYPE REPRESENTATIONS IMMEDIATE: B,D,E,F=data bit, C=flag code, P=pointer address bit ................................ pointer PPPPPPPPPPPPPPPPPPPPPPPPPPPPP000 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 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 MALLOCs: 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 ...........SCM (*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 00wroxxxxxxxxG1110111 ..........FILE *stream.......... inport uuuuuuuuuuU00011xxxxxxxxG1110111 ..........FILE *stream.......... outport 0000000000000101xxxxxxxxG1110111 ..........FILE *stream.......... ioport uuuuuuuuuuU00111xxxxxxxxG1110111 ..........FILE *stream.......... fport 00 00000000G1110111 ..........FILE *stream.......... pipe 00 00000001G1110111 ..........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...xxxxxxxxG1111111 ............*array.............. SMOBs SMOBs are a collection of miscellaneous types. 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. Inexact 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. To add a new type to scm: [1] long tc16_???; The type code which will be used to identify the new type. [2] static smobfuns ???smob = {mark???,free???,print???,equalp???}; smobfuns is a structure composed of 4 functions: 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 the CDR. 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 tc16???. 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?. [3] tc16_??? = newsmob(&???smob); Allocates the new type with the functions from [2]. Promises and macros in eval.c and arbiters in repl.c provide examples of SMOBs. There are a maximum of 256 SMOBs. PTOBs PTOBs are similar to SMOBs but define new types of port to which SCM can read or write. The following functions are defined in the ptobfuns: typedef struct { SCM (*mark)(); int (*free)(); int (*print)(); SCM (*equalp)(); int (*putc)(); int (*fputs)(); sizet (*fwrite)(); int (*fflush)(); int (*getc)(); int (*fclose)(); } ptobfuns; The "free" component to the structure takes a FILE * or other C construct as its argument, unlike "free" in 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. 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 SCM 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. INTERRUPTS If they are supported by the C implementation, init_signals() in scm.c sets up handlers for SIGINT and SIGALRM. The low level handlers for SIGINT and SIGALRM are int_signal() and alrm_signal(). All of the signal handlers immediately reestablish themselves by a call to signal(). 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. 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. When ALLOW_INTS is executed the deferred variables are checked and if set the appropriate handler is called. 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. CHANGING SCM When writing C-code a precaution is recommended. If your routine allocates a malloc type header from the heap make sure that a local SCM variable in your routine points to the type-header cell of the malloc object as long as you are using the malloced storage. This will prevent the malloc object from being freed before you are done with 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. The 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 `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' or 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). 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 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 calls 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. See MANUAL for details. To use scm from another program call init_scm or run_scm as is done in main() in "scm.c". 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: [1] the stack is grown larger than the saved stack, if neccessary. [2] the saved stack is copied back into it's original position. [3] 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". INTEGERS Scm has 30 bit immediate signed numbers called INUMs. An INUM instead of a pointer to a cell is flagged by a `1' in the second to low order bit position. Since cells are always 8 byte aligned a pointer to a cell has the low order 3 bits `0'. The high order 30 bits are used for the integer's value. 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. Scm also has large precision integers called bignums. They are stored as sign-magnitude 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. EVALUATION Top level symbol values are stored in the symhash table. Symhash is an array of lists of ISYMs and pairs of symbols and values. 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. 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. 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. IMPROVEMENTS TO MAKE 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. 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. 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. lgcd() needs to generate at most one bignum. divide() could use shifts instead of multiply and divide when scaling. (get-universal-time) should return the time since Midnight Jan 1, 1900 GMT when BIGDIG is #defined. DYNAMIC LINKING This note should help with porting to Windows NT and finishing the port for dynamic linking under VMS. ================================================================ Return-Path: <gjc@newjak.mitech.com> Date: Fri, 3 Sep 93 10:38:18 EDT From: gjc@newjak.mitech.com To: jaffer@ai.mit.edu Subject: dynamic linking. George Carrette GJC@MITECH.COM 3-SEP-1993 Dear Aubrey: On the subject of shared libraries and dynamic linking for VMS and WINDOWS NT, and SunOs. Let us take VMS first. (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$SHARE. The other consideration is that you will want to turn off CONTROL-C or other interrupt handling while you are inside most lib$ 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: 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. SUN-OS. I haven't tried any of this yet. The loading looks simple enough from the man command, but I do not know of the complexity or restrictions of creating the shared libraries. man dlopen DLOPEN(3X) MISCELLANEOUS LIBRARY FUNCTIONS DLOPEN(3X) NAME dlopen, dlsym, dlerror, dlclose - simple programmatic inter- face to the dynamic linker If you decide to employ this technology on SUN please let send me a summary so I can update this note. Obviously you want to hack your lisp compiler to automatically generate all the required linking and options files for the particular operating systems you are going to be using.