--- gcc/gcc.info-10 2018/04/24 18:10:57 1.1.1.6 +++ gcc/gcc.info-10 2018/04/24 18:41:43 1.1.1.9 @@ -1,12 +1,13 @@ -This is Info file gcc.info, produced by Makeinfo-1.54 from the input -file gcc.texi. +This is Info file gcc.info, produced by Makeinfo version 1.67 from the +input file gcc.texi. This file documents the use and the internals of the GNU compiler. - Published by the Free Software Foundation 675 Massachusetts Avenue -Cambridge, MA 02139 USA + Published by the Free Software Foundation 59 Temple Place - Suite 330 +Boston, MA 02111-1307 USA - Copyright (C) 1988, 1989, 1992, 1993 Free Software Foundation, Inc. + 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 @@ -14,807 +15,855 @@ 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" 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. +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" 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. +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: Bug Reporting, Next: Sending Patches, Prev: Bug Lists, Up: Bugs +File: gcc.info, Node: Asm Labels, Next: Explicit Reg Vars, Prev: Extended Asm, Up: C Extensions -How to Report Bugs -================== +Controlling Names Used in Assembler Code +======================================== - 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. - - 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. + You can specify the name to be used in the assembler code for a C +function or variable by writing the `asm' (or `__asm__') keyword after +the declarator as follows: + + int foo asm ("myfoo") = 2; + +This specifies that the name to be used for the variable `foo' in the +assembler code should be `myfoo' rather than the usual `_foo'. + + On systems where an underscore is normally prepended to the name of +a C function or variable, this feature allows you to define names for +the linker that do not start with an underscore. + + You cannot use `asm' in this way in a function *definition*; but you +can get the same effect by writing a declaration for the function +before its definition and putting `asm' there, like this: + + extern func () asm ("FUNC"); + + func (x, y) + int x, y; + ... + + It is up to you to make sure that the assembler names you choose do +not conflict with any other assembler symbols. Also, you must not use a +register name; that would produce completely invalid assembler code. +GNU CC does not as yet have the ability to store static variables in +registers. Perhaps that will be added.  -File: gcc.info, Node: Sending Patches, Prev: Bug Reporting, Up: Bugs +File: gcc.info, Node: Explicit Reg Vars, Next: Alternate Keywords, Prev: Asm Labels, Up: C Extensions -Sending Patches for GNU CC -========================== +Variables in Specified Registers +================================ - If you would like to write bug fixes or improvements for the GNU C -compiler, that is very helpful. When you send your changes, please -follow these guidelines to avoid causing extra work for us in studying -the patches. - - 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. + GNU C allows you to put a few global variables into specified +hardware registers. You can also specify the register in which an +ordinary register variable should be allocated. + + * Global register variables reserve registers throughout the program. + This may be useful in programs such as programming language + interpreters which have a couple of global variables that are + accessed very often. + + * Local register variables in specific registers do not reserve the + registers. The compiler's data flow analysis is capable of + determining where the specified registers contain live values, and + where they are available for other uses. + + These local variables are sometimes convenient for use with the + extended `asm' feature (*note Extended Asm::.), if you want to + write one output of the assembler instruction directly into a + particular register. (This will work provided the register you + specify fits the constraints specified for that operand in the + `asm'.) + +* Menu: + +* Global Reg Vars:: +* Local Reg Vars::  -File: gcc.info, Node: VMS, Next: Portability, Prev: Service, Up: Top +File: gcc.info, Node: Global Reg Vars, Next: Local Reg Vars, Up: Explicit Reg Vars -Using GNU CC on VMS -******************* +Defining Global Register Variables +---------------------------------- -* Menu: + You can define a global register variable in GNU C like this: + + register int *foo asm ("a5"); -* 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. +Here `a5' is the name of the register which should be used. Choose a +register which is normally saved and restored by function calls on your +machine, so that library routines will not clobber it. + + Naturally the register name is cpu-dependent, so you would need to +conditionalize your program according to cpu type. The register `a5' +would be a good choice on a 68000 for a variable of pointer type. On +machines with register windows, be sure to choose a "global" register +that is not affected magically by the function call mechanism. + + In addition, operating systems on one type of cpu may differ in how +they name the registers; then you would need additional conditionals. +For example, some 68000 operating systems call this register `%a5'. + + Eventually there may be a way of asking the compiler to choose a +register automatically, but first we need to figure out how it should +choose and how to enable you to guide the choice. No solution is +evident. + + Defining a global register variable in a certain register reserves +that