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GNU Info File | 1995-03-30 | 42KB | 729 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 675 Massachusetts Avenue Cambridge, MA 02139 USA Copyright (C) 1988, 1989, 1992, 1993, 1994 Free Software Foundation, 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: External Bugs, Next: Incompatibilities, Prev: Interoperation, Up: Trouble Problems Compiling Certain Programs =================================== Certain programs have problems compiling. * Parse errors may occur compiling X11 on a Decstation running Ultrix 4.2 because of problems in DEC's versions of the X11 header files `X11/Xlib.h' and `X11/Xutil.h'. People recommend adding `-I/usr/include/mit' to use the MIT versions of the header files, using the `-traditional' switch to turn off ANSI C, or fixing the header files by adding this: #ifdef __STDC__ #define NeedFunctionPrototypes 0 #endif * If you have trouble compiling Perl on a SunOS 4 system, it may be because Perl specifies `-I/usr/ucbinclude'. This accesses the unfixed header files. Perl specifies the options -traditional -Dvolatile=__volatile__ -I/usr/include/sun -I/usr/ucbinclude -fpcc-struct-return most of which are unnecessary with GCC 2.4.5 and newer versions. You can make a properly working Perl by setting `ccflags' to `-fwritable-strings' (implied by the `-traditional' in the original options) and `cppflags' to empty in `config.sh', then typing `./doSH; make depend; make'. * On various 386 Unix systems derived from System V, including SCO, ISC, and ESIX, you may get error messages about running out of virtual memory while compiling certain programs. You can prevent this problem by linking GNU CC with the GNU malloc (which thus replaces the malloc that comes with the system). GNU malloc is available as a separate package, and also in the file `src/gmalloc.c' in the GNU Emacs 19 distribution. If you have installed GNU malloc as a separate library package, use this option when you relink GNU CC: MALLOC=/usr/local/lib/libgmalloc.a Alternatively, if you have compiled `gmalloc.c' from Emacs 19, copy the object file to `gmalloc.o' and use this option when you relink GNU CC: MALLOC=gmalloc.o File: gcc.info, Node: Incompatibilities, Next: Fixed Headers, Prev: External Bugs, Up: Trouble Incompatibilities of GNU CC =========================== There are several noteworthy incompatibilities between GNU C and most existing (non-ANSI) versions of C. The `-traditional' option eliminates many of these incompatibilities, *but not all*, by telling GNU C to behave like the other C compilers. * GNU CC normally makes string constants read-only. If several identical-looking string constants are used, GNU CC stores only one copy of the string. One consequence is that you cannot call `mktemp' with a string constant argument. The function `mktemp' always alters the string its argument points to. Another consequence is that `sscanf' does not work on some systems when passed a string constant as its format control string or input. This is because `sscanf' incorrectly tries to write into the string constant. Likewise `fscanf' and `scanf'. The best solution to these problems is to change the program to use `char'-array variables with initialization strings for these purposes instead of string constants. But if this is not possible, you can use the `-fwritable-strings' flag, which directs GNU CC to handle string constants the same way most C compilers do. `-traditional' also has this effect, among others. * `-2147483648' is positive. This is because 2147483648 cannot fit in the type `int', so (following the ANSI C rules) its data type is `unsigned long int'. Negating this value yields 2147483648 again. * GNU CC does not substitute macro arguments when they appear inside of string constants. For example, the following macro in GNU CC #define foo(a) "a" will produce output `"a"' regardless of what the argument A is. The `-traditional' option directs GNU CC to handle such cases (among others) in the old-fashioned (non-ANSI) fashion. * When you use `setjmp' and `longjmp', the only automatic variables guaranteed to remain valid are those declared `volatile'. This is a consequence of automatic register allocation. Consider this function: jmp_buf j; foo () { int a, b; a = fun1 (); if (setjmp (j)) return a; a = fun2 (); /* `longjmp (j)' may occur in `fun3'. */ return a + fun3 (); } Here `a' may or may not be restored to its first value when the `longjmp' occurs. If `a' is allocated in a register, then its first value is restored; otherwise, it keeps the last value stored in it. If you use the `-W' option with the `-O' option, you will get a warning when GNU CC thinks such a problem might be possible. The `-traditional' option directs GNU C to put variables in the stack by default, rather than in registers, in functions that call `setjmp'. This results in the behavior found in traditional C compilers. * Programs that use preprocessor directives in the middle of macro arguments do not work with GNU CC. For example, a program like this will not work: foobar ( #define luser hack) ANSI C does not permit such a construct. It would make sense to support it when `-traditional' is used, but it is too much work to implement. * Declarations of external variables and functions within a block apply only to the block containing the declaration. In other words, they have the same scope as any other declaration in the same place. In some other C compilers, a `extern' declaration affects all the rest of the file even if it happens within a block. The `-traditional' option directs GNU C to treat all `extern' declarations as global, like traditional compilers. * In traditional C, you can combine `long', etc., with a typedef name, as shown