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GNU Info File | 2001-07-15 | 19.5 KB | 456 lines

This is Info file f/g77.info, produced by Makeinfo version 1.68 from the input file ./f/g77.texi. INFO-DIR-SECTION Programming START-INFO-DIR-ENTRY * g77: (g77). The GNU Fortran compiler. END-INFO-DIR-ENTRY This file documents the use and the internals of the GNU Fortran (`g77') compiler. It corresponds to the GCC-2.95 version of `g77'. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1995-1999 Free Software Foundation, Inc. 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. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English. Contributed by James Craig Burley (<craig@jcb-sc.com>). Inspired by a first pass at translating `g77-0.5.16/f/DOC' that was contributed to Craig by David Ronis (<ronis@onsager.chem.mcgill.ca>). File: g77.info, Node: Code Gen Options, Next: Environment Variables, Prev: Directory Options, Up: Invoking G77 Options for Code Generation Conventions ======================================= These machine-independent options control the interface conventions used in code generation. Most of them have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed--the one which is not the default. You can figure out the other form by either removing `no-' or adding it. `-fno-automatic' Treat each program unit as if the `SAVE' statement was specified for every local variable and array referenced in it. Does not affect common blocks. (Some Fortran compilers provide this option under the name `-static'.) `-finit-local-zero' Specify that variables and arrays that are local to a program unit (not in a common block and not passed as an argument) are to be initialized to binary zeros. Since there is a run-time penalty for initialization of variables that are not given the `SAVE' attribute, it might be a good idea to also use `-fno-automatic' with `-finit-local-zero'. `-fno-f2c' Do not generate code designed to be compatible with code generated by `f2c'; use the GNU calling conventions instead. The `f2c' calling conventions require functions that return type `REAL(KIND=1)' to actually return the C type `double', and functions that return type `COMPLEX' to return the values via an extra argument in the calling sequence that points to where to store the return value. Under the GNU calling conventions, such functions simply return their results as they would in GNU C--`REAL(KIND=1)' functions return the C type `float', and `COMPLEX' functions return the GNU C type `complex' (or its `struct' equivalent). This does not affect the generation of code that interfaces with the `libg2c' library. However, because the `libg2c' library uses `f2c' calling conventions, `g77' rejects attempts to pass intrinsics implemented by routines in this library as actual arguments when `-fno-f2c' is used, to avoid bugs when they are actually called by code expecting the GNU calling conventions to work. For example, `INTRINSIC ABS;CALL FOO(ABS)' is rejected when `-fno-f2c' is in force. (Future versions of the `g77' run-time library might offer routines that provide GNU-callable versions of the routines that implement the `f2c'-callable intrinsics that may be passed as actual arguments, so that valid programs need not be rejected when `-fno-f2c' is used.) *Caution:* If `-fno-f2c' is used when compiling any source file used in a program, it must be used when compiling *all* Fortran source files used in that program. `-ff2c-library' Specify that use of `libg2c' (or the original `libf2c') is required. This is the default for the current version of `g77'. Currently it is not valid to specify `-fno-f2c-library'. This option is provided so users can specify it in shell scripts that build programs and libraries that require the `libf2c' library, even when being compiled by future versions of `g77' that might otherwise default to generating code for an incompatible library. `-fno-underscoring' Do not transform names of entities specified in the Fortran source file by appending underscores to them. With `-funderscoring' in effect, `g77' appends two underscores to names with underscores and one underscore to external names with no underscores. (`g77' also appends two underscores to internal names with underscores to avoid naming collisions with external names. The `-fno-second-underscore' option disables appending of the second underscore in all cases.) This is done to ensure compatibility with code produced by many UNIX Fortran compilers, including `f2c', which perform the same transformations. Use of `-fno-underscoring' is not recommended unless you are experimenting with issues such as integration of (GNU) Fortran into existing system environments (vis-a-vis existing libraries, tools, and so on). For example, with `-funderscoring', and assuming other defaults like `-fcase-lower' and that `j()' and `max_count()' are external functions while `my_var' and `lvar' are local variables, a statement like I = J() + MAX_COUNT (MY_VAR, LVAR) is implemented as something akin to: i = j_() + max_count__(&my_var__, &lvar); With `-fno-underscoring', the same statement is implemented as: i = j() + max_count(&my_var, &lvar); Use of `-fno-underscoring' allows direct specification of user-defined names while debugging and when interfacing `g77'-compiled code with other languages. Note that just because the names match does *not* mean that the interface implemented by `g77' for an external name matches the interface implemented by some other language for that same name. That is, getting code produced by `g77' to link to code produced by some other compiler using this or any other method can be only a small part of the overall solution--getting the code generated