register entirely for this use, at least within the current +compilation. The register will not be allocated for any other purpose +in the functions in the current compilation. The register will not be +saved and restored by these functions. Stores into this register are +never deleted even if they would appear to be dead, but references may +be deleted or moved or simplified. + + It is not safe to access the global register variables from signal +handlers, or from more than one thread of control, because the system +library routines may temporarily use the register for other things +(unless you recompile them specially for the task at hand). + + It is not safe for one function that uses a global register variable +to call another such function `foo' by way of a third function `lose' +that was compiled without knowledge of this variable (i.e. in a +different source file in which the variable wasn't declared). This is +because `lose' might save the register and put some other value there. +For example, you can't expect a global register variable to be +available in the comparison-function that you pass to `qsort', since +`qsort' might have put something else in that register. (If you are +prepared to recompile `qsort' with the same global register variable, +you can solve this problem.) + + If you want to recompile `qsort' or other source files which do not +actually use your global register variable, so that they will not use +that register for any other purpose, then it suffices to specify the +compiler option `-ffixed-REG'. You need not actually add a global +register declaration to their source code. + + A function which can alter the value of a global register variable +cannot safely be called from a function compiled without this variable, +because it could clobber the value the caller expects to find there on +return. Therefore, the function which is the entry point into the part +of the program that uses the global register variable must explicitly +save and restore the value which belongs to its caller. + + On most machines, `longjmp' will restore to each global register +variable the value it had at the time of the `setjmp'. On some +machines, however, `longjmp' will not change the value of global +register variables. To be portable, the function that called `setjmp' +should make other arrangements to save the values of the global register +variables, and to restore them in a `longjmp'. This way, the same +thing will happen regardless of what `longjmp' does. + + All global register variable declarations must precede all function +definitions. If such a declaration could appear after function +definitions, the declaration would be too late to prevent the register +from being used for other purposes in the preceding functions. + + Global register variables may not have initial values, because an +executable file has no means to supply initial contents for a register. + + On the Sparc, there are reports that g3 ... g7 are suitable +registers, but certain library functions, such as `getwd', as well as +the subroutines for division and remainder, modify g3 and g4. g1 and +g2 are local temporaries. + + On the 68000, a2 ... a5 should be suitable, as should d2 ... d7. Of +course, it will not do to use more than a few of those.  -File: gcc.info, Node: Include Files and VMS, Next: Global Declarations, Up: VMS +File: gcc.info, Node: Local Reg Vars, Prev: Global Reg Vars, Up: Explicit Reg Vars -Include Files and VMS -===================== +Specifying Registers for Local Variables +---------------------------------------- - 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 ' 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 - - 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 - -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 + You can define a local register variable with a specified register +like this: -(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 + register int *foo asm ("a5"); + +Here `a5' is the name of the register which should be used. Note that +this is the same syntax used for defining global register variables, +but for a local variable it would appear within a function. + + Naturally the register name is cpu-dependent, but this is not a +problem, since specific registers are most often useful with explicit +assembler instructions (*note Extended Asm::.). Both of these things +generally require that you conditionalize your program according to cpu +type. + + In addition, operating systems on one type of cpu may differ in how +they name the registers; then you would need additional conditionals. +For example, some 68000 operating systems call this register `%a5'. + + Eventually there may be a way of asking the compiler to choose a +register automatically, but first we need to figure out how it should +choose and how to enable you to guide the choice. No solution is +evident. + + Defining such a register variable does not reserve the register; it +remains available for other uses in places where flow control determines +the variable's value is not live. However, these registers are made +unavailable for use in the reload pass. I would not be surprised if +excessive use of this feature leaves the compiler too few available +registers to compile certain functions. + + +File: gcc.info, Node: Alternate Keywords, Next: Incomplete Enums, Prev: Explicit Reg Vars, Up: C Extensions - 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}; +Alternate Keywords +================== + + The option `-traditional' disables certain keywords; `-ansi' +disables certain others. This