here: typedef int foo; typedef long foo bar; In ANSI C, this is not allowed: `long' and other type modifiers require an explicit `int'. Because this criterion is expressed by Bison grammar rules rather than C code, the `-traditional' flag cannot alter it. * PCC allows typedef names to be used as function parameters. The difficulty described immediately above applies here too. * PCC allows whitespace in the middle of compound assignment operators such as `+='. GNU CC, following the ANSI standard, does not allow this. The difficulty described immediately above applies here too. * GNU CC complains about unterminated character constants inside of preprocessor conditionals that fail. Some programs have English comments enclosed in conditionals that are guaranteed to fail; if these comments contain apostrophes, GNU CC will probably report an error. For example, this code would produce an error: #if 0 You can't expect this to work. #endif The best solution to such a problem is to put the text into an actual C comment delimited by `/*...*/'. However, `-traditional' suppresses these error messages. * Many user programs contain the declaration `long time ();'. In the past, the system header files on many systems did not actually declare `time', so it did not matter what type your program declared it to return. But in systems with ANSI C headers, `time' is declared to return `time_t', and if that is not the same as `long', then `long time ();' is erroneous. The solution is to change your program to use `time_t' as the return type of `time'. * When compiling functions that return `float', PCC converts it to a double. GNU CC actually returns a `float'. If you are concerned with PCC compatibility, you should declare your functions to return `double'; you might as well say what you mean. * When compiling functions that return structures or unions, GNU CC output code normally uses a method different from that used on most versions of Unix. As a result, code compiled with GNU CC cannot call a structure-returning function compiled with PCC, and vice versa. The method used by GNU CC is as follows: a structure or union which is 1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union with any other size is stored into an address supplied by the caller (usually in a special, fixed register, but on some machines it is passed on the stack). 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. GNU CC does not use this method because it is slower and nonreentrant. On some newer machines, PCC uses a reentrant convention for all structure and union returning. GNU CC on most of these machines uses a compatible convention when returning structures and unions in memory, but still returns small structures and unions in registers. You can tell GNU CC to use a compatible convention for all structure and union returning with the option `-fpcc-struct-return'. * GNU C complains about program fragments such as `0x74ae-0x4000' which appear to be two hexadecimal constants separated by the minus operator. Actually, this string is a single "preprocessing token". Each such token must correspond to one token in C. Since this does not, GNU C prints an error message. Although it may appear obvious that what is meant is an operator and two values, the ANSI C standard specifically requires that this be treated as erroneous. A "preprocessing token" is a "preprocessing number" if it begins with a digit and is followed by letters, underscores, digits, periods and `e+', `e-', `E+', or `E-' character sequences. To make the above program fragment valid, place whitespace in front of the minus sign. This whitespace will end the preprocessing number. File: gcc.info, Node: Fixed Headers, Next: Disappointments, Prev: Incompatibilities, Up: Trouble Fixed Header Files ================== GNU CC needs to install corrected versions of some system header files. This is because most target systems have some header files that won't work with GNU CC unless they are changed. Some have bugs, some are incompatible with ANSI C, and some depend on special features of other compilers. Installing GNU CC automatically creates and installs the fixed header files, by running a program called `fixincludes' (or for certain targets an alternative such as `fixinc.svr4'). Normally, you don't need to pay attention to this. But there are cases where it doesn't do the right thing automatically. * If you update the system's header files, such as by installing a new system version, the fixed header files of GNU CC are not automatically updated. The easiest way to update them is to reinstall GNU CC. (If you want to be clever, look in the makefile and you can find a shortcut.) * On some systems, in particular SunOS 4, header file directories contain machine-specific symbolic links in certain places. This makes it possible to share most of the header files among hosts running the same version of SunOS 4 on different machine models. The programs that fix the header files do not understand this special way of using symbolic links; therefore, the directory of fixed header files is good only for the machine model used to build it. In SunOS 4, only programs that look inside the kernel will notice the difference between machine models. Therefore, for most purposes, you need not be concerned about this. It is possible to make separate sets of fixed header files for the different machine models, and arrange a structure of symbolic links so as to use the proper set, but you'll have to do this by hand. * On Lynxos, GNU CC by default does not fix the header files. This is because bugs in the shell cause the `fixincludes' script to fail. This means you will encounter problems due to bugs in the system header files. It may be no comfort that they aren't