This is Info file f/g77.info, produced by Makeinfo version 1.68 from the input file ./f/g77.texi. This file explains how to use the GNU Fortran system. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1995-1997 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 (). Inspired by a first pass at translating `g77-0.5.16/f/DOC' that was contributed to Craig by David Ronis (). INFO-DIR-SECTION Programming START-INFO-DIR-ENTRY * g77: (g77). The GNU Fortran compiler. END-INFO-DIR-ENTRY  File: g77.info, Node: Actual Bugs, Next: Missing Features, Prev: But-bugs, Up: Trouble Actual Bugs We Haven't Fixed Yet ================================ This section identifies bugs that `g77' *users* might run into. This includes bugs that are actually in the `gcc' back end (GBE) or in `libf2c', because those sets of code are at least somewhat under the control of (and necessarily intertwined with) `g77', so it isn't worth separating them out. For information on bugs that might afflict people who configure, port, build, and install `g77', *Note Problems Installing::. * `g77' sometimes crashes when compiling code containing the construct `CMPLX(0.)' or similar. This is a `gcc' back-end bug. It can be worked around using `-fno-emulate-complex', though that might trigger other, older bugs. Compiling without optimization is another work-around. Fixed in `egcs' 1.1. * Automatic arrays aren't working on HP-UX systems, at least in HP-UX version 10.20. Writing into them apparently causes over-writing of statically declared data in the main program. This probably means the arrays themselves are being under-allocated, or pointers to them being improperly handled, e.g. not passed to other procedures as they should be. * Some Fortran code has been found to be miscompiled by `g77' built on `gcc' version 2.8.1 on m68k-next-nextstep3 configurations when using the `-O2' option. Even a C function is known to miscompile on that configuration when using the `-O2 -funroll-loops' options. Fixed in `egcs'. * A code-generation bug afflicts Intel x86 targets when `-O2' is specified compiling, for example, an old version of the `DNRM2' routine. The x87 coprocessor stack is being mismanaged in cases where assigned `GOTO' and `ASSIGN' are involved. Fixed in `egcs' version 1.1. * `g77' fails to warn about use of a "live" iterative-DO variable as an implied-DO variable in a `WRITE' or `PRINT' statement (although it does warn about this in a `READ' statement). * A compiler crash, or apparently infinite run time, can result when compiling complicated expressions involving `COMPLEX' arithmetic (especially multiplication). Fixed in `egcs' version 1.1. * Something about `g77''s straightforward handling of label references and definitions sometimes prevents the GBE from unrolling loops. Until this is solved, try inserting or removing `CONTINUE' statements as the terminal statement, using the `END DO' form instead, and so on. * Some confusion in diagnostics concerning failing `INCLUDE' statements from within `INCLUDE''d or `#include''d files. * `g77' assumes that `INTEGER(KIND=1)' constants range from `-2**31' to `2**31-1' (the range for two's-complement 32-bit values), instead of determining their range from the actual range of the type for the configuration (and, someday, for the constant). Further, it generally doesn't implement the handling of constants very well in that it makes assumptions about the configuration that it no longer makes regarding variables (types). Included with this item is the fact that `g77' doesn't recognize that, on IEEE-754/854-compliant systems, `0./0.' should produce a NaN and no warning instead of the value `0.' and a warning. This is to be fixed in version 0.6, when `g77' will use the `gcc' back end's constant-handling mechanisms to replace its own. * `g77' uses way too much memory and CPU time to process large aggregate areas having any initialized elements. For example, `REAL A(1000000)' followed by `DATA A(1)/1/' takes up way too much time and space, including the size of the generated assembler file. This is to be mitigated somewhat in version 0.6. Version 0.5.18 improves cases like this--specifically, cases of *sparse* initialization that leave large, contiguous areas uninitialized--significantly. However, even with the improvements, these cases still require too much memory and CPU time. (Version 0.5.18 also improves cases where the initial values are zero to a much greater degree, so if the above example ends with `DATA A(1)/0/', the compile-time performance will be about as good as it will ever get, aside from unrelated improvements to the compiler.) Note that `g77' does display a warning message to notify the user before the compiler appears to hang. *Note Initialization of Large Aggregate Areas: Large Initialization, for information on how to change the point at which `g77' decides to issue this warning. * `g77' doesn't emit variable and array members of common blocks for use with a debugger (the `-g' command-line option). The code is present to do this, but doesn't work with at least one debug format--perhaps it works with others. And it turns out there's a similar bug for local equivalence areas, so that has been disabled as well. As of Version 0.5.19, a temporary kludge solution is provided whereby some rudimentary information on a member is written as a string that is the member's value as a character string. *Note Options for Code Generation Conventions: Code Gen Options, for information on the `-fdebug-kludge' option. * When debugging, after starting up the debugger but before being able to see the source code