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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: Multiple Definitions of External Names, Next: Limitation on Implicit Declarations, Prev: Mangling of Names, Up: Disappointments Multiple Definitions of External Names -------------------------------------- `g77' doesn't allow a common block and an external procedure or `BLOCK DATA' to have the same name. Some systems allow this, but `g77' does not, to be compatible with `f2c'. `g77' could special-case the way it handles `BLOCK DATA', since it is not compatible with `f2c' in this particular area (necessarily, since `g77' offers an important feature here), but it is likely that such special-casing would be very annoying to people with programs that use `EXTERNAL FOO', with no other mention of `FOO' in the same program unit, to refer to external procedures, since the result would be that `g77' would treat these references as requests to force-load BLOCK DATA program units. In that case, if `g77' modified names of `BLOCK DATA' so they could have the same names as `COMMON', users would find that their programs wouldn't link because the `FOO' procedure didn't have its name translated the same way. (Strictly speaking, `g77' could emit a null-but-externally-satisfying definition of `FOO' with its name transformed as if it had been a `BLOCK DATA', but that probably invites more trouble than it's worth.) File: g77.info, Node: Limitation on Implicit Declarations, Prev: Multiple Definitions of External Names, Up: Disappointments Limitation on Implicit Declarations ----------------------------------- `g77' disallows `IMPLICIT CHARACTER*(*)'. This is not standard-conforming. File: g77.info, Node: Non-bugs, Next: Warnings and Errors, Prev: Disappointments, Up: Trouble Certain Changes We Don't Want to Make ===================================== This section lists changes that people frequently request, but which we do not make because we think GNU Fortran is better without them. * Menu: * Backslash in Constants:: Why `'\\'' is a constant that is one, not two, characters long. * Initializing Before Specifying:: Why `DATA VAR/1/' can't precede `COMMON VAR'. * Context-Sensitive Intrinsicness:: Why `CALL SQRT' won't work. * Context-Sensitive Constants:: Why `9.435784839284958' is a single-precision constant, and might be interpreted as `9.435785' or similar. * Equivalence Versus Equality:: Why `.TRUE. .EQ. .TRUE.' won't work. * Order of Side Effects:: Why `J = IFUNC() - IFUNC()' might not behave as expected. File: g77.info, Node: Backslash in Constants, Next: Initializing Before Specifying, Up: Non-bugs Backslash in Constants ---------------------- In the opinion of many experienced Fortran users, `-fno-backslash' should be the default, not `-fbackslash', as currently set by `g77'. First of all, you can always specify `-fno-backslash' to turn off this processing. Despite not being within the spirit (though apparently within the letter) of the ANSI FORTRAN 77 standard, `g77' defaults to `-fbackslash' because that is what most UNIX `f77' commands default to, and apparently lots of code depends on this feature. This is a particularly troubling issue. The use of a C construct in the midst of Fortran code is bad enough, worse when it makes existing Fortran programs stop working (as happens when programs written for non-UNIX systems are ported to UNIX systems with compilers that provide the `-fbackslash' feature as the default--sometimes with no option to turn it off). The author of GNU Fortran wished, for reasons of linguistic purity, to make `-fno-backslash' the default for GNU Fortran and thus require users of UNIX `f77' and `f2c' to specify `-fbackslash' to get the UNIX behavior. However, the realization that `g77' is intended as a replacement for *UNIX* `f77', caused the author to choose to make `g77' as compatible with `f77' as feasible, which meant making `-fbackslash' the default. The primary focus on compatibility is at the source-code level, and the question became "What will users expect a replacement for `f77' to do, by default?" Although at least one UNIX `f77' does not provide `-fbackslash' as a default, it appears that the majority of them do, which suggests that the majority of code that is compiled by UNIX `f77' compilers expects `-fbackslash' to be the default. It is probably the case that more code exists that would *not* work with `-fbackslash' in force than code that requires it be in force. However, most of *that* code is not being compiled with `f77', and when it is, new build procedures (shell scripts, makefiles, and so on) must be set up anyway so that they work under UNIX. That makes a much more natural and safe opportunity for non-UNIX users to adapt their build procedures for `g77''s default of `-fbackslash' than would exist for the majority of UNIX `f77' users who would have to modify existing, working build procedures to explicitly specify `-fbackslash' if that was not the default. One suggestion has been to configure the default for `-fbackslash' (and perhaps other options as well) based on the configuration of `g77'. This is technically quite straightforward, but will be avoided even in cases where not configuring defaults to be dependent on a particular configuration greatly inconveniences some users of legacy code. Many users appreciate the GNU compilers because they provide an environment that is uniform across machines. These users would be inconvenienced if the compiler treated things like the format of the source code differently on certain machines. Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU Fortran compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility. (This is consistent with the design goals for `gcc'. To change them for `g77', you must first change them for `gcc'. Do not ask the maintainers of `g77' to do this for you, or to disassociate `g77' from the widely understood, if not widely agreed-upon, goals for GNU compilers in general.) This is why GNU Fortran does and will treat backslashes in the same fashion on all types of machines (by default). *Note Direction of Language Development::, for more information on this overall philosophy guiding the development of the GNU Fortran language. Of course, users strongly concerned about portability should indicate explicitly in their build procedures which options are expected by their source code, or write source code that has as few such expectations as possible. For example, avoid writing code that depends on backslash (`\') being interpreted either way in particular, such as by starting a program unit with: CHARACTER BACKSL PARAMETER (BACKSL = '\\') Then, use concatenation of `BACKSL' anyplace a backslash is desired. In this way, users can write programs which have the same meaning in many Fortran dialects. (However, this technique does not work for Hollerith constants--which is just as well, since the only generally portable uses for Hollerith constants are in places where character constants can and should be used instead, for readability.) File: g77.info, Node: Initializing Before Specifying, Next: Context-Sensitive Intrinsicness, Prev: Backslash in Constants, Up: Non-bugs Initializing Before Specifying ------------------------------ `g77' does not allow `DATA VAR/1/' to appear in the source code before `COMMON VAR', `DIMENSION VAR(10)', `INTEGER VAR', and so on. In general, `g77' requires initialization of a variable or array to be specified *after* all other specifications of attributes (type, size, placement, and so on) of that variable or array are specified (though *confirmation* of data type is permitted). It is *possible* `g77' will someday allow all of this, even though it is not allowed by the FORTRAN 77 standard. Then again, maybe it is better to have `g77' always require placement of `DATA' so that it can possibly immediately write constants to the output file, thus saving time and space. That is, `DATA A/1000000*1/' should perhaps always be immediately writable to canonical assembler, unless it's already known to be in a `COMMON' area following as-yet-uninitialized stuff, and to do this it cannot be followed by `COMMON A'. File: g77.info, Node: Context-Sensitive Intrinsicness, Next: Context-Sensitive Constants, Prev: Initializing Before Specifying, Up: Non-bugs Context-Sensitive Intrinsicness ------------------------------- `g77' treats procedure references to *possible* intrinsic names as always enabling their intrinsic nature, regardless of whether the *form* of the reference is valid for that intrinsic. For example, `CALL SQRT' is interpreted by `g77' as an invalid reference to the `SQRT' intrinsic function, because the reference is a subroutine invocation. First, `g77' recognizes the statement `CALL SQRT' as a reference to a *procedure* named `SQRT', not to a *variable* with that name (as it would for a statement such as `V = SQRT'). Next, `g77' establishes that, in the program unit being compiled, `SQRT' is an intrinsic--not a subroutine that happens to have the same name as an intrinsic (as would be the case if, for example, `EXTERNAL SQRT' was present). Finally, `g77' recognizes that the *form* of the reference is invalid for that particular intrinsic. That is, it recognizes that it is invalid for an intrinsic *function*, such as `SQRT', to be invoked as a *subroutine*. At that point, `g77' issues a diagnostic. Some users claim that it is "obvious" that `CALL SQRT' references an external subroutine of their own, not an intrinsic function. However, `g77' knows about intrinsic subroutines, not just functions, and is able to support both having the same names, for example. As a result of this, `g77' rejects calls to intrinsics that are not subroutines, and function invocations of intrinsics that are not functions, just as it (and most compilers) rejects invocations of intrinsics with the wrong number (or types) of arguments. So, use the `EXTERNAL SQRT' statement in a program unit that calls a user-written subroutine named `SQRT'. File: g77.info, Node: Context-Sensitive Constants, Next: Equivalence Versus Equality, Prev: Context-Sensitive Intrinsicness, Up: Non-bugs Context-Sensitive Constants --------------------------- `g77' does not use context to determine the types of constants or named constants (`PARAMETER'), except for (non-standard) typeless constants such as `'123'O'. For example, consider the following statement: PRINT *, 9.435784839284958 * 2D0 `g77' will interpret the (truncated) constant `9.435784839284958' as a `REAL(KIND=1)', not `REAL(KIND=2)', constant, because the suffix `D0' is not specified. As a result, the output of the above statement when compiled by `g77' will appear to have "less precision" than when compiled by other compilers. In these and other cases, some compilers detect the fact that a single-precision constant is used in a double-precision context and therefore interpret the single-precision constant as if it was *explicitly* specified as a double-precision constant. (This has the effect of appending *decimal*, not *binary*, zeros to the fractional part of the number--producing different computational results.) The reason this misfeature is dangerous is that a slight, apparently