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INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* gcc: (gcc).                  The GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the use and the internals of the GNU compiler.


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   A GNU Manual

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   You have freedom to copy and modify this GNU Manual, like GNU
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File: gcc.info,  Node: Incompatibilities,  Next: Fixed Headers,  Prev: External Bugs,  Up: Trouble

Incompatibilities of GCC
========================

   There are several noteworthy incompatibilities between GNU C and K&R
(non-ISO) versions of C.  The `-traditional' option eliminates many of
these incompatibilities, _but not all_, by telling GCC to behave like a
K&R C compiler.

   * GCC normally makes string constants read-only.  If several
     identical-looking string constants are used, GCC 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 GCC 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 ISO C rules) its data type is `unsigned long int'.
     Negating this value yields 2147483648 again.

   * GCC does not substitute macro arguments when they appear inside of
     string constants.  For example, the following macro in GCC

          #define foo(a) "a"

     will produce output `"a"' regardless of what the argument A is.

     The `-traditional' option directs GCC to handle such cases (among
     others) in the old-fashioned (non-ISO) 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 GCC thinks such a problem might be possible.

     The `-traditional' option directs GCC 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 preprocessing directives in the middle of macro
     arguments do not work with GCC.  For example, a program like this
     will not work:

          foobar (
          #define luser
                  hack)

     ISO 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.

   * K&R compilers allow comments to cross over an inclusion boundary
     (i.e. started in an include file and ended in the including file).
     I think this would be quite ugly and can't imagine it could be
     needed.

   * 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 GCC 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 ISO 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.

   * When in `-traditional' mode, GCC allows the following erroneous
     pair of declarations to appear together in a given scope:

          typedef int foo;
          typedef foo foo;

   * GCC treats all characters of identifiers as significant, even when
     in `-traditional' mode.  According to K&R-1 (2.2), "No more than
     the first eight characters are significant, although more may be
     used.".  Also according to K&R-1 (2.2), "An identifier is a
     sequence of letters and digits; the first character must be a
     letter.  The underscore _ counts as a letter.", but GCC also
     allows dollar signs in identifiers.

   * PCC allows whitespace in the middle of compound assignment
     operators such as `+='.  GCC, following the ISO standard, does not
     allow this.  The difficulty described immediately above applies
     here too.

   * GCC complains about unterminated character constants inside of
     preprocessing conditionals that fail.  Some programs have English
     comments enclosed in conditionals that are guaranteed to fail; if
     these comments contain apostrophes, GCC 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 ISO 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 appropriate system
     headers (`<time.h>' on systems with ISO C headers) and not to
     declare `time' if the system header files declare it, or failing
     that to use `time_t' as the return type of `time'.

   * When compiling functions that return `float', PCC converts it to a
     double.  GCC 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, GCC
     output code normally uses a method different from that used on most
     versions of Unix.  As a result, code compiled with GCC cannot call
     a structure-returning function compiled with PCC, and vice versa.

     The method used by GCC 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 GCC 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.  GCC 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.  GCC 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 GCC to use a compatible convention for all structure
     and union returning with the option `-fpcc-struct-return'.

   * GCC 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, GCC prints an error message.  Although it may appear
     obvious that what is meant is an operator and two values, the ISO
     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+', `E-', `p+', `p-', `P+', or `P-'
     character sequences.  (In strict C89 mode, the sequences `p+',
     `p-', `P+' and `P-' cannot appear in preprocessing numbers.)

     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: Standard Libraries,  Prev: Incompatibilities,  Up: Trouble

Fixed Header Files
==================

   GCC 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 GCC unless they are changed.  Some have bugs, some are
incompatible with ISO C, and some depend on special features of other
compilers.

   Installing GCC 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 GCC are not
     automatically updated.  The easiest way to update them is to
     reinstall GCC.  (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, GCC 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 GCC's fault,
     but it does mean that there's nothing for us to do about them.


File: gcc.info,  Node: Standard Libraries,  Next: Disappointments,  Prev: Fixed Headers,  Up: Trouble

Standard Libraries
==================

   GCC by itself attempts to be a conforming freestanding
implementation.  *Note Language Standards Supported by GCC: Standards,
for details of what this means.  Beyond the library facilities required
of such an implementation, the rest of the C library is supplied by the
vendor of the operating system.  If that C library doesn't conform to
the C standards, then your programs might get warnings (especially when
using `-Wall') that you don't expect.

