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GNU Info File | 1994-02-07 | 49.1 KB | 1,118 lines

This is Info file gcc.info, produced by Makeinfo-1.49 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Copyright (C) 1988, 1989, 1992 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License" 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. File: gcc.info, Node: Interoperation, Next: Incompatibilities, Prev: Cross-Compiler Problems, Up: Trouble Interoperation ============== This section lists various difficulties encountered in using GNU C or GNU C++ together with other compilers or with the assemblers, linkers, libraries and debuggers on certain systems. * GNU C normally compiles functions to return small structures and unions in registers. Most other compilers arrange to return them just like larger structures and unions. This can lead to trouble when you link together code compiled by different compilers. To avoid the problem, you can use the option `-fpcc-struct-return' when compiling with GNU CC. * GNU C++ does not do name mangling in the same way as other C++ compilers. This means that object files compiled with one compiler cannot be used with another. GNU C++ also uses different techniques for arranging virtual function tables and the layout of class instances. In general, therefore, linking code compiled with different C++ compilers does not work. * Older GDB versions sometimes fail to read the output of GNU CC version 2. If you have trouble, get GDB version 4.4 or later. * DBX rejects some files produced by GNU CC, though it accepts similar constructs in output from PCC. Until someone can supply a coherent description of what is valid DBX input and what is not, there is nothing I can do about these problems. You are on your own. * The GNU assembler (GAS) does not support PIC. To generate PIC code, you must use some other assembler, such as `/bin/as'. * On some BSD systems including some versions of Ultrix, use of profiling causes static variable destructors (currently used only in C++) not to be run. * Use of `-I/usr/include' may cause trouble. Many systems come with header files that won't work with GNU CC unless corrected by `fixincludes'. The corrected header files go in a new directory; GNU CC searches this directory before `/usr/include'. If you use `-I/usr/include', this tells GNU CC to search `/usr/include' earlier on, before the corrected headers. The result is that you get the uncorrected header files. Instead, you should use these options: -I/usr/local/lib/gcc-lib/TARGET/VERSION/include -I/usr/include * On a Sparc, GNU CC aligns all values of type `double' on an 8-byte boundary, and it expects every `double' to be so aligned. The Sun compiler usually gives `double' values 8-byte alignment, with one exception: function arguments of type `double' may not be aligned. As a result, if a function compiled with Sun CC takes the address of an argument of type `double' and passes this pointer of type `double *' to a function compiled with GNU CC, dereferencing the pointer may cause a fatal signal. One way to solve this problem is to compile your entire program with GNU CC. Another solution is to modify the function that is compiled with Sun CC to copy the argument into a local variable; local variables are always properly aligned. A third solution is to modify the function that uses the pointer to dereference it via the following function `access_double' instead of directly with `*': inline double access_double (double *unaligned_ptr) { union d2i { double d; int i[2]; }; union d2i *p = (union d2i *) unaligned_ptr; union d2i u; u.i[0] = p->i[0]; u.i[1] = p->i[1]; return u.d; } Storing into the pointer can be done likewise with the same union. * On a Sun, linking using GNU CC fails to find a shared library and reports that the library doesn't exist at all. This happens if you are using the GNU linker, because it does only static linking and looks only for unshared libraries. If you have a shared library with no unshared counterpart, the GNU linker won't find anything. We hope to make a linker which supports Sun shared libraries, but please don't ask when it will be finished--we don't know. * Sun forgot to include a static version of `libdl.a' with some versions of SunOS (mainly 4.1). This results in undefined symbols when linking static binaries (that is, if you use `-static'). If you see undefined symbols `_dlclose', `_dlsym' or `_dlopen' when linking, compile and link against the file `mit/util/misc/dlsym.c' from the MIT version of X windows. * On the HP PA machine, ADB sometimes fails to work on functions compiled with GNU CC. Specifically, it fails to work on functions that use `alloca' or variable-size arrays. This is because GNU CC doesn't generate HP-UX unwind descriptors for such functions. It may even be impossible to generate them. * Debugging (`-g') is not supported on the HP PA machine, unless you use the preliminary GNU tools (*note Installation::.). * The HP-UX linker has a bug which can cause programs which make use of `const' variables to fail in unusual ways. If your program makes use of global `const' variables, we suggest you compile with the following additional options: -Dconst="" -D__const="" -D__const__="" -fwritable-strings This will force the `const' variables into the DATA subspace which will avoid the linker bug. Another option one might use to work around this problem is `-mkernel'. `-mkernel' changes how the address of variables is computed to a sequence less likely to tickle the HP-UX linker bug. We hope to work around this problem in GNU CC 2.4, if HP does not fix it. * Taking the address of a label may generate errors from the HP-UX PA assembler. GAS for the PA does not have this problem. * GNU CC produced code will not yet link against HP-UX 8.0 shared libraries. We expect to fix this problem in GNU CC 2.4. * The current version of the assembler (`/bin/as') for the RS/6000 has certain problems that prevent the `-g' option in GCC from working. IBM has produced a fixed version