innocuous change to the source code can change the computational results. Consider: REAL ALMOST, CLOSE DOUBLE PRECISION FIVE PARAMETER (ALMOST = 5.000000000001) FIVE = 5 CLOSE = 5.000000000001 PRINT *, 5.000000000001 - FIVE PRINT *, ALMOST - FIVE PRINT *, CLOSE - FIVE END Running the above program should result in the same value being printed three times. With `g77' as the compiler, it does. However, compiled by many other compilers, running the above program would print two or three distinct values, because in two or three of the statements, the constant `5.000000000001', which on most systems is exactly equal to `5.' when interpreted as a single-precision constant, is instead interpreted as a double-precision constant, preserving the represented precision. However, this "clever" promotion of type does not extend to variables or, in some compilers, to named constants. Since programmers often are encouraged to replace manifest constants or permanently-assigned variables with named constants (`PARAMETER' in Fortran), and might need to replace some constants with variables having the same values for pertinent portions of code, it is important that compilers treat code so modified in the same way so that the results of such programs are the same. `g77' helps in this regard by treating constants just the same as variables in terms of determining their types in a context-independent way. Still, there is a lot of existing Fortran code that has been written to depend on the way other compilers freely interpret constants' types based on context, so anything `g77' can do to help flag cases of this in such code could be very helpful. File: g77.info, Node: Equivalence Versus Equality, Next: Order of Side Effects, Prev: Context-Sensitive Constants, Up: Non-bugs Equivalence Versus Equality --------------------------- Use of `.EQ.' and `.NE.' on `LOGICAL' operands is not supported, except via `-fugly-logint', which is not recommended except for legacy code (where the behavior expected by the *code* is assumed). Legacy code should be changed, as resources permit, to use `.EQV.' and `.NEQV.' instead, as these are permitted by the various Fortran standards. New code should never be written expecting `.EQ.' or `.NE.' to work if either of its operands is `LOGICAL'. The problem with supporting this "feature" is that there is unlikely to be consensus on how it works, as illustrated by the following sample program: LOGICAL L,M,N DATA L,M,N /3*.FALSE./ IF (L.AND.M.EQ.N) PRINT *,'L.AND.M.EQ.N' END The issue raised by the above sample program is: what is the precedence of `.EQ.' (and `.NE.') when applied to `LOGICAL' operands? Some programmers will argue that it is the same as the precedence for `.EQ.' when applied to numeric (such as `INTEGER') operands. By this interpretation, the subexpression `M.EQ.N' must be evaluated first in the above program, resulting in a program that, when run, does not execute the `PRINT' statement. Other programmers will argue that the precedence is the same as the precedence for `.EQV.', which is restricted by the standards to `LOGICAL' operands. By this interpretation, the subexpression `L.AND.M' must be evaluated first, resulting in a program that *does* execute the `PRINT' statement. Assigning arbitrary semantic interpretations to syntactic expressions that might legitimately have more than one "obvious" interpretation is generally unwise. The creators of the various Fortran standards have done a good job in this case, requiring a distinct set of operators (which have their own distinct precedence) to compare `LOGICAL' operands. This requirement results in expression syntax with more certain precedence (without requiring substantial context), making it easier for programmers to read existing code. `g77' will avoid muddying up elements of the Fortran language that were well-designed in the first place. (Ask C programmers about the precedence of expressions such as `(a) & (b)' and `(a) - (b)'--they cannot even tell you, without knowing more context, whether the `&' and `-' operators are infix (binary) or unary!) Most dangerous of all is the fact that, even assuming consensus on its meaning, an expression like `L.AND.M.EQ.N', if it is the result of a typographical error, doesn't *look* like it has such a typo. Even experienced Fortran programmers would not likely notice that `L.AND.M.EQV.N' was, in fact, intended. So, this is a prime example of a circumstance in which a quality compiler diagnoses the code, instead of leaving it up to someone debugging it to know to turn on special compiler options that might diagnose it. File: g77.info, Node: Order of Side Effects, Prev: Equivalence Versus Equality, Up: Non-bugs Order of Side Effects --------------------- `g77' does not necessarily produce code that, when run, performs side effects (such as those performed by function invocations) in the same order as in some other compiler--or even in the same order as another version, port, or invocation (using different command-line options) of `g77'. It is never safe to depend on the order of evaluation of side effects. For example, an expression like this may very well behave differently from one compiler to another: J = IFUNC() - IFUNC() There is no guarantee that `IFUNC' will be evaluated in any particular order. Either invocation might happen first. If `IFUNC' returns 5 the first time it is invoked, and returns 12 the second time, `J' might end up with the value `7', or it might end up with `-7'. Generally, in Fortran, procedures with side-effects intended to be visible to the caller are best designed as *subroutines*, not functions. Examples of such side-effects include: * The generation of random numbers that are intended to influence return values. * Performing I/O (other than internal I/O to local variables). * Updating information in common blocks. An example of a side-effect that is not intended to be visible to the caller is a function that maintains a cache of recently calculated results, intended solely to speed repeated invocations of the function with identical arguments. Such a function can be safely used in expressions, because if the compiler optimizes away one or more calls to the function, operation of the program is unaffected (aside from being speeded up). File: g77.info, Node: Warnings and Errors, Prev: Non-bugs, Up: Trouble Warning Messages and Error Messages =================================== The GNU compiler can produce two kinds of diagnostics: errors and warnings. Each kind has a different purpose: *Errors* report problems that make it impossible to compile your program. GNU Fortran reports errors with the source file name, line number, and column within the line where the problem is apparent. *Warnings* report other unusual conditions in your code that *might* indicate a problem, although compilation can (and does) proceed. Warning messages also report the source file name, line number, and column information, but include the text `warning:' to distinguish them from error messages. Warnings might indicate danger points where you should check to make sure that your program really does what you intend; or the use of obsolete features; or the use of nonstandard features of GNU Fortran. Many warnings are issued only if you ask for them, with one of the `-W' options (for instance, `-Wall' requests a variety of useful warnings). *Note:* Currently, the text of the line and a pointer to the column is printed in most `g77' diagnostics. Probably, as of version 0.6, `g77' will no longer print the text of the source line, instead printing the column number following the file name and line number in a form that GNU Emacs recognizes. This change is expected to speed up and reduce the memory usage of the `g77' compiler. *Note Options to Request or Suppress Warnings: Warning Options, for more detail on these and related command-line options. File: g77.info, Node: Open Questions, Next: Bugs, Prev: Trouble, Up: Top Open Questions ************** Please consider offering useful answers to these questions! * `LOC()' and other intrinsics are probably somewhat misclassified. Is the a need for more precise classification of intrinsics, and if so, what are the appropriate groupings? Is there a need to individually enable/disable/delete/hide intrinsics from the command line? File: g77.info, Node: Bugs, Next: Service, Prev: Open Questions, Up: Top Reporting Bugs ************** Your bug reports play an essential role in making GNU Fortran reliable. When you encounter a problem, the first thing to do is to see if it is already known. *Note Trouble::. If it isn't known, then you should report the problem. Reporting a bug might help you by bringing a solution to your problem, or it might not. (If it does not, look in the service directory; see *Note Service::..) In any case, the principal function of a bug report is to help the entire community by making the next version of GNU Fortran work better. Bug reports are your contribution to the maintenance of GNU Fortran. Since the maintainers are very overloaded, we cannot respond to every bug report. However, if the bug has not been fixed, we are likely to send you a patch and ask you to tell us whether it works. In order for a bug report to serve its purpose, you must include the information that makes for fixing the bug. * Menu: * Criteria: Bug Criteria. Have you really found a bug? * Where: Bug Lists. Where to send your bug report. * Reporting: Bug Reporting. How to report a bug effectively. * Patches: Sending Patches. How to send a patch for GNU Fortran. *Note Known Causes of Trouble with GNU Fortran: Trouble, for information on problems we already know about. *Note How To Get Help with GNU Fortran: Service, for information on where to ask for help. File: g77.info, Node: Bug Criteria, Next: Bug Lists, Up: Bugs Have You Found a Bug? ===================== If you are not sure whether you have found a bug, here are some guidelines: * If the compiler gets a fatal signal, for any input whatever, that is a compiler bug. Reliable compilers never crash--they just remain obsolete. * If the compiler produces invalid assembly code, for any input whatever, that is a compiler bug, unless the compiler reports errors (not just warnings) which would ordinarily prevent the assembler from being run. * If the compiler produces valid assembly code that does not correctly execute the input source code, that is a compiler bug. However, you must double-check to make sure, because you might have run into an incompatibility between GNU Fortran and traditional Fortran. These incompatibilities might be considered bugs, but they are inescapable consequences of valuable features. Or you might have a program whose behavior is undefined, which happened by chance to give the desired results with another Fortran compiler. It is best to check the relevant Fortran standard thoroughly if it is possible that the program indeed does something undefined. After you have localized the error to a single source line, it should be easy to check for these things. If your program is correct and well defined, you have found a compiler bug. It might help if, in your submission, you identified the specific language in the