causes trouble when you want to use GNU C +extensions, or ANSI C features, in a general-purpose header file that +should be usable by all programs, including ANSI C programs and +traditional ones. The keywords `asm', `typeof' and `inline' cannot be +used since they won't work in a program compiled with `-ansi', while +the keywords `const', `volatile', `signed', `typeof' and `inline' won't +work in a program compiled with `-traditional'. + + The way to solve these problems is to put `__' at the beginning and +end of each problematical keyword. For example, use `__asm__' instead +of `asm', `__const__' instead of `const', and `__inline__' instead of +`inline'. + + Other C compilers won't accept these alternative keywords; if you +want to compile with another compiler, you can define the alternate +keywords as macros to replace them with the customary keywords. It +looks like this: + + #ifndef __GNUC__ + #define __asm__ asm #endif + `-pedantic' causes warnings for many GNU C extensions. You can +prevent such warnings within one expression by writing `__extension__' +before the expression. `__extension__' has no effect aside from this. +  -File: gcc.info, Node: VMS Misc, Prev: Global Declarations, Up: VMS +File: gcc.info, Node: Incomplete Enums, Next: Function Names, Prev: Alternate Keywords, Up: C Extensions + +Incomplete `enum' Types +======================= + + You can define an `enum' tag without specifying its possible values. +This results in an incomplete type, much like what you get if you write +`struct foo' without describing the elements. A later declaration +which does specify the possible values completes the type. + + You can't allocate variables or storage using the type while it is +incomplete. However, you can work with pointers to that type. -Other VMS Issues -================ + This extension may not be very useful, but it makes the handling of +`enum' more consistent with the way `struct' and `union' are handled. - 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. - - -File: gcc.info, Node: Interface, Next: Passes, Prev: Portability, Up: Top - -Interfacing to GNU CC Output -**************************** - - GNU CC is normally configured to use the same function calling -convention normally in use on the target system. This is done with the -machine-description macros described (*note Target Macros::.). - - However, returning of structure and union values is done differently -on some target machines. As a result, functions compiled with PCC -returning such types cannot be called from code compiled with GNU CC, -and vice versa. This does not cause trouble often because few Unix -library routines return structures or unions. - - GNU CC code returns structures and unions that are 1, 2, 4 or 8 bytes -long in the same registers used for `int' or `double' return values. -(GNU CC typically allocates variables of such types in registers also.) -Structures and unions of other sizes are returned by storing them into -an address passed by the caller (usually in a register). The -machine-description macros `STRUCT_VALUE' and `STRUCT_INCOMING_VALUE' -tell GNU CC where to pass this address. - - By contrast, PCC on most target machines returns structures and -unions of any size by copying the data into an area of static storage, -and then returning the address of that storage as if it were a pointer -value. The caller must copy the data from that memory area to the -place where the value is wanted. This is slower than the method used -by GNU CC, and fails to be reentrant. - - On some target machines, such as RISC machines and the 80386, the -standard system convention is to pass to the subroutine the address of -where to return the value. On these machines, GNU CC has been -configured to be compatible with the standard compiler, when this method -is used. It may not be compatible for structures of 1, 2, 4 or 8 bytes. - - GNU CC uses the system's standard convention for passing arguments. -On some machines, the first few arguments are passed in registers; in -others, all are passed on the stack. It would be possible to use -registers for argument passing on any machine, and this would probably -result in a significant speedup. But the result would be complete -incompatibility with code that follows the standard convention. So this -change is practical only if you are switching to GNU CC as the sole C -compiler for the system. We may implement register argument passing on -certain machines once we have a complete GNU system so that we can -compile the libraries with GNU CC. - - On some machines (particularly the Sparc), certain types of arguments -are passed "by invisible reference". This means that the value is -stored in memory, and the address of the memory location is passed to -the subroutine. - - If you use `longjmp', beware of automatic variables. ANSI C says -that automatic variables that are not declared `volatile' have undefined -values after a `longjmp'. And this is all GNU CC promises to do, -because it is very difficult to restore register variables correctly, -and one of GNU CC's features is that it can put variables in registers -without your asking it to. - - If you want a variable to be unaltered by `longjmp', and you don't -want to write `volatile' because old C compilers don't accept it, just -take the address of the variable. If a variable's address is ever -taken, even if just to compute it and ignore it, then the variable -cannot go in a register: + This extension is not supported by GNU C++. + +File: gcc.info, Node: Function Names, Prev: Incomplete Enums, Up: C Extensions + +Function Names as Strings +========================= + + GNU CC predefines two string variables to be the name of the current +function. The variable `__FUNCTION__' is the name of the function as +it appears in the source. The variable `__PRETTY_FUNCTION__' is the +name of the function pretty printed in a language specific fashion. + + These names are always the same in a C function, but in a C++ +function they may be different. For example, this program: + + extern "C" { + extern int printf (char *, ...); + } + + class a { + public: + sub (int i) + { + printf ("__FUNCTION__ = %s\n", __FUNCTION__); + printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__); + } + }; + + int + main (void) { - int careful; - &careful; + a ax; + ax.sub (0); + return 0; + } + +gives this output: + + __FUNCTION__ = sub + __PRETTY_FUNCTION__ = int a::sub (int) + + These names are not macros: they are predefined string variables. +For example, `#ifdef __FUNCTION__' does not have any special meaning +inside a function, since the preprocessor does not do anything special +with the identifier `__FUNCTION__'. + + +File: gcc.info, Node: C++ Extensions, Next: Trouble, Prev: C Extensions, Up: Top + +Extensions to the C++ Language +****************************** + + The GNU compiler provides these extensions to the C++ language (and +you can also use most of the C language extensions in your C++ +programs). If you want to write code that checks whether these +features are available, you can test for the GNU compiler the same way +as for C programs: check for a predefined macro `__GNUC__'. You can +also use `__GNUG__' to test specifically for GNU C++ (*note Standard +Predefined Macros: (cpp.info)Standard Predefined.). + +* Menu: + +* Naming Results:: Giving a name to C++ function return values. +* Min and Max:: C++ Minimum and maximum operators. +* Destructors and Goto:: Goto is safe to use in C++ even when destructors + are needed. +* C++ Interface:: You can use a single C++ header file for both + declarations and definitions. +* Template Instantiation:: Methods for ensuring that exactly one copy of + each needed template instantiation is emitted. +* C++ Signatures:: You can specify abstract types to get subtype + polymorphism independent from inheritance. + + +File: gcc.info, Node: Naming Results, Next: Min and Max, Up: C++ Extensions + +Named Return Values in C++ +========================== + + GNU C++ extends the function-definition syntax to allow you to +specify a name for the result of a function outside the body of the +definition, in C++ programs: + + TYPE + FUNCTIONNAME (ARGS) return RESULTNAME; + { + ... + BODY ... } - Code compiled with GNU CC may call certain library routines. Most of -them handle arithmetic for which there are no instructions. This -includes multiply and divide on some machines, and floating point -operations on any machine for which floating point support is disabled -with `-msoft-float'. Some standard parts of the C library, such as -`bcopy' or `memcpy', are also called automatically. The usual function -call interface is used for calling the library routines. - - These library routines should be defined in the library `libgcc.a', -which GNU CC automatically searches whenever it links a program. On -machines that have multiply and divide instructions, if hardware -floating point is in use, normally `libgcc.a' is not needed, but it is -searched just in case. - - Each arithmetic function is defined in `libgcc1.c' to use the -corresponding C arithmetic operator. As long as the file is compiled -with another C compiler, which supports all the C arithmetic operators, -this file will work portably. However, `libgcc1.c' does not work if -compiled with GNU CC, because each arithmetic function would compile -into a call to itself! + You can use this feature to avoid an extra constructor call when a +function result has a class type. For example, consider a function +`m', declared as `X v = m ();', whose result is of class `X': + + X + m () + { + X b; + b.a = 23; + return b; + } + + Although `m' appears to have no arguments, in fact it has one +implicit argument: the address of the return value. At invocation, the +address of enough space to hold `v' is sent in as the implicit argument. +Then `b' is constructed and its `a' field is set to the value 23. +Finally, a copy constructor (a constructor of the form `X(X&)') is +applied to `b', with the (implicit) return value location as the +target, so that `v' is now bound to the return value. + + But this is wasteful. The local `b' is declared just to hold +something that will be copied right out. While a compiler that +combined an "elision" algorithm with interprocedural data flow analysis +could conceivably eliminate all of this, it is much more practical to +allow you to assist the compiler in generating efficient code by +manipulating the return value explicitly, thus avoiding the local +variable and copy constructor altogether. + + Using the extended GNU C++ function-definition syntax, you can avoid +the temporary allocation and copying by naming `r' as your return value +at the outset, and assigning to its `a' field directly: + + X + m () return r; + { + r.a = 23; + } + +The declaration of `r' is a standard, proper declaration, whose effects +are executed *before* any of the body of `m'. + + Functions of this type impose no additional restrictions; in +particular, you can execute `return' statements, or return implicitly by +reaching the end of the function body ("falling off the edge"). Cases +like + + X + m () return r (23); + { + return; + } + +(or even `X m () return r (23); { }') are unambiguous, since the return +value `r' has been initialized in either case. The following code may +be hard to read, but also works predictably: + + X + m () return r; + { + X b; + return b; + } + + The return value slot denoted by `r' is initialized at the outset, +but the statement `return b;' overrides this value. The compiler deals +with this by destroying `r' (calling the destructor if there is one, or +doing nothing if there is not), and then reinitializing `r' with `b'. + + This extension is provided primarily to help people who use +overloaded operators, where there is a great need to control not just +the arguments, but the return values of functions. For classes where +the copy constructor incurs a heavy performance penalty (especially in +the common case where there is a quick default constructor), this is a +major savings. The disadvantage of this extension is that you do not +control when the default constructor for the return value is called: it +is always called at the beginning. + + +File: gcc.info, Node: Min and Max, Next: Destructors and Goto, Prev: Naming Results, Up: C++ Extensions + +Minimum and Maximum Operators in C++ +==================================== + + It is very convenient to have operators which return the "minimum" +or the "maximum" of two arguments. In GNU C++ (but not in GNU C), + +`A ? B' + is the "maximum", returning the larger of the numeric values A and + B. + + These operations are not primitive in ordinary C++, since you can +use a macro to return the minimum of two things in C++, as in the +following example. + + #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y)) + +You might then use `int min = MIN (i, j);' to set MIN to the minimum +value of variables I and J. + + However, side effects in `X' or `Y' may cause unintended behavior. +For example, `MIN (i++, j++)' will fail, incrementing the smaller +counter twice. A GNU C extension allows you to write safe macros that +avoid this kind of problem (*note Naming an Expression's Type: Naming +Types.). However, writing `MIN' and `MAX' as macros also forces you to +use function-call notation notation for a fundamental arithmetic +operation. Using GNU C++ extensions, you can write `int min = i ?' are built into the compiler, they properly +handle expressions with side-effects; `int min = i++ ; + template ostream& operator << (ostream&, const A&); + + This strategy will work with code written for either model. If + you are using code written for the Cfront model, the file + containing a class template and the file containing its member + templates should be implemented in the same translation unit. + + A slight variation on this approach is to use the flag + -falt-external-templates instead; this flag causes template + instances to be emitted in the translation unit that implements + the header where they are first instantiated, rather than the one + which implements the file where the templates are defined. This + header must be the same in all translation units, or things are + likely to break. + + *Note Declarations and Definitions in One Header: C++ Interface, + for more discussion of these pragmas. + + 3. Explicitly instantiate all the template instances you use, and + compile with -fno-implicit-templates. This is probably your best + bet; it may require more knowledge of exactly which templates you + are using, but it's less mysterious than the previous approach, + and it doesn't require any `#pragma's or other g++-specific code. + You can scatter the instantiations throughout your program, you + can create one big file to do all the instantiations, or you can + create tiny files like + + #include "Foo.h" + #include "Foo.cc" + + template class Foo; + + for each instance you need, and create a template instantiation + library from those. I'm partial to the last, but your mileage may + vary. If you are using Cfront-model code, you can probably get + away with not using -fno-implicit-templates when compiling files + that don't `#include' the member template definitions. + + +File: gcc.info, Node: C++ Signatures, Prev: Template Instantiation, Up: C++ Extensions + +Type Abstraction using Signatures +================================= + + In GNU C++, you can use the keyword `signature' to define a +completely abstract class interface as a datatype. You can connect this +abstraction with actual classes using signature pointers. If you want +to use signatures, run the GNU compiler with the `-fhandle-signatures' +command-line option. (With this option, the compiler reserves a second +keyword `sigof' as well, for a future extension.) + + Roughly, signatures are type abstractions or interfaces of classes. +Some other languages have similar facilities. C++ signatures are +related to ML's signatures, Haskell's type classes, definition modules +in Modula-2, interface modules in Modula-3, abstract types in Emerald, +type modules in Trellis/Owl, categories in Scratchpad II, and types in +POOL-I. For a more detailed discussion of signatures, see `Signatures: +A Language Extension for Improving Type Abstraction and Subtype +Polymorphism in C++' by Gerald Baumgartner and Vincent F. Russo (Tech +report CSD-TR-95-051, Dept. of Computer Sciences, Purdue University, +August 1995, a slightly improved version appeared in +*Software--Practice & Experience*, 25(8), pp. 863-889, August 1995). +You can get the tech report by anonymous FTP from `ftp.cs.purdue.edu' +in `pub/gb/Signature-design.ps.gz'. + + Syntactically, a signature declaration is a collection of member +function declarations and