for the main program unit, the user must currently set a breakpoint at `MAIN__' (or `MAIN___' or `MAIN_' if `MAIN__' doesn't exist) and run the program until it hits the breakpoint. At that point, the main program unit is activated and about to execute its first executable statement, but that's the state in which the debugger should start up, as is the case for languages like C. * Debugging `g77'-compiled code using debuggers other than `gdb' is likely not to work. Getting `g77' and `gdb' to work together is a known problem--getting `g77' to work properly with other debuggers, for which source code often is unavailable to `g77' developers, seems like a much larger, unknown problem, and is a lower priority than making `g77' and `gdb' work together properly. On the other hand, information about problems other debuggers have with `g77' output might make it easier to properly fix `g77', and perhaps even improve `gdb', so it is definitely welcome. Such information might even lead to all relevant products working together properly sooner. * `g77' doesn't work perfectly on 64-bit configurations such as the Digital Semiconductor ("DEC") Alpha. This problem is largely resolved as of version 0.5.23. Version 0.6 should solve most or all remaining problems (such as cross-compiling involving 64-bit machines). * Maintainers of gcc report that the back end definitely has "broken" support for `COMPLEX' types. Based on their input, it seems many of the problems affect only the more-general facilities for gcc's `__complex__' type, such as `__complex__ int' (where the real and imaginary parts are integers) that GNU Fortran does not use. Version 0.5.20 of `g77' works around this problem by not using the back end's support for `COMPLEX'. The new option `-fno-emulate-complex' avoids the work-around, reverting to using the same "broken" mechanism as that used by versions of `g77' prior to 0.5.20. * `g77' sometimes produces invalid assembler code when using the `-fPIC' option (such as compiling for ELF targets) on the Intel x86 architecture target. The symptom is that the assembler complains about invalid opcodes. This bug is in the gcc back end. Fixed in `egcs' version 1.0.2. * `g77' currently inserts needless padding for things like `COMMON A,IPAD' where `A' is `CHARACTER*1' and `IPAD' is `INTEGER(KIND=1)' on machines like x86, because the back end insists that `IPAD' be aligned to a 4-byte boundary, but the processor has no such requirement (though it is usually good for performance). The `gcc' back end needs to provide a wider array of specifications of alignment requirements and preferences for targets, and front ends like `g77' should take advantage of this when it becomes available. * The x86 target's `-malign-double' option no longer reliably aligns double-precision variables and arrays when they are placed in the stack frame. This can significantly reduce the performance of some applications, even on a run-to-run basis (that is, performance measurements can vary fairly widely depending on whether frequently used variables are properly aligned, and that can change from one program run to the next, even from one procedure call to the next). Versions 0.5.22 and earlier of `g77' included a patch to `gcc' that enabled this, but that patch has been deemed an improper (probably buggy) one for version 2.8 of `gcc' and for `egcs'. Note that version 1.1 of `egcs' aligns double-precision variables and arrays when they are in static storage even if `-malign-double' is not specified. There is ongoing investigation into how to make `-malign-double' work properly, also into how to make it unnecessary to get all double-precision variables and arrays aligned when such alignment would not violate the relevant specifications for processor and inter-procedural interfaces. For a suite of programs to test double-precision alignment, see `ftp://alpha.gnu.org/gnu/g77/align/'. * The `libf2c' routines that perform some run-time arithmetic on `COMPLEX' operands were modified circa version 0.5.20 of `g77' to work properly even in the presence of aliased operands. While the `g77' and `netlib' versions of `libf2c' differ on how this is accomplished, the main differences are that we believe the `g77' version works properly even in the presence of *partially* aliased operands. However, these modifications have reduced performance on targets such as x86, due to the extra copies of operands involved.  File: g77.info, Node: Missing Features, Next: Disappointments, Prev: Actual Bugs, Up: Trouble Missing Features ================ This section lists features we know are missing from `g77', and which we want to add someday. (There is no priority implied in the ordering below.) * Menu: GNU Fortran language: * Better Source Model:: * Fortran 90 Support:: * Intrinsics in PARAMETER Statements:: * SELECT CASE on CHARACTER Type:: * RECURSIVE Keyword:: * Popular Non-standard Types:: * Full Support for Compiler Types:: * Array Bounds Expressions:: * POINTER Statements:: * Sensible Non-standard Constructs:: * FLUSH Statement:: * Expressions in FORMAT Statements:: * Explicit Assembler Code:: * Q Edit Descriptor:: GNU Fortran dialects: * Old-style PARAMETER Statements:: * TYPE and ACCEPT I/O Statements:: * STRUCTURE UNION RECORD MAP:: * OPEN CLOSE and INQUIRE Keywords:: * ENCODE and DECODE:: * Suppressing Space Padding:: * Fortran Preprocessor:: * Bit Operations on Floating-point Data:: New facilities: * POSIX Standard:: * Floating-point Exception Handling:: * Nonportable Conversions:: * Large Automatic Arrays:: * Support for Threads:: * Increasing Precision/Range:: Better