by both compilers to agree on issues other than naming can require significant effort, and, unlike naming disagreements, linkers normally cannot detect disagreements in these other areas. Also, note that with `-fno-underscoring', the lack of appended underscores introduces the very real possibility that a user-defined external name will conflict with a name in a system library, which could make finding unresolved-reference bugs quite difficult in some cases--they might occur at program run time, and show up only as buggy behavior at run time. In future versions of `g77', we hope to improve naming and linking issues so that debugging always involves using the names as they appear in the source, even if the names as seen by the linker are mangled to prevent accidental linking between procedures with incompatible interfaces. `-fno-second-underscore' Do not append a second underscore to names of entities specified in the Fortran source file. This option has no effect if `-fno-underscoring' is in effect. Otherwise, with this option, an external name such as `MAX_COUNT' is implemented as a reference to the link-time external symbol `max_count_', instead of `max_count__'. `-fno-ident' Ignore the `#ident' directive. `-fzeros' Treat initial values of zero as if they were any other value. As of version 0.5.18, `g77' normally treats `DATA' and other statements that are used to specify initial values of zero for variables and arrays as if no values were actually specified, in the sense that no diagnostics regarding multiple initializations are produced. This is done to speed up compiling of programs that initialize large arrays to zeros. Use `-fzeros' to revert to the simpler, slower behavior that can catch multiple initializations by keeping track of all initializations, zero or otherwise. *Caution:* Future versions of `g77' might disregard this option (and its negative form, the default) or interpret it somewhat differently. The interpretation changes will affect only non-standard programs; standard-conforming programs should not be affected. `-fdebug-kludge' Emit information on `COMMON' and `EQUIVALENCE' members that might help users of debuggers work around lack of proper debugging information on such members. As of version 0.5.19, `g77' offers this option to emit information on members of aggregate areas to help users while debugging. This information consists of establishing the type and contents of each such member so that, when a debugger is asked to print the contents, the printed information provides rudimentary debugging information. This information identifies the name of the aggregate area (either the `COMMON' block name, or the `g77'-assigned name for the `EQUIVALENCE' name) and the offset, in bytes, of the member from the beginning of the area. Using `gdb', this information is not coherently displayed in the Fortran language mode, so temporarily switching to the C language mode to display the information is suggested. Use `set language c' and `set language fortran' to accomplish this. For example: COMMON /X/A,B EQUIVALENCE (C,D) CHARACTER XX*50 EQUIVALENCE (I,XX(20:20)) END GDB is free software and you are welcome to distribute copies of it under certain conditions; type "show copying" to see the conditions. There is absolutely no warranty for GDB; type "show warranty" for details. GDB 4.16 (lm-gnits-dwim), Copyright 1996 Free Software Foundation, Inc... (gdb) b MAIN__ Breakpoint 1 at 0t1200000201120112: file cd.f, line 5. (gdb) r Starting program: /home/user/a.out Breakpoint 1, MAIN__ () at cd.f:5 Current language: auto; currently fortran (gdb) set language c Warning: the current language does not match this frame. (gdb) p a $2 = "At (COMMON) `x_' plus 0 bytes" (gdb) p b $3 = "At (COMMON) `x_' plus 4 bytes" (gdb) p c $4 = "At (EQUIVALENCE) `__g77_equiv_c' plus 0 bytes" (gdb) p d $5 = "At (EQUIVALENCE) `__g77_equiv_c' plus 0 bytes" (gdb) p i $6 = "At (EQUIVALENCE) `__g77_equiv_xx' plus 20 bytes" (gdb) p xx $7 = "At (EQUIVALENCE) `__g77_equiv_xx' plus 1 bytes" (gdb) set language fortran (gdb) Use `-fdebug-kludge' to generate this information, which might make some programs noticeably larger. *Caution:* Future versions of `g77' might disregard this option (and its negative form). Current plans call for this to happen when published versions of `g77' and `gdb' exist that provide proper access to debugging information on `COMMON' and `EQUIVALENCE' members. `-femulate-complex' Implement `COMPLEX' arithmetic via emulation, instead of using the facilities of the `gcc' back end that provide direct support of `complex' arithmetic. (`gcc' had some bugs in its back-end support for `complex' arithmetic, due primarily to the support not being completed as of version 2.8.1 and `egcs' 1.1.2.) Use `-femulate-complex' if you suspect code-generation bugs, or experience compiler crashes, that might result from `g77' using the `COMPLEX' support in the `gcc' back end. If using that option fixes the bugs or crashes you are seeing, that indicates a likely `g77' bugs (though, all compiler crashes are considered bugs), so, please report it. (Note that the known bugs, now believed fixed, produced compiler crashes rather than causing the generation of incorrect code.) Use of this option should not affect how Fortran code compiled by `g77' works in terms of its interfaces to other code, e.g. that compiled by `f2c'. *Caution:* Future versions of `g77' might ignore both forms of this option. `-falias-check' `-fargument-alias' `-fargument-noalias' `-fno-argument-noalias-global' *Version info:* These options are not supported by versions of `g77' based on `gcc' version 2.8. These options specify to what degree aliasing (overlap) is permitted