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GNU Info File | 1995-06-16 | 49.1 KB | 1,102 lines

This is Info file gcc.info, produced by Makeinfo-1.55 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995 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. File: gcc.info, Node: Non-bugs, Next: Warnings and Errors, Prev: Protoize Caveats, 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 CC is better without them. * Checking the number and type of arguments to a function which has an old-fashioned definition and no prototype. Such a feature would work only occasionally--only for calls that appear in the same file as the called function, following the definition. The only way to check all calls reliably is to add a prototype for the function. But adding a prototype eliminates the motivation for this feature. So the feature is not worthwhile. * Warning about using an expression whose type is signed as a shift count. Shift count operands are probably signed more often than unsigned. Warning about this would cause far more annoyance than good. * Warning about assigning a signed value to an unsigned variable. Such assignments must be very common; warning about them would cause more annoyance than good. * Warning about unreachable code. It's very common to have unreachable code in machine-generated programs. For example, this happens normally in some files of GNU C itself. * Warning when a non-void function value is ignored. Coming as I do from a Lisp background, I balk at the idea that there is something dangerous about discarding a value. There are functions that return values which some callers may find useful; it makes no sense to clutter the program with a cast to `void' whenever the value isn't useful. * Assuming (for optimization) that the address of an external symbol is never zero. This assumption is false on certain systems when `#pragma weak' is used. * Making `-fshort-enums' the default. This would cause storage layout to be incompatible with most other C compilers. And it doesn't seem very important, given that you can get the same result in other ways. The case where it matters most is when the enumeration-valued object is inside a structure, and in that case you can specify a field width explicitly. * Making bitfields unsigned by default on particular machines where "the ABI standard" says to do so. The ANSI C standard leaves it up to the implementation whether a bitfield declared plain `int' is signed or not. This in effect creates two alternative dialects of C. The GNU C compiler supports both dialects; you can specify the signed dialect with `-fsigned-bitfields' and the unsigned dialect with `-funsigned-bitfields'. However, this leaves open the question of which dialect to use by default. Currently, the preferred dialect makes plain bitfields signed, because this is simplest. Since `int' is the same as `signed int' in every other context, it is cleanest for them to be the same in bitfields as well. Some computer manufacturers have published Application Binary Interface standards which specify that plain bitfields should be unsigned. It is a mistake, however, to say anything about this issue in an ABI. This is because the handling of plain bitfields distinguishes two dialects of C. Both dialects are meaningful on every type of machine. Whether a particular object file was compiled using signed bitfields or unsigned is of no concern to other object files, even if they access the same bitfields in the same data structures. A given program is written in one or the other of these two dialects. The program stands a chance to work on most any machine if it is compiled with the proper dialect. It is unlikely to work at all if compiled with the wrong dialect. Many users appreciate the GNU C compiler because it provides an environment that is uniform across machines. These users would be inconvenienced if the compiler treated plain bitfields 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 C 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 why GNU CC does and will treat plain bitfields in the same fashion on all types of machines (by default). There are some arguments for making bitfields unsigned by default on all machines. If, for example, this becomes a universal de facto standard, it would make sense for GNU CC to go along with it. This is something to be considered in the future. (Of course, users strongly concerned about portability should indicate explicitly in each bitfield whether it is signed or not. In this way, they write programs which have the same meaning in both C dialects.) * Undefining `__STDC__' when `-ansi' is not used. Currently, GNU CC defines `__STDC__' as long as you don't use `-traditional'. This provides good results in practice. Programmers normally use conditionals on `__STDC__' to ask whether it is safe to use certain features of ANSI C, such as function prototypes or ANSI token concatenation. Since plain `gcc' supports all the features of ANSI C, the correct answer to these questions is "yes". Some users try to use `__STDC__' to check for the availability of certain library facilities. This is actually incorrect usage in an ANSI C program, because the ANSI C standard says that a conforming freestanding implementation should define `__STDC__' even though it does not have the library facilities. `gcc -ansi -pedantic' is a conforming freestanding implementation, and it is therefore required to define `__STDC__', even though it does not come with an ANSI C library. Sometimes people say that defining `__STDC__' in a compiler that does not completely conform to the ANSI C standard somehow violates the standard. This is illogical. The standard is a standard for compilers that claim to support ANSI C, such as `gcc -ansi'--not for other compilers such as plain `gcc'. Whatever the ANSI C standard says is relevant to the design of plain `gcc' without `-ansi' only for pragmatic reasons, not as a requirement. * Undefining `__STDC__' in C++. Programs written to compile with C++-to-C translators get the value of `__STDC__' that goes with the C compiler that is subsequently used. These programs must test `__STDC__' to determine what kind of C preprocessor that compiler uses: whether they should concatenate tokens in the ANSI C fashion or in the traditional fashion. These programs work properly with GNU C++ if `__STDC__' is defined. They would not work otherwise. In addition, many header files are written to provide prototypes in ANSI C but not in traditional C. Many of these header files can work without change in C++ provided `__STDC__' is defined. If `__STDC__' is not defined, they will all fail, and will all need to be changed to test explicitly for C++ as well. * Deleting "empty" loops. GNU CC does not delete "empty" loops because the most likely reason you would put one in a program is to have a delay. Deleting them will not make real programs run any faster, so it would be pointless. It would be different if optimization of a nonempty loop could produce an empty one. But this generally can't happen. * Making side effects happen in the same order as in some other compiler. It is never safe to depend on the order of evaluation of side effects. For example, a function call like this may very well behave differently from one compiler to another: void func (int, int); int i = 2; func (i++, i++); There is no guarantee (in either the C or the C++ standard language definitions) that the increments will be evaluated in any particular order. Either increment might happen first. `func' might get the arguments `2, 3', or it might get `3, 2', or even `2, 2'. * Not allowing structures with volatile fields in registers. Strictly speaking, there is no prohibition in the ANSI C standard against allowing structures with volatile fields in registers, but it does not seem to make any sense and is probably not what you wanted to do. So the compiler will give an error message in this case. File: gcc.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 CC reports errors with the source file name and line number where the problem is apparent. *Warnings* report other unusual conditions in your code that *may* indicate a problem, although compilation can (and does) proceed. Warning messages also report the source file name and line number, but include the text `warning:' to distinguish them from error messages. Warnings may 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 C or C++. 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). GNU CC always tries to compile your program if possible; it never gratuitously rejects a program whose meaning is clear merely because (for instance) it fails to conform to a standard. In some cases, however, the C and C++ standards specify that certain extensions are forbidden, and a diagnostic *must* be issued by a conforming compiler. The `-pedantic' option tells GNU CC to issue warnings in such cases; `-pedantic-errors' says to make them errors instead. This does not mean that *all* non-ANSI constructs get warnings or errors. *Note Options to Request or Suppress Warnings: Warning Options, for more detail on these and related command-line options. File: gcc.info, Node: Bugs, Next: Service, Prev: Trouble, Up: Top Reporting Bugs ************** Your bug reports play an essential role in making GNU CC 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 may help you by bringing a solution to your problem, or it may 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 CC work better. Bug reports are your contribution to the maintenance of GNU CC. 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 CC. * Known: Trouble. Known problems. * Help: Service. Where to ask for help. File: gcc.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. * If the compiler produces invalid assembly code, for any input whatever (except an `asm' statement), 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 may have run into an incompatibility between GNU C and traditional C (*note Incompatibilities::.). These incompatibilities might be considered bugs, but they are inescapable consequences of valuable features. Or you may have a program whose behavior is undefined, which happened by chance to give the desired results with another C or C++ compiler. For example, in many nonoptimizing compilers, you can write `x;' at the end of a function instead of `return x;', with the same results. But the value of the function is undefined if `return' is omitted; it is not a bug when GNU CC produces different results. Problems often result from expressions with two increment operators, as in `f (*p++, *p++)'. Your previous compiler might have interpreted that expression the way you intended; GNU CC might interpret it another way. Neither compiler is wrong. The bug is in your code. 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. * 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 my idea of "an extension" or "support for traditional practice". * If you are an experienced user of C or C++ compilers, your suggestions for improvement of GNU CC or GNU C++ are welcome in any case. File: gcc.info, Node: Bug Lists, Next: Bug Reporting, Prev: Bug Criteria, Up: Bugs Where to Report Bugs ==================== Send bug reports for GNU C to `bug-gcc@prep.ai.mit.edu'. Send bug reports for GNU C++ to `bug-g++@prep.ai.mit.edu'. If your bug involves the C++ class library libg++, send mail to `bug-lib-g++@prep.ai.mit.edu'. If you're not sure, you can send the bug report to both lists. *Do not send bug reports to `help-gcc@prep.ai.mit.edu' or to the newsgroup `gnu.gcc.help'.