diagnostics: * Gracefully Handle Sensible Bad Code:: * Non-standard Conversions:: * Non-standard Intrinsics:: * Modifying DO Variable:: * Better Pedantic Compilation:: * Warn About Implicit Conversions:: * Invalid Use of Hollerith Constant:: * Dummy Array Without Dimensioning Dummy:: * Invalid FORMAT Specifiers:: * Ambiguous Dialects:: * Unused Labels:: * Informational Messages:: Run-time facilities: * Uninitialized Variables at Run Time:: * Bounds Checking at Run Time:: * Portable Unformatted Files:: Debugging: * Labels Visible to Debugger::  File: g77.info, Node: Better Source Model, Next: Fortran 90 Support, Up: Missing Features Better Source Model ------------------- `g77' needs to provide, as the default source-line model, a "pure visual" mode, where the interpretation of a source program in this mode can be accurately determined by a user looking at a traditionally displayed rendition of the program (assuming the user knows whether the program is fixed or free form). The design should assume the user cannot tell tabs from spaces and cannot see trailing spaces on lines, but has canonical tab stops and, for fixed-form source, has the ability to always know exactly where column 72 is (since the Fortran standard itself requires this for fixed-form source). This would change the default treatment of fixed-form source to not treat lines with tabs as if they were infinitely long--instead, they would end at column 72 just as if the tabs were replaced by spaces in the canonical way. As part of this, provide common alternate models (Digital, `f2c', and so on) via command-line options. This includes allowing arbitrarily long lines for free-form source as well as fixed-form source and providing various limits and diagnostics as appropriate. Also, `g77' should offer, perhaps even default to, warnings when characters beyond the last valid column are anything other than spaces. This would mean code with "sequence numbers" in columns 73 through 80 would be rejected, and there's a lot of that kind of code around, but one of the most frequent bugs encountered by new users is accidentally writing fixed-form source code into and beyond column 73. So, maybe the users of old code would be able to more easily handle having to specify, say, a `-Wno-col73to80' option.  File: g77.info, Node: Fortran 90 Support, Next: Intrinsics in PARAMETER Statements, Prev: Better Source Model, Up: Missing Features Fortran 90 Support ------------------ `g77' does not support many of the features that distinguish Fortran 90 (and, now, Fortran 95) from ANSI FORTRAN 77. Some Fortran 90 features are supported, because they make sense to offer even to die-hard users of F77. For example, many of them codify various ways F77 has been extended to meet users' needs during its tenure, so `g77' might as well offer them as the primary way to meet those same needs, even if it offers compatibility with one or more of the ways those needs were met by other F77 compilers in the industry. Still, many important F90 features are not supported, because no attempt has been made to research each and every feature and assess its viability in `g77'. In the meantime, users who need those features must use Fortran 90 compilers anyway, and the best approach to adding some F90 features to GNU Fortran might well be to fund a comprehensive project to create GNU Fortran 95.  File: g77.info, Node: Intrinsics in PARAMETER Statements, Next: SELECT CASE on CHARACTER Type, Prev: Fortran 90 Support, Up: Missing Features Intrinsics in `PARAMETER' Statements ------------------------------------ `g77' doesn't allow intrinsics in `PARAMETER' statements. This feature is considered to be absolutely vital, even though it is not standard-conforming, and is scheduled for version 0.6. Related to this, `g77' doesn't allow non-integral exponentiation in `PARAMETER' statements, such as `PARAMETER (R=2**.25)'. It is unlikely `g77' will ever support this feature, as doing it properly requires complete emulation of a target computer's floating-point facilities when building `g77' as a cross-compiler. But, if the `gcc' back end is enhanced to provide such a facility, `g77' will likely use that facility in implementing this feature soon afterwards.  File: g77.info, Node: SELECT CASE on CHARACTER Type, Next: RECURSIVE Keyword, Prev: Intrinsics in PARAMETER Statements, Up: Missing Features `SELECT CASE' on `CHARACTER' Type --------------------------------- Character-type selector/cases for `SELECT CASE' currently are not supported.  File: g77.info, Node: RECURSIVE Keyword, Next: Popular Non-standard Types, Prev: SELECT CASE on CHARACTER Type, Up: Missing Features `RECURSIVE' Keyword ------------------- `g77' doesn't support the `RECURSIVE' keyword that F90 compilers do. Nor does it provide any means for compiling procedures designed to do recursion. All recursive code can be rewritten to not use recursion, but the result is not pretty.  