nested type declarations. For example, this +signature declaration defines a new abstract type `S' with member +functions `int foo ()' and `int bar (int)': + + signature S + { + int foo (); + int bar (int); + }; + + Since signature types do not include implementation definitions, you +cannot write an instance of a signature directly. Instead, you can +define a pointer to any class that contains the required interfaces as a +"signature pointer". Such a class "implements" the signature type. + + To use a class as an implementation of `S', you must ensure that the +class has public member functions `int foo ()' and `int bar (int)'. +The class can have other member functions as well, public or not; as +long as it offers what's declared in the signature, it is suitable as +an implementation of that signature type. + + For example, suppose that `C' is a class that meets the requirements +of signature `S' (`C' "conforms to" `S'). Then + + C obj; + S * p = &obj; + +defines a signature pointer `p' and initializes it to point to an +object of type `C'. The member function call `int i = p->foo ();' +executes `obj.foo ()'. + + Abstract virtual classes provide somewhat similar facilities in +standard C++. There are two main advantages to using signatures +instead: + + 1. Subtyping becomes independent from inheritance. A class or + signature type `T' is a subtype of a signature type `S' + independent of any inheritance hierarchy as long as all the member + functions declared in `S' are also found in `T'. So you can + define a subtype hierarchy that is completely independent from any + inheritance (implementation) hierarchy, instead of being forced to + use types that mirror the class inheritance hierarchy. + + 2. Signatures allow you to work with existing class hierarchies as + implementations of a signature type. If those class hierarchies + are only available in compiled form, you're out of luck with + abstract virtual classes, since an abstract virtual class cannot + be retrofitted on top of existing class hierarchies. So you would + be required to write interface classes as subtypes of the abstract + virtual class. + + There is one more detail about signatures. A signature declaration +can contain member function *definitions* as well as member function +declarations. A signature member function with a full definition is +called a *default implementation*; classes need not contain that +particular interface in order to conform. For example, a class `C' can +conform to the signature + + signature T + { + int f (int); + int f0 () { return f (0); }; + }; + +whether or not `C' implements the member function `int f0 ()'. If you +define `C::f0', that definition takes precedence; otherwise, the +default implementation `S::f0' applies. + + +File: gcc.info, Node: Trouble, Next: Bugs, Prev: C++ Extensions, Up: Top + +Known Causes of Trouble with GNU CC +*********************************** + + This section describes known problems that affect users of GNU CC. +Most of these are not GNU CC bugs per se--if they were, we would fix +them. But the result for a user may be like the result of a bug. + + Some of these problems are due to bugs in other software, some are +missing features that are too much work to add, and some are places +where people's opinions differ as to what is best. + +* Menu: + +* Actual Bugs:: Bugs we will fix later. +* Installation Problems:: Problems that manifest when you install GNU CC. +* Cross-Compiler Problems:: Common problems of cross compiling with GNU CC. +* Interoperation:: Problems using GNU CC with other compilers, + and with certain linkers, assemblers and debuggers. +* External Bugs:: Problems compiling certain programs. +* Incompatibilities:: GNU CC is incompatible with traditional C. +* Fixed Headers:: GNU C uses corrected versions of system header files. + This is necessary, but doesn't always work smoothly. +* Standard Libraries:: GNU C uses the system C library, which might not be + compliant with the ISO/ANSI C standard. +* Disappointments:: Regrettable things we can't change, but not quite bugs. +* C++ Misunderstandings:: Common misunderstandings with GNU C++. +* Protoize Caveats:: Things to watch out for when using `protoize'. +* Non-bugs:: Things we think are right, but some others disagree. +* Warnings and Errors:: Which problems in your code get warnings, + and which get errors. + + +File: gcc.info, Node: Actual Bugs, Next: Installation Problems, Up: Trouble + +Actual Bugs We Haven't Fixed Yet +================================ + + * The `fixincludes' script interacts badly with automounters; if the + directory of system header files is automounted, it tends to be + unmounted while `fixincludes' is running. This would seem to be a + bug in the automounter. We don't know any good way to work around + it. + + * The `fixproto' script will sometimes add prototypes for the + `sigsetjmp' and `siglongjmp' functions that reference the + `jmp_buf' type before that type is defined. To work around this, + edit the offending file and place the typedef in front of the + prototypes. + + * There are several obscure case of mis-using struct, union, and + enum tags that are not detected as errors by the compiler. + + * When `-pedantic-errors' is specified, GNU C will incorrectly give + an error message when a function name is specified in an expression + involving the comma operator. + + * Loop unrolling doesn't work properly for certain C++ programs. + This is a bug in the C++ front end. It sometimes emits incorrect + debug info, and the loop unrolling code is unable to recover from + this error.