   For example, the `sprintf' function on SunOS 4.1.3 returns `char *'
while the C standard says that `sprintf' returns an `int'.  The
`fixincludes' program could make the prototype for this function match
the Standard, but that would be wrong, since the function will still
return `char *'.

   If you need a Standard compliant library, then you need to find one,
as GCC does not provide one.  The GNU C library (called `glibc')
provides ISO C, POSIX, BSD, SystemV and X/Open compatibility for
GNU/Linux and HURD-based GNU systems; no recent version of it supports
other systems, though some very old versions did.  Version 2.2 of the
GNU C library includes nearly complete C99 support.  You could also ask
your operating system vendor if newer libraries are available.


File: gcc.info,  Node: Disappointments,  Next: C++ Misunderstandings,  Prev: Standard Libraries,  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 GCC 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 GCC 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 GCC 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 ISO 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 ISO C just to avoid an
     error for the example shown above.

   * Accesses to bit-fields 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
     bit-field; it may even vary for a given bit-field according to the
     precise usage.

     If you care about controlling the amount of memory that is
     accessed, use volatile but do not use bit-fields.

   * GCC 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 GCC 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 GCC 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 and x86 systems, for instance, 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
     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 ISO standard `stdarg.h' method
     of variable arguments, and the prototype is in scope at the time
     of the call, everything will work fine.

   * On the H8/300 and H8/300H, variable argument functions must be
     implemented using the ISO standard `stdarg.h' method of variable
     arguments.  Furthermore, calls to functions using `stdarg.h'
     variable arguments must have a prototype for the called function
     in scope at the time of the call.

   * On AIX and other platforms without weak symbol support, templates
     need to be instantiated explicitly and symbols for static members
     of templates will not be generated.


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 ISO C++ standard) was only recently completed.  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
* Copy Assignment::     Copy Assignment operators copy virtual bases twice


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 ISO 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,  Next: Copy Assignment,  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 string classes,
especially ones that define a conversion function to type `char *' or
`const char *'--which is one reason why the standard `string' class
requires you to call the `c_str' member function.  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 *);
     
     void
     f ()
     {
       const char *p = strfunc().c_str();
       ...
       charfunc (p);
       ...
       charfunc (p);
     }

In this situation, it may seem reasonable to save a pointer to the C
string returned by the `c_str' member function and use that rather than
call `c_str' repeatedly.  However, the temporary string created by the
call to `strfunc' is destroyed after `p' is initialized, at which point
`p' is left pointing to freed memory.

   Code like this may run successfully under some other compilers,
particularly obsolete cfront-based compilers that delete temporaries
along with normal local variables.  However, the GNU C++ behavior is
standard-conforming, so if your program depends on late destruction of
temporaries it is not portable.

   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.c_str ());


File: gcc.info,  Node: Copy Assignment,  Prev: Temporaries,  Up: C++ Misunderstandings

Implicit Copy-Assignment for Virtual Bases
------------------------------------------

   When a base class is virtual, only one subobject of the base class
belongs to each full object.  Also, the constructors and destructors are
invoked only once, and called from the most-derived class.  However,
such objects behave unspecified when being assigned.  For example:

     struct Base{
       char *name;
       Base(char *n) : name(strdup(n)){}
       Base& operator= (const Base& other){
        free (name);
        name = strdup (other.name);
       }
     };
     
     struct A:virtual Base{
       int val;
       A():Base("A"){}
     };
     
     struct B:virtual Base{
       int bval;
       B():Base("B"){}
     };
     
     struct Derived:public A, public B{
       Derived():Base("Derived"){}
     };
     
     void func(Derived &d1, Derived &d2)
     {
       d1 = d2;
     }

   The C++ standard specifies that `Base::Base' is only called once
when constructing or copy-constructing a Derived object.  It is
unspecified whether `Base::operator=' is called more than once when the
implicit copy-assignment for Derived objects is invoked (as it is
inside `func' in the example).

   g++ implements the "intuitive" algorithm for copy-assignment: assign
all direct bases, then assign all members.  In that algorithm, the
virtual base subobject can be encountered many times.  In the example,
copying proceeds in the following order: `val', `name' (via `strdup'),
`bval', and `name' again.

   If application code relies on copy-assignment, a user-defined
copy-assignment operator removes any uncertainties.  With such an
operator, the application can define whether and how the virtual base
subobject is assigned.


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.  ISO 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 preprocessing
     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 recommend 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 GCC 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 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.

   * 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 bit-fields unsigned by default on particular machines where
     "the ABI standard" says to do so.

     The ISO C standard leaves it up to the implementation whether a
     bit-field 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 bit-fields 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
     bit-fields as well.