GNU CC's fault, but it does mean that there's nothing for us to do about them. File: gcc.info, Node: Disappointments, Next: C++ Misunderstandings, Prev: Fixed Headers, Up: Trouble Disappointments and Misunderstandings ===================================== These problems are perhaps regrettable, but we don't know any practical way around them. * Certain local variables aren't recognized by debuggers when you compile with optimization. This occurs because sometimes GNU CC optimizes the variable out of existence. There is no way to tell the debugger how to compute the value such a variable "would have had", and it is not clear that would be desirable anyway. So GNU CC simply does not mention the eliminated variable when it writes debugging information. You have to expect a certain amount of disagreement between the executable and your source code, when you use optimization. * Users often think it is a bug when GNU CC reports an error for code like this: int foo (struct mumble *); struct mumble { ... }; int foo (struct mumble *x) { ... } This code really is erroneous, because the scope of `struct mumble' in the prototype is limited to the argument list containing it. It does not refer to the `struct mumble' defined with file scope immediately below--they are two unrelated types with similar names in different scopes. But in the definition of `foo', the file-scope type is used because that is available to be inherited. Thus, the definition and the prototype do not match, and you get an error. This behavior may seem silly, but it's what the ANSI standard specifies. It is easy enough for you to make your code work by moving the definition of `struct mumble' above the prototype. It's not worth being incompatible with ANSI C just to avoid an error for the example shown above. * Accesses to bitfields even in volatile objects works by accessing larger objects, such as a byte or a word. You cannot rely on what size of object is accessed in order to read or write the bitfield; it may even vary for a given bitfield according to the precise usage. If you care about controlling the amount of memory that is accessed, use volatile but do not use bitfields. * GNU CC comes with shell scripts to fix certain known problems in system header files. They install corrected copies of various header files in a special directory where only GNU CC will normally look for them. The scripts adapt to various systems by searching all the system header files for the problem cases that we know about. If new system header files are installed, nothing automatically arranges to update the corrected header files. You will have to reinstall GNU CC to fix the new header files. More specifically, go to the build directory and delete the files `stmp-fixinc' and `stmp-headers', and the subdirectory `include'; then do `make install' again. * On 68000 systems, you can get paradoxical results if you test the precise values of floating point numbers. For example, you can find that a floating point value which is not a NaN is not equal to itself. This results from the fact that the the floating point registers hold a few more bits of precision than fit in a `double' in memory. Compiled code moves values between memory and floating point registers at its convenience, and moving them into memory truncates them. You can partially avoid this problem by using the `-ffloat-store' option (*note Optimize Options::.). * On the MIPS, variable argument functions using `varargs.h' cannot have a floating point value for the first argument. The reason for this is that in the absence of a prototype in scope, if the first argument is a floating point, it is passed in a floating point register, rather than an integer register. If the code is rewritten to use the ANSI standard `stdarg.h' method of variable arguments, and the prototype is in scope at the time of the call, everything will work fine. File: gcc.info, Node: C++ Misunderstandings, Next: Protoize Caveats, Prev: Disappointments, Up: Trouble Common Misunderstandings with GNU C++ ===================================== C++ is a complex language and an evolving one, and its standard definition (the ANSI C++ draft standard) is also evolving. As a result, your C++ compiler may occasionally surprise you, even when its behavior is correct. This section discusses some areas that frequently give rise to questions of this sort. * Menu: * Static Definitions:: Static member declarations are not definitions * Temporaries:: Temporaries may vanish before you expect File: gcc.info, Node: Static Definitions, Next: Temporaries, Up: C++ Misunderstandings Declare *and* Define Static Members ----------------------------------- When a class has static data members, it is not enough to *declare* the static member; you must also *define* it. For example: class Foo { ... void method(); static int bar; }; This declaration only establishes that the class `Foo' has an `int' named `Foo::bar', and a member function named `Foo::method'. But you still need to define *both* `method' and `bar' elsewhere. According to the draft ANSI standard, you must supply an initializer in one (and only one) source file, such as: int Foo::bar = 0; Other C++ compilers may not correctly implement the standard behavior. As a result, when you switch to `g++' from one of these compilers, you may discover that a program that appeared to work correctly in fact does not conform to the standard: `g++' reports as undefined symbols any static data members that lack definitions. File: gcc.info, Node: Temporaries, Prev: Static Definitions, Up: C++ Misunderstandings Temporaries May Vanish Before You Expect ---------------------------------------- It is dangerous to use pointers or references to *portions* of a temporary object. The