of the assembler. The replacement assembler is not a standard component of either AIX 3.1.5 or AIX 3.2, but is expected to become standard in a future distribution. This assembler is available from IBM as APAR IX22829. Yet more bugs have been fixed in a newer assembler, which will shortly be available as APAR IX26107. See the file `README.RS6000' for more details on these assemblers. * On the IBM RS/6000, compiling code of the form extern int foo; ... foo ... static int foo; will cause the linker to report an undefined symbol `foo'. Although this behavior differs from most other systems, it is not a bug because redefining an `extern' variable as `static' is undefined in ANSI C. * On VMS, GAS versions 1.38.1 and earlier may cause spurious warning messages from the linker. These warning messages complain of mismatched psect attributes. You can ignore them. *Note VMS Install::. * On NewsOS version 3, if you include both `stddef.h' and `sys/types.h', you get an error because there are two typedefs of `size_t'. You should change `sys/types.h' by adding these lines around the definition of `size_t': #ifndef _SIZE_T #define _SIZE_T ACTUAL TYPEDEF HERE #endif * On the Alliant, the system's own convention for returning structures and unions is unusual, and is not compatible with GNU CC no matter what options are used. * On the IBM RT PC, the MetaWare HighC compiler (hc) uses yet another convention for structure and union returning. Use `-mhc-struct-return' to tell GNU CC to use a convention compatible with it. * On Ultrix, the Fortran compiler expects registers 2 through 5 to be saved by function calls. However, the C compiler uses conventions compatible with BSD Unix: registers 2 through 5 may be clobbered by function calls. GNU CC uses the same convention as the Ultrix C compiler. You can use these options to produce code compatible with the Fortran compiler: -fcall-saved-r2 -fcall-saved-r3 -fcall-saved-r4 -fcall-saved-r5 * On the WE32k, you may find that programs compiled with GNU CC do not work with the standard shared C ilbrary. You may need to link with the ordinary C compiler. If you do so, you must specify the following options: -L/usr/local/lib/gcc-lib/we32k-att-sysv/2.3 -lgcc -lc_s The first specifies where to find the library `libgcc.a' specified with the `-lgcc' option. GNU CC does linking by invoking `ld', just as `cc' does, and there is no reason why it *should* matter which compilation program you use to invoke `ld'. If someone tracks this problem down, it can probably be fixed easily. File: gcc.info, Node: Incompatibilities, Next: Disappointments, Prev: Interoperation, 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'. File: gcc.info, Node: Disappointments, Next: Protoize Caveats, Prev: Incompatibilities, 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. * 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 option `-ffloat-store' (*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 resgister. 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: Protoize Caveats, Next: Non-bugs, Prev: Disappointments, 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, 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 dialect you want with the option `-fsigned-bitfields' or `-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. 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. 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 one of these addresses: bug-gcc@prep.ai.mit.edu {ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-gcc Send bug reports for GNU C++ to one of these addresses: bug-g++@prep.ai.mit.edu {ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-g++ *Do not send bug reports to `help-gcc', or to the newsgroup `gnu.gcc.help'.* Most users of GNU CC do not want to receive bug reports. Those that do, have asked to be on `bug-gcc' and/or `bug-g++'. The mailing lists `bug-gcc' and `bug-g++' both have newsgroups which serve as repeaters: `gnu.gcc.bug' and `gnu.g++.bug'. Each mailing list and its newsgroup carry exactly the same messages. Often people think of posting bug reports to the newsgroup instead of mailing them. This appears to work, but it has one problem which can be crucial: a newsgroup posting does not contain a mail path back to the sender. Thus, if maintainers need more information, they may be unable to reach you. For this reason, you should always send bug reports by mail to the proper mailing list. As a last resort, send bug reports on paper to: GNU Compiler Bugs Free Software Foundation 675 Mass Ave Cambridge, MA 02139 File: gcc.info, Node: Bug Reporting, Next: Sending Patches, Prev: Bug Lists, Up: Bugs How to Report Bugs ================== The fundamental principle of reporting bugs usefully is this: *report all the facts*. If you are not sure whether to state a fact or leave it out, state it! Often people omit facts because they think they know what causes the problem and they conclude that some details don't matter. Thus, you might assume that the name of the variable you use in an example does not matter. Well, probably it doesn't, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the compiler into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful. Keep in mind that the purpose of a bug report is to enable someone to fix the bug if it is not known. It isn't very important what happens if the bug is already known. Therefore, always write your bug reports on the assumption that the bug is not known. Sometimes people give a few sketchy facts and ask, "Does this ring a bell?" This cannot help us fix a bug, so it is basically useless. We respond by asking for enough details to enable us to investigate. You might as well expedite matters by sending them to begin with. Try to make your bug report self-contained. If we have to ask you for more information, it is best if you include all the previous information in your response, as well as the information that was missing. 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. * 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.