File: g77.info, Node: Increasing Precision/Range, Next: Gracefully Handle Sensible Bad Code, Prev: Support for Threads, Up: Missing Features Increasing Precision/Range -------------------------- Some compilers, such as `f2c', have an option (`-r8' or similar) that provides automatic treatment of `REAL' entities such that they have twice the storage size, and a corresponding increase in the range and precision, of what would normally be the `REAL(KIND=1)' (default `REAL') type. (This affects `COMPLEX' the same way.) They also typically offer another option (`-i8') to increase `INTEGER' entities so they are twice as large (with roughly twice as much range). (There are potential pitfalls in using these options.) `g77' does not yet offer any option that performs these kinds of transformations. Part of the problem is the lack of detailed specifications regarding exactly how these options affect the interpretation of constants, intrinsics, and so on. Until `g77' addresses this need, programmers could improve the portability of their code by modifying it to not require compile-time options to produce correct results. Some free tools are available which may help, specifically in Toolpack (which one would expect to be sound) and the `fortran' section of the Netlib repository. Use of preprocessors can provide a fairly portable means to work around the lack of widely portable methods in the Fortran language itself (though increasing acceptance of Fortran 90 would alleviate this problem).  File: g77.info, Node: Popular Non-standard Types, Next: Full Support for Compiler Types, Prev: RECURSIVE Keyword, Up: Missing Features Popular Non-standard Types -------------------------- `g77' doesn't fully support `INTEGER*2', `LOGICAL*1', and similar. Version 0.6 will provide full support for this very popular set of features. In the meantime, version 0.5.18 provides rudimentary support for them.  File: g77.info, Node: Full Support for Compiler Types, Next: Array Bounds Expressions, Prev: Popular Non-standard Types, Up: Missing Features Full Support for Compiler Types ------------------------------- `g77' doesn't support `INTEGER', `REAL', and `COMPLEX' equivalents for *all* applicable back-end-supported types (`char', `short int', `int', `long int', `long long int', and `long double'). This means providing intrinsic support, and maybe constant support (using F90 syntax) as well, and, for most machines will result in automatic support of `INTEGER*1', `INTEGER*2', `INTEGER*8', maybe even `REAL*16', and so on. This is scheduled for version 0.6.  File: g77.info, Node: Array Bounds Expressions, Next: POINTER Statements, Prev: Full Support for Compiler Types, Up: Missing Features Array Bounds Expressions ------------------------ `g77' doesn't support more general expressions to dimension arrays, such as array element references, function references, etc. For example, `g77' currently does not accept the following: SUBROUTINE X(M, N) INTEGER N(10), M(N(2), N(1))  File: g77.info, Node: POINTER Statements, Next: Sensible Non-standard Constructs, Prev: Array Bounds Expressions, Up: Missing Features POINTER Statements ------------------ `g77' doesn't support pointers or allocatable objects (other than automatic arrays). This set of features is probably considered just behind intrinsics in `PARAMETER' statements on the list of large, important things to add to `g77'. In the meantime, consider using the `INTEGER(KIND=7)' declaration to specify that a variable must be able to hold a pointer. This construct is not portable to other non-GNU compilers, but it is portable to all machines GNU Fortran supports when `g77' is used. *Note Functions and Subroutines::, for information on `%VAL()', `%REF()', and `%DESCR()' constructs, which are useful for passing pointers to procedures written in languages other than Fortran.  File: g77.info, Node: Sensible Non-standard Constructs, Next: FLUSH Statement, Prev: POINTER Statements, Up: Missing Features Sensible Non-standard Constructs -------------------------------- `g77' rejects things other compilers accept, like `INTRINSIC SQRT,SQRT'. As time permits in the future, some of these things that are easy for humans to read and write and unlikely to be intended to mean something else will be accepted by `g77' (though `-fpedantic' should trigger warnings about such non-standard constructs). Until `g77' no longer gratuitously rejects sensible code, you might as well fix your code to be more standard-conforming and portable. The kind of case that is important to except from the recommendation to change your code is one where following good coding rules would force you to write non-standard code that nevertheless has a clear meaning. For example, when writing an `INCLUDE' file that defines a common block, it might be appropriate to include a `SAVE' statement for the common block (such as `SAVE /CBLOCK/'), so that variables defined in the common block retain their values even when all procedures declaring the common block become inactive (return to their callers). However, putting `SAVE' statements in an `INCLUDE' file would prevent otherwise standard-conforming code from also specifying the `SAVE' statement, by itself, to indicate that all local variables and arrays are to have the `SAVE' attribute. For this reason, `g77' already has been changed to allow this combination, because although the general problem of gratuitously rejecting unambiguous and "safe" constructs still exists in `g77', this particular construct was deemed useful enough that it was worth fixing `g77' for just this case. So, while there is no need to change your code to avoid using this particular construct, there might be other, equally appropriate but non-standard constructs, that you shouldn't have to stop using just because `g77' (or any other compiler) gratuitously rejects it. Until the general problem is solved, if you have any such construct you believe is worthwhile using (e.g. not just an arbitrary, redundant specification of an attribute), please submit a bug report with an explanation, so we can consider fixing `g77' just for cases like yours.  