* Most users of GNU CC do not want to receive bug reports. Those that do, have asked to be on `bug-gcc' and/or `bug-g++'. The mailing lists `bug-gcc' and `bug-g++' both have newsgroups which serve as repeaters: `gnu.gcc.bug' and `gnu.g++.bug'. Each mailing list and its newsgroup carry exactly the same messages. Often people think of posting bug reports to the newsgroup instead of mailing them. This 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 may 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 675 Mass Ave Cambridge, MA 02139 File: gcc.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 basically useless. We respond by asking for enough details to enable us to investigate. You might as well expedite matters by sending them to begin with. 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. To enable someone to investigate the bug, you should include all these things: * The version of GNU CC. You can get this by running it with the `-v' option. Without this, we won't know whether there is any point in looking for the bug in the current version of GNU CC. * A complete input file that will reproduce the bug. If the bug is in the C preprocessor, send a source file and any header files that it requires. If the bug is in the compiler proper (`cc1'), run your source file through the C preprocessor by doing `gcc -E SOURCEFILE > OUTFILE', then include the contents of OUTFILE in the bug report. (When you do this, use the same `-I', `-D' or `-U' options 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; and 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 frequently depend on every little detail of the function they happen in. Even if the input file that fails comes from a GNU program, you should still send the complete test case. Don't ask the GNU CC maintainers to do the extra work of obtaining the program in question--they are all overworked as it is. Also, the problem may depend on what is in the header files on your system; it is unreliable for the GNU CC maintainers to try the problem with the header files available to them. By sending CPP output, you can eliminate this source of uncertainty and save us a certain percentage of wild goose chases. * The command arguments you gave GNU CC or GNU C++ 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. * 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 CC. * 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 C 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 compiling GNU CC 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 CC or GNU C++, 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 CC 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 useful 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 CC 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 CC 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 may 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 cpp 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 CC 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 I 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: gcc.info, Node: Sending Patches, Prev: Bug Reporting, Up: Bugs Sending Patches for GNU CC ========================== If you would like to write bug fixes or improvements for the GNU C compiler, that is very helpful. Send suggested fixes to the bug report mailing list, `bug-gcc@prep.ai.mit.edu'. 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 C 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. * 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 CC 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: gcc.info, Node: Service, Next: VMS, Prev: Bugs, Up: Top How To Get Help with GNU CC *************************** If you need help installing, using or changing GNU CC, there are two ways to find it: * Send a message to a suitable network mailing list. First try `bug-gcc@prep.ai.mit.edu', and if that brings no response, try `help-gcc@prep.ai.mit.edu'. * 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. File: gcc.info, Node: VMS, Next: Portability, Prev: Service, Up: Top Using GNU CC on VMS ******************* Here is how to use GNU CC on VMS. * Menu: * Include Files and VMS:: Where the preprocessor looks for the include files. * Global Declarations:: How to do globaldef, globalref and globalvalue with GNU CC. * VMS Misc:: Misc information. File: gcc.info, Node: Include Files and VMS, Next: Global Declarations, Up: VMS Include Files and VMS ===================== Due to the differences between the filesystems of Unix and VMS, GNU CC attempts to translate file names in `#include' into names that VMS will understand. The basic strategy is to prepend a prefix to the specification of the include file, convert the whole filename to a VMS filename, and then try to open the file. GNU CC tries various prefixes one by one until one of them succeeds: 1. The first prefix is the `GNU_CC_INCLUDE:' logical name: this is where GNU C header files are traditionally stored. If you wish to store header files in non-standard locations, then you can assign the logical `GNU_CC_INCLUDE' to be a search list, where each element of the list is suitable for use with a rooted logical. 2. The next prefix tried is `SYS$SYSROOT:[SYSLIB.]'. This is where VAX-C header files are traditionally stored. 3. If the include file specification by itself is a valid VMS filename, the preprocessor then uses this name with no prefix in an attempt to open the include file. 4. If the file specification is not a valid VMS filename (i.e. does not contain a device or a directory specifier, and contains a `/' character), the preprocessor tries to convert it from Unix syntax to VMS syntax. Conversion works like this: the first directory name becomes a device, and the rest of the directories are converted into VMS-format directory names. For example, the name `X11/foobar.h' is translated to `X11:[000000]foobar.h' or `X11:foobar.h', whichever one can be opened. This strategy allows you to assign a logical name to point to the actual location of the header files. 