     Some computer manufacturers have published Application Binary
     Interface standards which specify that plain bit-fields should be
     unsigned.  It is a mistake, however, to say anything about this
     issue in an ABI.  This is because the handling of plain bit-fields
     distinguishes two dialects of C.  Both dialects are meaningful on
     every type of machine.  Whether a particular object file was
     compiled using signed bit-fields or unsigned is of no concern to
     other object files, even if they access the same bit-fields 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 bit-fields
     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 GCC does and will treat plain bit-fields in the same
     fashion on all types of machines (by default).

     There are some arguments for making bit-fields unsigned by default
     on all machines.  If, for example, this becomes a universal de
     facto standard, it would make sense for GCC 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 bit-field 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, GCC 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 ISO C, such as function
     prototypes or ISO token concatenation.  Since plain `gcc' supports
     all the features of ISO 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 ISO C program, because the ISO 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 ISO C library.

     Sometimes people say that defining `__STDC__' in a compiler that
     does not completely conform to the ISO C standard somehow violates
     the standard.  This is illogical.  The standard is a standard for
     compilers that claim to support ISO C, such as `gcc -ansi'--not
     for other compilers such as plain `gcc'.  Whatever the ISO C
     standard says is relevant to the design of plain `gcc' without
     `-ansi' only for pragmatic reasons, not as a requirement.

     GCC normally defines `__STDC__' to be 1, and in addition defines
     `__STRICT_ANSI__' if you specify the `-ansi' option, or a `-std'
     option for strict conformance to some version of ISO C.  On some
     hosts, system include files use a different convention, where
     `__STDC__' is normally 0, but is 1 if the user specifies strict
     conformance to the C Standard.  GCC follows the host convention
     when processing system include files, but when processing user
     files it follows the usual GNU C convention.

   * 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 ISO 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 ISO 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.

     Historically, GCC has not deleted "empty" loops under the
     assumption that the most likely reason you would put one in a
     program is to have a delay, so deleting them will not make real
     programs run any faster.

     However, the rationale here is that optimization of a nonempty loop
     cannot produce an empty one, which holds for C but is not always
     the case for C++.

     Moreover, with `-funroll-loops' small "empty" loops are already
     removed, so the current behavior is both sub-optimal and
     inconsistent and will change in the future.

   * Making side effects happen in the same order as in some other
     compiler.

     It is never safe to depend on the order of evaluation of side
     effects.  For example, a function call like this may very well
     behave differently from one compiler to another:

          void func (int, int);
          
          int i = 2;
          func (i++, i++);

     There is no guarantee (in either the C or the C++ standard language
     definitions) that the increments will be evaluated in any
     particular order.  Either increment might happen first.  `func'
     might get the arguments `2, 3', or it might get `3, 2', or even
     `2, 2'.

   * Not allowing structures with volatile fields in registers.

     Strictly speaking, there is no prohibition in the ISO 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.

   * Making certain warnings into errors by default.

     Some ISO C testsuites report failure when the compiler does not
     produce an error message for a certain program.

     ISO C requires a "diagnostic" message for certain kinds of invalid
     programs, but a warning is defined by GCC to count as a
     diagnostic.  If GCC produces a warning but not an error, that is
     correct ISO C support.  If test suites call this "failure", they
     should be run with the GCC option `-pedantic-errors', which will
     turn these warnings into errors.



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.  GCC 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).

   GCC always tries to compile your program if possible; it never
gratuitously rejects a program whose meaning is clear merely because
(for instance) it fails to conform to a standard.  In some cases,
however, the C and C++ standards specify that certain extensions are
forbidden, and a diagnostic _must_ be issued by a conforming compiler.
The `-pedantic' option tells GCC to issue warnings in such cases;
`-pedantic-errors' says to make them errors instead.  This does not
mean that _all_ non-ISO 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 GCC 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 GCC work
better.  Bug reports are your contribution to the maintenance of GCC.

   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.
* GNATS: gccbug.             You can use a bug reporting tool.
* 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 GCC 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; GCC 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 one of the languages GCC
     supports, your suggestions for improvement of GCC 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 the GNU Compiler Collection to
<gcc-bugs@gcc.gnu.org>.  In accordance with the GNU-wide convention, in
which bug reports for tool "foo" are sent to `bug-foo@gnu.org', the
address <bug-gcc@gnu.org> may also be used; it will forward to the
address given above.

   Please read `http://gcc.gnu.org/bugs.html' for additional and/or
more up-to-date bug reporting instructions before you post a bug report.