File: g77.info, Node: FLUSH Statement, Next: Expressions in FORMAT Statements, Prev: Sensible Non-standard Constructs, Up: Missing Features `FLUSH' Statement ----------------- `g77' could perhaps use a `FLUSH' statement that does what `CALL FLUSH' does, but that supports `*' as the unit designator (same unit as for `PRINT') and accepts `ERR=' and/or `IOSTAT=' specifiers.  File: g77.info, Node: Expressions in FORMAT Statements, Next: Explicit Assembler Code, Prev: FLUSH Statement, Up: Missing Features Expressions in `FORMAT' Statements ---------------------------------- `g77' doesn't support `FORMAT(I)' and the like. Supporting this requires a significant redesign or replacement of `libg2c'. However, `g77' does support this construct when the expression is constant (as of version 0.5.22). For example: PARAMETER (IWIDTH = 12) 10 FORMAT (I) Otherwise, at least for output (`PRINT' and `WRITE'), Fortran code making use of this feature can be rewritten to avoid it by constructing the `FORMAT' string in a `CHARACTER' variable or array, then using that variable or array in place of the `FORMAT' statement label to do the original `PRINT' or `WRITE'. Many uses of this feature on input can be rewritten this way as well, but not all can. For example, this can be rewritten: READ 20, I 20 FORMAT (I) However, this cannot, in general, be rewritten, especially when `ERR=' and `END=' constructs are employed: READ 30, J, I 30 FORMAT (I)  File: g77.info, Node: Explicit Assembler Code, Next: Q Edit Descriptor, Prev: Expressions in FORMAT Statements, Up: Missing Features Explicit Assembler Code ----------------------- `g77' needs to provide some way, a la `gcc', for `g77' code to specify explicit assembler code.  File: g77.info, Node: Q Edit Descriptor, Next: Old-style PARAMETER Statements, Prev: Explicit Assembler Code, Up: Missing Features Q Edit Descriptor ----------------- The `Q' edit descriptor in `FORMAT's isn't supported. (This is meant to get the number of characters remaining in an input record.) Supporting this requires a significant redesign or replacement of `libg2c'. A workaround might be using internal I/O or the stream-based intrinsics. *Note FGetC Intrinsic (subroutine)::.  File: g77.info, Node: Old-style PARAMETER Statements, Next: TYPE and ACCEPT I/O Statements, Prev: Q Edit Descriptor, Up: Missing Features Old-style PARAMETER Statements ------------------------------ `g77' doesn't accept `PARAMETER I=1'. Supporting this obsolete form of the `PARAMETER' statement would not be particularly hard, as most of the parsing code is already in place and working. Until time/money is spent implementing it, you might as well fix your code to use the standard form, `PARAMETER (I=1)' (possibly needing `INTEGER I' preceding the `PARAMETER' statement as well, otherwise, in the obsolete form of `PARAMETER', the type of the variable is set from the type of the constant being assigned to it).  File: g77.info, Node: TYPE and ACCEPT I/O Statements, Next: STRUCTURE UNION RECORD MAP, Prev: Old-style PARAMETER Statements, Up: Missing Features `TYPE' and `ACCEPT' I/O Statements ---------------------------------- `g77' doesn't support the I/O statements `TYPE' and `ACCEPT'. These are common extensions that should be easy to support, but also are fairly easy to work around in user code. Generally, any `TYPE fmt,list' I/O statement can be replaced by `PRINT fmt,list'. And, any `ACCEPT fmt,list' statement can be replaced by `READ fmt,list'.  File: g77.info, Node: STRUCTURE UNION RECORD MAP, Next: OPEN CLOSE and INQUIRE Keywords, Prev: TYPE and ACCEPT I/O Statements, Up: Missing Features `STRUCTURE', `UNION', `RECORD', `MAP' ------------------------------------- `g77' doesn't support `STRUCTURE', `UNION', `RECORD', `MAP'. This set of extensions is quite a bit lower on the list of large, important things to add to `g77', partly because it requires a great deal of work either upgrading or replacing `libg2c'.  File: g77.info, Node: OPEN CLOSE and INQUIRE Keywords, Next: ENCODE and DECODE, Prev: STRUCTURE UNION RECORD MAP, Up: Missing Features `OPEN', `CLOSE', and `INQUIRE' Keywords --------------------------------------- `g77' doesn't have support for keywords such as `DISP='DELETE'' in the `OPEN', `CLOSE', and `INQUIRE' statements. These extensions are easy to add to `g77' itself, but require much more work on `libg2c'. `g77' doesn't support `FORM='PRINT'' or an equivalent to translate the traditional `carriage control' characters in column 1 of output to use backspaces, carriage returns and the like. However programs exist to translate them in output files (or standard output). These are typically called either `fpr' or `asa'. You can get a version of `asa' from `ftp://sunsite.unc.edu/pub/Linux/devel/lang/fortran' for GNU systems which will probably build easily on other systems. Alternatively, `fpr' is in BSD distributions in various archive sites.  File: g77.info, Node: ENCODE and DECODE, Next: Suppressing Space Padding, Prev: OPEN CLOSE and INQUIRE Keywords, Up: Missing Features `ENCODE' and `DECODE' --------------------- `g77' doesn't support `ENCODE' or `DECODE'. These statements are best replaced by READ and WRITE statements involving internal files (CHARACTER variables and arrays). For example, replace a code fragment like INTEGER*1 LINE(80) ... DECODE (80, 9000, LINE) A, B, C ... 9000 FORMAT (1X, 3(F10.5)) with: CHARACTER*80 LINE ... READ (UNIT=LINE, FMT=9000) A, B, C ... 9000 FORMAT (1X, 3(F10.5)) Similarly, replace a code fragment like INTEGER*1 LINE(80) ... ENCODE (80, 9000, LINE) A, B, C ... 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5)) with: CHARACTER*80 LINE ... WRITE (UNIT=LINE, FMT=9000) A, B, C ... 