5. If none of these strategies succeeds, the `#include' fails. Include directives of the form: #include foobar are a common source of incompatibility between VAX-C and GNU CC. VAX-C treats this much like a standard `#include <foobar.h>' directive. That is incompatible with the ANSI C behavior implemented by GNU CC: to expand the name `foobar' as a macro. Macro expansion should eventually yield one of the two standard formats for `#include': #include "FILE" #include <FILE> If you have this problem, the best solution is to modify the source to convert the `#include' directives to one of the two standard forms. That will work with either compiler. If you want a quick and dirty fix, define the file names as macros with the proper expansion, like this: #define stdio <stdio.h> This will work, as long as the name doesn't conflict with anything else in the program. Another source of incompatibility is that VAX-C assumes that: #include "foobar" is actually asking for the file `foobar.h'. GNU CC does not make this assumption, and instead takes what you ask for literally; it tries to read the file `foobar'. The best way to avoid this problem is to always specify the desired file extension in your include directives. GNU CC for VMS is distributed with a set of include files that is sufficient to compile most general purpose programs. Even though the GNU CC distribution does not contain header files to define constants and structures for some VMS system-specific functions, there is no reason why you cannot use GNU CC with any of these functions. You first may have to generate or create header files, either by using the public domain utility `UNSDL' (which can be found on a DECUS tape), or by extracting the relevant modules from one of the system macro libraries, and using an editor to construct a C header file. A `#include' file name cannot contain a DECNET node name. The preprocessor reports an I/O error if you attempt to use a node name, whether explicitly, or implicitly via a logical name. File: gcc.info, Node: Global Declarations, Next: VMS Misc, Prev: Include Files and VMS, Up: VMS Global Declarations and VMS =========================== GNU CC does not provide the `globalref', `globaldef' and `globalvalue' keywords of VAX-C. You can get the same effect with an obscure feature of GAS, the GNU assembler. (This requires GAS version 1.39 or later.) The following macros allow you to use this feature in a fairly natural way: #ifdef __GNUC__ #define GLOBALREF(TYPE,NAME) \ TYPE NAME \ asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) #define GLOBALDEF(TYPE,NAME,VALUE) \ TYPE NAME \ asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) \ = VALUE #define GLOBALVALUEREF(TYPE,NAME) \ const TYPE NAME[1] \ asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) #define GLOBALVALUEDEF(TYPE,NAME,VALUE) \ const TYPE NAME[1] \ asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) \ = {VALUE} #else #define GLOBALREF(TYPE,NAME) \ globalref TYPE NAME #define GLOBALDEF(TYPE,NAME,VALUE) \ globaldef TYPE NAME = VALUE #define GLOBALVALUEDEF(TYPE,NAME,VALUE) \ globalvalue TYPE NAME = VALUE #define GLOBALVALUEREF(TYPE,NAME) \ globalvalue TYPE NAME #endif (The `_$$PsectAttributes_GLOBALSYMBOL' prefix at the start of the name is removed by the assembler, after it has modified the attributes of the symbol). These macros are provided in the VMS binaries distribution in a header file `GNU_HACKS.H'. An example of the usage is: GLOBALREF (int, ijk); GLOBALDEF (int, jkl, 0); The macros `GLOBALREF' and `GLOBALDEF' cannot be used straightforwardly for arrays, since there is no way to insert the array dimension into the declaration at the right place. However, you can declare an array with these macros if you first define a typedef for the array type, like this: typedef int intvector[10]; GLOBALREF (intvector, foo); Array and structure initializers will also break the macros; you can define the initializer to be a macro of its own, or you can expand the `GLOBALDEF' macro by hand. You may find a case where you wish to use the `GLOBALDEF' macro with a large array, but you are not interested in explicitly initializing each element of the array. In such cases you can use an initializer like: `{0,}', which will initialize the entire array to `0'. A shortcoming of this implementation is that a variable declared with `GLOBALVALUEREF' or `GLOBALVALUEDEF' is always an array. For example, the declaration: GLOBALVALUEREF(int, ijk); declares the variable `ijk' as an array of type `int [1]'. This is done because a globalvalue is actually a constant; its "value" is what the linker would normally consider an address. That is not how an integer value works in C, but it is how an array works. So treating the symbol as an array name gives consistent results--with the exception that the value seems to have the wrong type. *Don't try to access an element of the array.* It doesn't have any elements. The array "address" may not be the address of actual storage. The fact that the symbol is an array may lead to warnings where the variable is used. Insert type casts to avoid the warnings. Here is an example; it takes advantage of the ANSI C feature allowing macros that expand to use the same name as the macro itself. GLOBALVALUEREF (int, ss$_normal); GLOBALVALUEDEF (int, xyzzy,123); #ifdef __GNUC__ #define ss$_normal ((int) ss$_normal) #define xyzzy ((int) xyzzy) #endif Don't use `globaldef' or `globalref' with a variable whose type is an enumeration type; this is not implemented. Instead, make the variable an integer, and use a `globalvaluedef' for each of the enumeration values. An example of this would be: #ifdef __GNUC__ GLOBALDEF (int, color, 0); GLOBALVALUEDEF (int, RED, 0); GLOBALVALUEDEF (int, BLUE, 1); GLOBALVALUEDEF (int, GREEN, 3); #else enum globaldef color {RED, BLUE, GREEN = 3}; #endif File: gcc.info, Node: VMS Misc, Prev: Global Declarations, Up: VMS Other VMS