relevant Fortran standard that specifies the desired behavior, if it isn't likely to be obvious and agreed-upon by all Fortran users. * If the compiler produces an error message for valid input, that is a compiler bug. * If the compiler does not produce an error message for invalid input, that is a compiler bug. However, you should note that your idea of "invalid input" might be someone else's idea of "an extension" or "support for traditional practice". * If you are an experienced user of Fortran compilers, your suggestions for improvement of GNU Fortran are welcome in any case. Many, perhaps most, bug reports against `g77' turn out to be bugs in the user's code. While we find such bug reports educational, they sometimes take a considerable amount of time to track down or at least respond to--time we could be spending making `g77', not some user's code, better. Some steps you can take to verify that the bug is not certainly in the code you're compiling with `g77': * Compile your code using the `g77' options `-W -Wall -O'. These options enable many useful warning; the `-O' option enables flow analysis that enables the uninitialized-variable warning. If you investigate the warnings and find evidence of possible bugs in your code, fix them first and retry `g77'. * Compile your code using the `g77' options `-finit-local-zero', `-fno-automatic', `-ffloat-store', and various combinations thereof. If your code works with any of these combinations, that is not proof that the bug isn't in `g77'--a `g77' bug exposed by your code might simply be avoided, or have a different, more subtle effect, when different options are used--but it can be a strong indicator that your code is making unwarranted assumptions about the Fortran dialect and/or underlying machine it is being compiled and run on. *Note Overly Convenient Command-Line Options: Overly Convenient Options, for information on the `-fno-automatic' and `-finit-local-zero' options and how to convert their use into selective changes in your own code. * Validate your code with `ftnchek' or a similar code-checking tool. `ftnchek' can be found at `ftp://ftp.netlib.org/fortran' or `ftp://ftp.dsm.fordham.edu'. Here are some sample `Makefile' rules using `ftnchek' "project" files to do cross-file checking and `sfmakedepend' (from `ftp://ahab.rutgers.edu/pub/perl/sfmakedepend') to maintain dependencies automatically. These assume the use of GNU `make'. # Dummy suffix for ftnchek targets: .SUFFIXES: .chek .PHONY: chekall # How to compile .f files (for implicit rule): FC = g77 # Assume `include' directory: FFLAGS = -Iinclude -g -O -Wall # Flags for ftnchek: CHEK1 = -array=0 -include=includes -noarray CHEK2 = -nonovice -usage=1 -notruncation CHEKFLAGS = $(CHEK1) $(CHEK2) # Run ftnchek with all the .prj files except the one corresponding # to the target's root: %.chek : %.f ; \ ftnchek $(filter-out $*.prj,$(PRJS)) $(CHEKFLAGS) \ -noextern -library $< # Derive a project file from a source file: %.prj : %.f ; \ ftnchek $(CHEKFLAGS) -noextern -project -library $< # The list of objects is assumed to be in variable OBJS. # Sources corresponding to the objects: SRCS = $(OBJS:%.o=%.f) # ftnchek project files: PRJS = $(OBJS:%.o=%.prj) # Build the program prog: $(OBJS) ; \ $(FC) -o $ $(OBJS) chekall: $(PRJS) ; \ ftnchek $(CHEKFLAGS) $(PRJS) prjs: $(PRJS) # For Emacs M-x find-tag: TAGS: $(SRCS) ; \ etags $(SRCS) # Rebuild dependencies: depend: ; \ sfmakedepend -I $(PLTLIBDIR) -I includes -a prj $(SRCS1) * Try your code out using other Fortran compilers, such as `f2c'. If it does not work on at least one other compiler (assuming the compiler supports the features the code needs), that is a strong indicator of a bug in the code. However, even if your code works on many compilers *except* `g77', that does *not* mean the bug is in `g77'. It might mean the bug is in your code, and that `g77' simply exposes it more readily than other compilers. File: g77.info, Node: Bug Lists, Next: Bug Reporting, Prev: Bug Criteria, Up: Bugs Where to Report Bugs ==================== Send bug reports for GNU Fortran to <gcc-bugs@gcc.gnu.org or bug-gcc@gnu.org>. Often people think of posting bug reports to a newsgroup instead of mailing them. This sometimes appears to work, but it has one problem which can be crucial: a newsgroup posting does not contain a mail path back to the sender. Thus, if maintainers need more information, they might be unable to reach you. For this reason, you should always send bug reports by mail to the proper mailing list. As a last resort, send bug reports on paper to: GNU Compiler Bugs Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307, USA File: g77.info, Node: Bug Reporting, Next: Sending Patches, Prev: Bug Lists, Up: Bugs How to Report Bugs ================== The fundamental principle of reporting bugs usefully is this: *report all the facts*. If you are not sure whether to state a fact or leave it out, state it! Often people omit facts because they think they know what causes the problem and they conclude that some details don't matter. Thus, you might assume that the name of the variable you use in an example does not matter. Well, probably it doesn't, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the compiler into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful. Keep in mind that the purpose of a bug report is to enable someone to fix the bug if it is not known. It isn't very important what happens if the bug is already known. Therefore, always write your bug reports on the assumption that the bug is not known. Sometimes people give a few sketchy facts and