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5)) It is entirely possible that `ENCODE' and `DECODE' will be supported by a future version of `g77'.  File: g77.info, Node: Suppressing Space Padding, Next: Fortran Preprocessor, Prev: ENCODE and DECODE, Up: Missing Features Suppressing Space Padding of Source Lines ----------------------------------------- `g77' should offer VXT-Fortran-style suppression of virtual spaces at the end of a source line if an appropriate command-line option is specified. This affects cases where a character constant is continued onto the next line in a fixed-form source file, as in the following example: 10 PRINT *,'HOW MANY 1 SPACES?' `g77', and many other compilers, virtually extend the continued line through column 72 with spaces that become part of the character constant, but Digital Fortran normally didn't, leaving only one space between `MANY' and `SPACES?' in the output of the above statement. Fairly recently, at least one version of Digital Fortran was enhanced to provide the other behavior when a command-line option is specified, apparently due to demand from readers of the USENET group `comp.lang.fortran' to offer conformance to this widespread practice in the industry. `g77' should return the favor by offering conformance to Digital's approach to handling the above example.  File: g77.info, Node: Fortran Preprocessor, Next: Bit Operations on Floating-point Data, Prev: Suppressing Space Padding, Up: Missing Features Fortran Preprocessor -------------------- `g77' should offer a preprocessor designed specifically for Fortran to replace `cpp -traditional'. There are several out there worth evaluating, at least. Such a preprocessor would recognize Hollerith constants, properly parse comments and character constants, and so on. It might also recognize, process, and thus preprocess files included via the `INCLUDE' directive.  File: g77.info, Node: Bit Operations on Floating-point Data, Next: POSIX Standard, Prev: Fortran Preprocessor, Up: Missing Features Bit Operations on Floating-point Data ------------------------------------- `g77' does not allow `REAL' and other non-integral types for arguments to intrinsics like `AND', `OR', and `SHIFT'. For example, this program is rejected by `g77', because the intrinsic `IAND' does not accept `REAL' arguments: DATA A/7.54/, B/9.112/ PRINT *, IAND(A, B) END  File: g77.info, Node: POSIX Standard, Next: Floating-point Exception Handling, Prev: Bit Operations on Floating-point Data, Up: Missing Features `POSIX' Standard ---------------- `g77' should support the POSIX standard for Fortran.  File: g77.info, Node: Floating-point Exception Handling, Next: Nonportable Conversions, Prev: POSIX Standard, Up: Missing Features Floating-point Exception Handling --------------------------------- The `gcc' backend and, consequently, `g77', currently provides no general control over whether or not floating-point exceptions are trapped or ignored. (Ignoring them typically results in NaN values being propagated in systems that conform to IEEE 754.) The behaviour is normally inherited from the system-dependent startup code, though some targets, such as the Alpha, have code generation options which change the behaviour. Most systems provide some C-callable mechanism to change this; this can be invoked at startup using `gcc''s `constructor' attribute. For example, just compiling and linking the following C code with your program will turn on exception trapping for the "common" exceptions on an x86-based GNU system: #include static void __attribute__ ((constructor)) trapfpe () { __setfpucw (_FPU_DEFAULT & ~(_FPU_MASK_IM | _FPU_MASK_ZM | _FPU_MASK_OM)); } A convenient trick is to compile this something like: gcc -o libtrapfpe.a trapfpe.c and then use it by adding `-trapfpe' to the `g77' command line when linking.  File: g77.info, Node: Nonportable Conversions, Next: Large Automatic Arrays, Prev: Floating-point Exception Handling, Up: Missing Features Nonportable Conversions ----------------------- `g77' doesn't accept some particularly nonportable, silent data-type conversions such as `LOGICAL' to `REAL' (as in `A=.FALSE.', where `A' is type `REAL'), that other compilers might quietly accept. Some of these conversions are accepted by `g77' when the `-fugly' option is specified. Perhaps it should accept more or all of them.  File: g77.info, Node: Large Automatic Arrays, Next: Support for Threads, Prev: Nonportable Conversions, Up: Missing Features Large Automatic Arrays ---------------------- Currently, automatic arrays always are allocated on the stack. For situations where the stack cannot be made large enough, `g77' should offer a compiler option that specifies allocation of automatic arrays in heap storage.  File: g77.info, Node: Support for Threads, Next: Increasing Precision/Range, Prev: Large Automatic Arrays, Up: Missing Features Support for Threads ------------------- Neither the code produced by `g77' nor the `libg2c' library are thread-safe, nor does `g77' have support for parallel processing (other than the instruction-level parallelism available on some processors). A package such as PVM might help here.  File: g77.info, Node: Gracefully Handle Sensible Bad Code, Next: Non-standard Conversions, Prev: Increasing Precision/Range, Up: Missing Features Gracefully Handle Sensible Bad Code ----------------------------------- `g77' generally should continue processing for warnings and recoverable (user) errors whenever possible--that is, it shouldn't gratuitously make bad or useless code. For example: INTRINSIC ZABS CALL FOO(ZABS) END When compiling the above with `-ff2c-intrinsics-disable', `g77' should indeed complain about passing `ZABS', but it still should compile, instead of rejecting the entire `CALL' statement. (Some of this is related to improving the compiler internals to improve how statements are analyzed.)  