between arguments (passed as pointers) and `COMMON' (external, or public) storage. The default for Fortran code, as mandated by the FORTRAN 77 and Fortran 90 standards, is `-fargument-noalias-global'. The default for code written in the C language family is `-fargument-alias'. Note that, on some systems, compiling with `-fforce-addr' in effect can produce more optimal code when the default aliasing options are in effect (and when optimization is enabled). *Note Aliasing Assumed To Work::, for detailed information on the implications of compiling Fortran code that depends on the ability to alias dummy arguments. `-fno-globals' Disable diagnostics about inter-procedural analysis problems, such as disagreements about the type of a function or a procedure's argument, that might cause a compiler crash when attempting to inline a reference to a procedure within a program unit. (The diagnostics themselves are still produced, but as warnings, unless `-Wno-globals' is specified, in which case no relevant diagnostics are produced.) Further, this option disables such inlining, to avoid compiler crashes resulting from incorrect code that would otherwise be diagnosed. As such, this option might be quite useful when compiling existing, "working" code that happens to have a few bugs that do not generally show themselves, but which `g77' diagnoses. Use of this option therefore has the effect of instructing `g77' to behave more like it did up through version 0.5.19.1, when it paid little or no attention to disagreements between program units about a procedure's type and argument information, and when it performed no inlining of procedures (except statement functions). Without this option, `g77' defaults to performing the potentially inlining procedures as it started doing in version 0.5.20, but as of version 0.5.21, it also diagnoses disagreements that might cause such inlining to crash the compiler as (fatal) errors, and warns about similar disagreements that are currently believed to not likely to result in the compiler later crashing or producing incorrect code. `-fflatten-arrays' Use back end's C-like constructs (pointer plus offset) instead of its `ARRAY_REF' construct to handle all array references. *Note:* This option is not supported. It is intended for use only by `g77' developers, to evaluate code-generation issues. It might be removed at any time. `-fbounds-check' `-ffortran-bounds-check' Enable generation of run-time checks for array subscripts and substring start and end points against the (locally) declared minimum and maximum values. The current implementation uses the `libf2c' library routine `s_rnge' to print the diagnostic. However, whereas `f2c' generates a single check per reference for a multi-dimensional array, of the computed offset against the valid offset range (0 through the size of the array), `g77' generates a single check per *subscript* expression. This catches some cases of potential bugs that `f2c' does not, such as references to below the beginning of an assumed-size array. `g77' also generates checks for `CHARACTER' substring references, something `f2c' currently does not do. Use the new `-ffortran-bounds-check' option to specify bounds-checking for only the Fortran code you are compiling, not necessarily for code written in other languages. *Note:* To provide more detailed information on the offending subscript, `g77' provides the `libg2c' run-time library routine `s_rnge' with somewhat differently-formatted information. Here's a sample diagnostic: Subscript out of range on file line 4, procedure rnge.f/bf. Attempt to access the -6-th element of variable b[subscript-2-of-2]. Aborted The above message indicates that the offending source line is line 4 of the file `rnge.f', within the program unit (or statement function) named `bf'. The offended array is named `b'. The offended array dimension is the second for a two-dimensional array, and the offending, computed subscript expression was `-6'. For a `CHARACTER' substring reference, the second line has this appearance: Attempt to access the 11-th element of variable a[start-substring]. This indicates that the offended `CHARACTER' variable or array is named `a', the offended substring position is the starting (leftmost) position, and the offending substring expression is `11'. (Though the verbage of `s_rnge' is not ideal for the purpose of the `g77' compiler, the above information should provide adequate diagnostic abilities to it users.) *Note Options for Code Generation Conventions: (gcc)Code Gen Options, for information on more options offered by the GBE shared by `g77', `gcc', and other GNU compilers. Some of these do *not* work when compiling programs written in Fortran: `-fpcc-struct-return' `-freg-struct-return' You should not use these except strictly the same way as you used them to build the version of `libg2c' with which you will be linking all code compiled by `g77' with the same option. `-fshort-double' This probably either has no effect on Fortran programs, or makes them act loopy. `-fno-common' Do not use this when compiling Fortran programs, or there will be Trouble. `-fpack-struct' This probably will break any calls to the `libg2c' library, at the very least, even if it is built with the same option. File: g77.info, Node: Environment Variables, Prev: Code Gen Options, Up: Invoking G77 Environment Variables Affecting GNU Fortran =========================================== GNU Fortran currently does not make use of any environment variables to control its operation above and beyond those that affect the operation of `gcc'. *Note Environment Variables Affecting GNU CC: (gcc)Environment Variables, for information on environment variables.