ask, "Does this ring a bell?" This cannot help us fix a bug, so it is rarely helpful. We respond by asking for enough details to enable us to investigate. You might as well expedite matters by sending them to begin with. (Besides, there are enough bells ringing around here as it is.) Try to make your bug report self-contained. If we have to ask you for more information, it is best if you include all the previous information in your response, as well as the information that was missing. Please report each bug in a separate message. This makes it easier for us to track which bugs have been fixed and to forward your bugs reports to the appropriate maintainer. Do not compress and encode any part of your bug report using programs such as `uuencode'. If you do so it will slow down the processing of your bug. If you must submit multiple large files, use `shar', which allows us to read your message without having to run any decompression programs. (As a special exception for GNU Fortran bug-reporting, at least for now, if you are sending more than a few lines of code, if your program's source file format contains "interesting" things like trailing spaces or strange characters, or if you need to include binary data files, it is acceptable to put all the files together in a `tar' archive, and, whether you need to do that, it is acceptable to then compress the single file (`tar' archive or source file) using `gzip' and encode it via `uuencode'. Do not use any MIME stuff--the current maintainer can't decode this. Using `compress' instead of `gzip' is acceptable, assuming you have licensed the use of the patented algorithm in `compress' from Unisys.) To enable someone to investigate the bug, you should include all these things: * The version of GNU Fortran. You can get this by running `g77' with the `-v' option. (Ignore any error messages that might be displayed when the linker is run.) Without this, we won't know whether there is any point in looking for the bug in the current version of GNU Fortran. * A complete input file that will reproduce the bug. If your source file(s) require preprocessing (for example, their names have suffixes like `.F', `.fpp', `.FPP', and `.r'), and the bug is in the compiler proper (`f771') or in a subsequent phase of processing, run your source file through the C preprocessor by doing `g77 -E SOURCEFILE > NEWFILE'. Then, include the contents of NEWFILE in the bug report. (When you do this, use the same preprocessor options--such as `-I', `-D', and `-U'--that you used in actual compilation.) A single statement is not enough of an example. In order to compile it, it must be embedded in a complete file of compiler input. The bug might depend on the details of how this is done. Without a real example one can compile, all anyone can do about your bug report is wish you luck. It would be futile to try to guess how to provoke the bug. For example, bugs in register allocation and reloading can depend on every little detail of the source and include files that trigger them. * Note that you should include with your bug report any files included by the source file (via the `#include' or `INCLUDE' directive) that you send, and any files they include, and so on. It is not necessary to replace the `#include' and `INCLUDE' directives with the actual files in the version of the source file that you send, but it might make submitting the bug report easier in the end. However, be sure to *reproduce* the bug using the *exact* version of the source material you submit, to avoid wild-goose chases. * The command arguments you gave GNU Fortran to compile that example and observe the bug. For example, did you use `-O'? To guarantee you won't omit something important, list all the options. If we were to try to guess the arguments, we would probably guess wrong and then we would not encounter the bug. * The type of machine you are using, and the operating system name and version number. (Much of this information is printed by `g77 -v'--if you include that, send along any additional info you have that you don't see clearly represented in that output.) * The operands you gave to the `configure' command when you installed the compiler. * A complete list of any modifications you have made to the compiler source. (We don't promise to investigate the bug unless it happens in an unmodified compiler. But if you've made modifications and don't tell us, then you are sending us on a wild-goose chase.) Be precise about these changes. A description in English is not enough--send a context diff for them. Adding files of your own (such as a machine description for a machine we don't support) is a modification of the compiler source. * Details of any other deviations from the standard procedure for installing GNU Fortran. * A description of what behavior you observe that you believe is incorrect. For example, "The compiler gets a fatal signal," or, "The assembler instruction at line 208 in the output is incorrect." Of course, if the bug is that the compiler gets a fatal signal, then one can't miss it. But if the bug is incorrect output, the maintainer might not notice unless it is glaringly wrong. None of us has time to study all the assembler code from a 50-line Fortran program just on the chance that one instruction might be wrong. We need *you* to do this part! Even if the problem you experience is a fatal signal, you should still say so explicitly. Suppose something strange is going on, such as, your copy of the compiler is out of synch, or you have encountered a bug in the C library on your system. (This has happened!) Your copy might