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(gcc-300.info)Disappointments

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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.