File: g77.info, Node: Non-standard Conversions, Next: Non-standard Intrinsics, Prev: Gracefully Handle Sensible Bad Code, Up: Missing Features Non-standard Conversions ------------------------ `-Wconversion' and related should flag places where non-standard conversions are found. Perhaps much of this would be part of `-Wugly*'.  File: g77.info, Node: Non-standard Intrinsics, Next: Modifying DO Variable, Prev: Non-standard Conversions, Up: Missing Features Non-standard Intrinsics ----------------------- `g77' needs a new option, like `-Wintrinsics', to warn about use of non-standard intrinsics without explicit `INTRINSIC' statements for them. This would help find code that might fail silently when ported to another compiler.  File: g77.info, Node: Modifying DO Variable, Next: Better Pedantic Compilation, Prev: Non-standard Intrinsics, Up: Missing Features Modifying `DO' Variable ----------------------- `g77' should warn about modifying `DO' variables via `EQUIVALENCE'. (The internal information gathered to produce this warning might also be useful in setting the internal "doiter" flag for a variable or even array reference within a loop, since that might produce faster code someday.) For example, this code is invalid, so `g77' should warn about the invalid assignment to `NOTHER': EQUIVALENCE (I, NOTHER) DO I = 1, 100 IF (I.EQ. 10) NOTHER = 20 END DO  File: g77.info, Node: Better Pedantic Compilation, Next: Warn About Implicit Conversions, Prev: Modifying DO Variable, Up: Missing Features Better Pedantic Compilation --------------------------- `g77' needs to support `-fpedantic' more thoroughly, and use it only to generate warnings instead of rejecting constructs outright. Have it warn: if a variable that dimensions an array is not a dummy or placed explicitly in `COMMON' (F77 does not allow it to be placed in `COMMON' via `EQUIVALENCE'); if specification statements follow statement-function-definition statements; about all sorts of syntactic extensions.  File: g77.info, Node: Warn About Implicit Conversions, Next: Invalid Use of Hollerith Constant, Prev: Better Pedantic Compilation, Up: Missing Features Warn About Implicit Conversions ------------------------------- `g77' needs a `-Wpromotions' option to warn if source code appears to expect automatic, silent, and somewhat dangerous compiler-assisted conversion of `REAL(KIND=1)' constants to `REAL(KIND=2)' based on context. For example, it would warn about cases like this: DOUBLE PRECISION FOO PARAMETER (TZPHI = 9.435784839284958) FOO = TZPHI * 3D0  File: g77.info, Node: Invalid Use of Hollerith Constant, Next: Dummy Array Without Dimensioning Dummy, Prev: Warn About Implicit Conversions, Up: Missing Features Invalid Use of Hollerith Constant --------------------------------- `g77' should disallow statements like `RETURN 2HAB', which are invalid in both source forms (unlike `RETURN (2HAB)', which probably still makes no sense but at least can be reliably parsed). Fixed-form processing rejects it, but not free-form, except in a way that is a bit difficult to understand.  File: g77.info, Node: Dummy Array Without Dimensioning Dummy, Next: Invalid FORMAT Specifiers, Prev: Invalid Use of Hollerith Constant, Up: Missing Features Dummy Array Without Dimensioning Dummy -------------------------------------- `g77' should complain when a list of dummy arguments containing an adjustable dummy array does not also contain every variable listed in the dimension list of the adjustable array. Currently, `g77' does complain about a variable that dimensions an array but doesn't appear in any dummy list or `COMMON' area, but this needs to be extended to catch cases where it doesn't appear in every dummy list that also lists any arrays it dimensions. For example, `g77' should warn about the entry point `ALT' below, since it includes `ARRAY' but not `ISIZE' in its list of arguments: SUBROUTINE PRIMARY(ARRAY, ISIZE) REAL ARRAY(ISIZE) ENTRY ALT(ARRAY)  File: g77.info, Node: Invalid FORMAT Specifiers, Next: Ambiguous Dialects, Prev: Dummy Array Without Dimensioning Dummy, Up: Missing Features Invalid FORMAT Specifiers ------------------------- `g77' should check `FORMAT' specifiers for validity as it does `FORMAT' statements. For example, a diagnostic would be produced for: PRINT 'HI THERE!' !User meant PRINT *, 'HI THERE!'  File: g77.info, Node: Ambiguous Dialects, Next: Unused Labels, Prev: Invalid FORMAT Specifiers, Up: Missing Features Ambiguous Dialects ------------------ `g77' needs a set of options such as `-Wugly*', `-Wautomatic', `-Wvxt', `-Wf90', and so on. These would warn about places in the user's source where ambiguities are found, helpful in resolving ambiguities in the program's dialect or dialects.  File: g77.info, Node: Unused Labels, Next: Informational Messages, Prev: Ambiguous Dialects, Up: Missing Features Unused Labels ------------- `g77' should warn about unused labels when `-Wunused' is in effect.  File: g77.info, Node: Informational Messages, Next: Uninitialized Variables at Run Time, Prev: Unused Labels, Up: Missing Features Informational Messages ---------------------- `g77' needs an option to suppress information messages (notes). `-w' does this but also suppresses warnings. The default should be to suppress info messages. Perhaps info messages should simply be eliminated.  