crash and the copy here would not. If you said to expect a crash, then when the compiler here fails to crash, we would know that the bug was not happening. If you don't say to expect a crash, then we would not know whether the bug was happening. We would not be able to draw any conclusion from our observations. If the problem is a diagnostic when building GNU Fortran with some other compiler, say whether it is a warning or an error. Often the observed symptom is incorrect output when your program is run. Sad to say, this is not enough information unless the program is short and simple. None of us has time to study a large program to figure out how it would work if compiled correctly, much less which line of it was compiled wrong. So you will have to do that. Tell us which source line it is, and what incorrect result happens when that line is executed. A person who understands the program can find this as easily as finding a bug in the program itself. * If you send examples of assembler code output from GNU Fortran, please use `-g' when you make them. The debugging information includes source line numbers which are essential for correlating the output with the input. * If you wish to mention something in the GNU Fortran source, refer to it by context, not by line number. The line numbers in the development sources don't match those in your sources. Your line numbers would convey no convenient information to the maintainers. * Additional information from a debugger might enable someone to find a problem on a machine which he does not have available. However, you need to think when you collect this information if you want it to have any chance of being useful. For example, many people send just a backtrace, but that is never useful by itself. A simple backtrace with arguments conveys little about GNU Fortran because the compiler is largely data-driven; the same functions are called over and over for different RTL insns, doing different things depending on the details of the insn. Most of the arguments listed in the backtrace are useless because they are pointers to RTL list structure. The numeric values of the pointers, which the debugger prints in the backtrace, have no significance whatever; all that matters is the contents of the objects they point to (and most of the contents are other such pointers). In addition, most compiler passes consist of one or more loops that scan the RTL insn sequence. The most vital piece of information about such a loop--which insn it has reached--is usually in a local variable, not in an argument. What you need to provide in addition to a backtrace are the values of the local variables for several stack frames up. When a local variable or an argument is an RTX, first print its value and then use the GDB command `pr' to print the RTL expression that it points to. (If GDB doesn't run on your machine, use your debugger to call the function `debug_rtx' with the RTX as an argument.) In general, whenever a variable is a pointer, its value is no use without the data it points to. Here are some things that are not necessary: * A description of the envelope of the bug. Often people who encounter a bug spend a lot of time investigating which changes to the input file will make the bug go away and which changes will not affect it. This is often time consuming and not very useful, because the way we will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. You might as well save your time for something else. Of course, if you can find a simpler example to report *instead* of the original one, that is a convenience. Errors in the output will be easier to spot, running under the debugger will take less time, etc. Most GNU Fortran bugs involve just one function, so the most straightforward way to simplify an example is to delete all the function definitions except the one where the bug occurs. Those earlier in the file may be replaced by external declarations if the crucial function depends on them. (Exception: inline functions might affect compilation of functions defined later in the file.) However, simplification is not vital; if you don't want to do this, report the bug anyway and send the entire test case you used. * In particular, some people insert conditionals `#ifdef BUG' around a statement which, if removed, makes the bug not happen. These are just clutter; we won't pay any attention to them anyway. Besides, you should send us preprocessor output, and that can't have conditionals. * A patch for the bug. A patch for the bug is useful if it is a good one. But don't omit the necessary information, such as the test case, on the assumption that a patch is all we need. We might see problems with your patch and decide to fix the problem another way, or we might not understand it at all. Sometimes with a program as complicated as GNU Fortran it is very hard to construct an example that will make the program follow a certain path through the code. If you don't send the example, we won't be able to construct one, so we won't be able to verify that the bug is fixed. And if we can't understand what bug you are trying to fix, or why your patch should be an improvement, we won't install it. A test case will help us to understand. *Note Sending Patches::, for guidelines on how to make it easy for us to understand and install your patches. * A guess about what the bug is or what it depends on. Such guesses are usually wrong. Even the maintainer can't guess right about such things without first using the debugger to find the facts. * A core dump file. We have no way of examining a core dump for your