Issues ================ GNU CC automatically arranges for `main' to return 1 by default if you fail to specify an explicit return value. This will be interpreted by VMS as a status code indicating a normal successful completion. Version 1 of GNU CC did not provide this default. GNU CC on VMS works only with the GNU assembler, GAS. You need version 1.37 or later of GAS in order to produce value debugging information for the VMS debugger. Use the ordinary VMS linker with the object files produced by GAS. Under previous versions of GNU CC, the generated code would occasionally give strange results when linked to the sharable `VAXCRTL' library. Now this should work. A caveat for use of `const' global variables: the `const' modifier must be specified in every external declaration of the variable in all of the source files that use that variable. Otherwise the linker will issue warnings about conflicting attributes for the variable. Your program will still work despite the warnings, but the variable will be placed in writable storage. Although the VMS linker does distinguish between upper and lower case letters in global symbols, most VMS compilers convert all such symbols into upper case and most run-time library routines also have upper case names. To be able to reliably call such routines, GNU CC (by means of the assembler GAS) converts global symbols into upper case like other VMS compilers. However, since the usual practice in C is to distinguish case, GNU CC (via GAS) tries to preserve usual C behavior by augmenting each name that is not all lower case. This means truncating the name to at most 23 characters and then adding more characters at the end which encode the case pattern of those 23. Names which contain at least one dollar sign are an exception; they are converted directly into upper case without augmentation. Name augmentation yields bad results for programs that use precompiled libraries (such as Xlib) which were generated by another compiler. You can use the compiler option `/NOCASE_HACK' to inhibit augmentation; it makes external C functions and variables case-independent as is usual on VMS. Alternatively, you could write all references to the functions and variables in such libraries using lower case; this will work on VMS, but is not portable to other systems. The compiler option `/NAMES' also provides control over global name handling. Function and variable names are handled somewhat differently with GNU C++. The GNU C++ compiler performs "name mangling" on function names, which means that it adds information to the function name to describe the data types of the arguments that the function takes. One result of this is that the name of a function can become very long. Since the VMS linker only recognizes the first 31 characters in a name, special action is taken to ensure that each function and variable has a unique name that can be represented in 31 characters. If the name (plus a name augmentation, if required) is less than 32 characters in length, then no special action is performed. If the name is longer than 31 characters, the assembler (GAS) will generate a hash string based upon the function name, truncate the function name to 23 characters, and append the hash string to the truncated name. If the `/VERBOSE' compiler option is used, the assembler will print both the full and truncated names of each symbol that is truncated. The `/NOCASE_HACK' compiler option should not be used when you are compiling programs that use libg++. libg++ has several instances of objects (i.e. `Filebuf' and `filebuf') which become indistinguishable in a case-insensitive environment. This leads to cases where you need to inhibit augmentation selectively (if you were using libg++ and Xlib in the same program, for example). There is no special feature for doing this, but you can get the result by defining a macro for each mixed case symbol for which you wish to inhibit augmentation. The macro should expand into the lower case equivalent of itself. For example: #define StuDlyCapS studlycaps These macro definitions can be placed in a header file to minimize the number of changes to your source code. File: gcc.info, Node: Portability, Next: Interface, Prev: VMS, Up: Top GNU CC and Portability ********************** The main goal of GNU CC was to make a good, fast compiler for machines in the class that the GNU system aims to run on: 32-bit machines that address 8-bit bytes and have several general registers. Elegance, theoretical power and simplicity are only secondary. GNU CC gets most of the information about the target machine from a machine description which gives an algebraic formula for each of the machine's instructions. This is a very clean way to describe the target. But when the compiler needs information that is difficult to express in this fashion, I have not hesitated to define an ad-hoc parameter to the machine description. The purpose of portability is to reduce the total work needed on the compiler; it was not of interest for its own sake. GNU CC does not contain machine dependent code, but it does contain code that depends on machine parameters such as endianness (whether the most significant byte has the highest or lowest address of the bytes in a word) and the availability of autoincrement addressing. In the RTL-generation pass, it is often necessary to have multiple strategies for generating code for a particular kind of syntax tree, strategies that are usable for different combinations of parameters. Often I have not tried to address all possible cases, but only the common ones or only the ones that I have encountered. As a result, a new target may require additional strategies. You will know if this happens because the compiler will call `abort'. Fortunately, the new strategies can be added in a machine-independent fashion, and will affect only the target machines that need them.