File: g77.info, Node: Uninitialized Variables at Run Time, Next: Bounds Checking at Run Time, Prev: Informational Messages, Up: Missing Features Uninitialized Variables at Run Time ----------------------------------- `g77' needs an option to initialize everything (not otherwise explicitly initialized) to "weird" (machine-dependent) values, e.g. NaNs, bad (non-`NULL') pointers, and largest-magnitude integers, would help track down references to some kinds of uninitialized variables at run time. Note that use of the options `-O -Wuninitialized' can catch many such bugs at compile time.  File: g77.info, Node: Bounds Checking at Run Time, Next: Portable Unformatted Files, Prev: Uninitialized Variables at Run Time, Up: Missing Features Bounds Checking at Run Time --------------------------- `g77' should offer run-time bounds-checking of array/subscript references in a fashion similar to `f2c'. Note that `g77' already warns about references to out-of-bounds elements of arrays when it detects these at compile time.  File: g77.info, Node: Portable Unformatted Files, Next: Labels Visible to Debugger, Prev: Bounds Checking at Run Time, Up: Missing Features Portable Unformatted Files -------------------------- `g77' has no facility for exchanging unformatted files with systems using different number formats--even differing only in endianness (byte order)--or written by other compilers. Some compilers provide facilities at least for doing byte-swapping during unformatted I/O. It is unrealistic to expect to cope with exchanging unformatted files with arbitrary other compiler runtimes, but the `g77' runtime should at least be able to read files written by `g77' on systems with different number formats, particularly if they differ only in byte order. In case you do need to write a program to translate to or from `g77' (`libf2c') unformatted files, they are written as follows: Sequential Unformatted sequential records consist of 1. A number giving the length of the record contents; 2. the length of record contents again (for backspace). The record length is of C type `long'; this means that it is 8 bytes on 64-bit systems such as Alpha GNU/Linux and 4 bytes on other systems, such as x86 GNU/Linux. Consequently such files cannot be exchanged between 64-bit and 32-bit systems, even with the same basic number format. Direct access Unformatted direct access files form a byte stream of length RECORDS*RECL bytes, where RECORDS is the maximum record number (`REC=RECORDS') written and RECL is the record length in bytes specified in the `OPEN' statement (`RECL=RECL'). Data appear in the records as determined by the relevant `WRITE' statement. Dummy records with arbitrary contents appear in the file in place of records which haven't been written. Thus for exchanging a sequential or direct access unformatted file between big- and little-endian 32-bit systems using IEEE 754 floating point it would be sufficient to reverse the bytes in consecutive words in the file *iff* only `REAL*4', `COMPLEX', `INTEGER*4' and/or `LOGICAL*4' data have been written to it by `g77'. If necessary, it is possible to do byte-oriented i/o with `g77''s `FGETC' and `FPUTC' intrinsics. Byte-swapping can be done in Fortran by equivalencing larger sized variables to an `INTEGER*1' array or a set of scalars. If you need to exchange binary data between arbitrary system and compiler variations, we recommend using a portable binary format with Fortran bindings, such as NCSA's HDF (`http://hdf.ncsa.uiuc.edu/') or PACT's PDB(1) (`http://www.llnl.gov/def_sci/pact/pact_homepage.html'). (Unlike, say, CDF or XDR, HDF-like systems write in the native number formats and only incur overhead when they are read on a system with a different format.) A future `g77' runtime library should use such techniques. ---------- Footnotes ---------- (1) No, not *that* one.  File: g77.info, Node: Labels Visible to Debugger, Prev: Portable Unformatted Files, Up: Missing Features Labels Visible to Debugger -------------------------- `g77' should output debugging information for statements labels, for use by debuggers that know how to support them. Same with weirder things like construct names. It is not yet known if any debug formats or debuggers support these.  File: g77.info, Node: Disappointments, Next: Non-bugs, Prev: Missing Features, Up: Trouble Disappointments and Misunderstandings ===================================== These problems are perhaps regrettable, but we don't know any practical way around them for now. * Menu: * Mangling of Names:: `SUBROUTINE FOO' is given external name `foo_'. * Multiple Definitions of External Names:: No doing both `COMMON /FOO/' and `SUBROUTINE FOO'. * Limitation on Implicit Declarations:: No `IMPLICIT CHARACTER*(*)'.  File: g77.info, Node: Mangling of Names, Next: Multiple Definitions of External Names, Up: Disappointments Mangling of Names in Source Code -------------------------------- The current external-interface design, which includes naming of external procedures, COMMON blocks, and the library interface, has various usability problems, including things like adding underscores where not really necessary (and preventing easier inter-language operability) and yet not providing complete namespace freedom for user C code linked with Fortran apps (due to the naming of functions in the library, among other things). Project GNU should at least get all this "right" for systems it fully controls, such as the Hurd, and provide defaults and options for compatibility with existing systems and interoperability with popular existing compilers.  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.