type of machine unless we have an identical system--and if we do have one, we should be able to reproduce the crash ourselves. File: g77.info, Node: Sending Patches, Prev: Bug Reporting, Up: Bugs Sending Patches for GNU Fortran =============================== If you would like to write bug fixes or improvements for the GNU Fortran compiler, that is very helpful. Send suggested fixes to the bug report mailing list, <gcc-bugs@gcc.gnu.org or bug-gcc@gnu.org>. Please follow these guidelines so we can study your patches efficiently. If you don't follow these guidelines, your information might still be useful, but using it will take extra work. Maintaining GNU Fortran is a lot of work in the best of circumstances, and we can't keep up unless you do your best to help. * Send an explanation with your changes of what problem they fix or what improvement they bring about. For a bug fix, just include a copy of the bug report, and explain why the change fixes the bug. (Referring to a bug report is not as good as including it, because then we will have to look it up, and we have probably already deleted it if we've already fixed the bug.) * Always include a proper bug report for the problem you think you have fixed. We need to convince ourselves that the change is right before installing it. Even if it is right, we might have trouble judging it if we don't have a way to reproduce the problem. * Include all the comments that are appropriate to help people reading the source in the future understand why this change was needed. * Don't mix together changes made for different reasons. Send them *individually*. If you make two changes for separate reasons, then we might not want to install them both. We might want to install just one. If you send them all jumbled together in a single set of diffs, we have to do extra work to disentangle them--to figure out which parts of the change serve which purpose. If we don't have time for this, we might have to ignore your changes entirely. If you send each change as soon as you have written it, with its own explanation, then the two changes never get tangled up, and we can consider each one properly without any extra work to disentangle them. Ideally, each change you send should be impossible to subdivide into parts that we might want to consider separately, because each of its parts gets its motivation from the other parts. * Send each change as soon as that change is finished. Sometimes people think they are helping us by accumulating many changes to send them all together. As explained above, this is absolutely the worst thing you could do. Since you should send each change separately, you might as well send it right away. That gives us the option of installing it immediately if it is important. * Use `diff -c' to make your diffs. Diffs without context are hard for us to install reliably. More than that, they make it hard for us to study the diffs to decide whether we want to install them. Unidiff format is better than contextless diffs, but not as easy to read as `-c' format. If you have GNU `diff', use `diff -cp', which shows the name of the function that each change occurs in. (The maintainer of GNU Fortran currently uses `diff -rcp2N'.) * Write the change log entries for your changes. We get lots of changes, and we don't have time to do all the change log writing ourselves. Read the `ChangeLog' file to see what sorts of information to put in, and to learn the style that we use. The purpose of the change log is to show people where to find what was changed. So you need to be specific about what functions you changed; in large functions, it's often helpful to indicate where within the function the change was. On the other hand, once you have shown people where to find the change, you need not explain its purpose. Thus, if you add a new function, all you need to say about it is that it is new. If you feel that the purpose needs explaining, it probably does--but the explanation will be much more useful if you put it in comments in the code. If you would like your name to appear in the header line for who made the change, send us the header line. * When you write the fix, keep in mind that we can't install a change that would break other systems. People often suggest fixing a problem by changing machine-independent files such as `toplev.c' to do something special that a particular system needs. Sometimes it is totally obvious that such changes would break GNU Fortran for almost all users. We can't possibly make a change like that. At best it might tell us how to write another patch that would solve the problem acceptably. Sometimes people send fixes that *might* be an improvement in general--but it is hard to be sure of this. It's hard to install such changes because we have to study them very carefully. Of course, a good explanation of the reasoning by which you concluded the change was correct can help convince us. The safest changes are changes to the configuration files for a particular machine. These are safe because they can't create new bugs on other machines. Please help us keep up with the workload by designing the patch in a form that is good to install. File: g77.info, Node: Service, Next: Adding Options, Prev: Bugs, Up: Top How To Get Help with GNU Fortran ******************************** If you need help installing, using or changing GNU Fortran, there are two ways to find it: * Look in the service directory for someone who might help you for a fee. The service directory is found in the file named `SERVICE' in the GNU CC distribution. * Send a message to <gcc@gcc.gnu.org>.