compiler may very well delete the object before you expect it to, leaving a pointer to garbage. The most common place where this problem crops up is in classes like the libg++ `String' class, that define a conversion function to type `char *' or `const char *'. However, any class that returns a pointer to some internal structure is potentially subject to this problem. For example, a program may use a function `strfunc' that returns `String' objects, and another function `charfunc' that operates on pointers to `char': String strfunc (); void charfunc (const char *); In this situation, it may seem natural to write `charfunc (strfunc ());' based on the knowledge that class `String' has an explicit conversion to `char' pointers. However, what really happens is akin to `charfunc (strfunc ().convert ());', where the `convert' method is a function to do the same data conversion normally performed by a cast. Since the last use of the temporary `String' object is the call to the conversion function, the compiler may delete that object before actually calling `charfunc'. The compiler has no way of knowing that deleting the `String' object will invalidate the pointer. The pointer then points to garbage, so that by the time `charfunc' is called, it gets an invalid argument. Code like this may run successfully under some other compilers, especially those that delete temporaries relatively late. However, the GNU C++ behavior is also standard-conformant, so if your program depends on late destruction of temporaries it is not portable. If you think this is surprising, you should be aware that the ANSI C++ committee continues to debate the lifetime-of-temporaries problem. For now, at least, the safe way to write such code is to give the temporary a name, which forces it to remain until the end of the scope of the name. For example: String& tmp = strfunc (); charfunc (tmp); File: gcc.info, Node: Protoize Caveats, Next: Non-bugs, Prev: C++ Misunderstandings, Up: Trouble Caveats of using `protoize' =========================== The conversion programs `protoize' and `unprotoize' can sometimes change a source file in a way that won't work unless you rearrange it. * `protoize' can insert references to a type name or type tag before the definition, or in a file where they are not defined. If this happens, compiler error messages should show you where the new references are, so fixing the file by hand is straightforward. * There are some C constructs which `protoize' cannot figure out. For example, it can't determine argument types for declaring a pointer-to-function variable; this you must do by hand. `protoize' inserts a comment containing `???' each time it finds such a variable; so you can find all such variables by searching for this string. ANSI C does not require declaring the argument types of pointer-to-function types. * Using `unprotoize' can easily introduce bugs. If the program relied on prototypes to bring about conversion of arguments, these conversions will not take place in the program without prototypes. One case in which you can be sure `unprotoize' is safe is when you are removing prototypes that were made with `protoize'; if the program worked before without any prototypes, it will work again without them. You can find all the places where this problem might occur by compiling the program with the `-Wconversion' option. It prints a warning whenever an argument is converted. * Both conversion programs can be confused if there are macro calls in and around the text to be converted. In other words, the standard syntax for a declaration or definition must not result from expanding a macro. This problem is inherent in the design of C and cannot be fixed. If only a few functions have confusing macro calls, you can easily convert them manually. * `protoize' cannot get the argument types for a function whose definition was not actually compiled due to preprocessor conditionals. When this happens, `protoize' changes nothing in regard to such a function. `protoize' tries to detect such instances and warn about them. You can generally work around this problem by using `protoize' step by step, each time specifying a different set of `-D' options for compilation, until all of the functions have been converted. There is no automatic way to verify that you have got them all, however. * Confusion may result if there is an occasion to convert a function declaration or definition in a region of source code where there is more than one formal parameter list present. Thus, attempts to convert code containing multiple (conditionally compiled) versions of a single function header (in the same vicinity) may not produce the desired (or expected) results. If you plan on converting source files which contain such code, it is recommended that you first make sure that each conditionally compiled region of source code which contains an alternative function header also contains at least one additional follower token (past the final right parenthesis of the function header). This should circumvent the problem. * `unprotoize' can become confused when trying to convert a function definition or declaration which contains a declaration for a pointer-to-function formal argument which has the same name as the function being defined or declared. We recommand you avoid such choices of formal parameter names. * You might also want to correct some of the indentation by hand and break long lines. (The conversion programs don't write lines longer than eighty characters in any case.) 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 `3, 4', or it might get `4, 3', or even `3, 3'. * 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 gratuituously 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