Using and Porting GNU CC

7.8: Disappointments and Misunderstandings

These problems are perhaps regrettable, but we don't know any practical way around them.

• Certain local variables aren't recognized by debuggers when you compile with optimization.

  This occurs because sometimes GNU CC optimizes the variable out of existence. There is no way to tell the debugger how to compute the value such a variable ``would have had'', and it is not clear that would be desirable anyway. So GNU CC simply does not mention the eliminated variable when it writes debugging information.

  You have to expect a certain amount of disagreement between the executable and your source code, when you use optimization.

• Users often think it is a bug when GNU CC reports an error for code like this:

  int foo (struct mumble *);
  
  struct mumble { ... };
  
  int foo (struct mumble *x)
  { ... }
  

  This code really is erroneous, because the scope of struct mumble in the prototype is limited to the argument list containing it. It does not refer to the struct mumble defined with file scope immediately below---they are two unrelated types with similar names in different scopes.

  But in the definition of foo, the file-scope type is used because that is available to be inherited. Thus, the definition and the prototype do not match, and you get an error.

  This behavior may seem silly, but it's what the ANSI standard specifies. It is easy enough for you to make your code work by moving the definition of struct mumble above the prototype. It's not worth being incompatible with ANSI C just to avoid an error for the example shown above.

• Accesses to bitfields even in volatile objects works by accessing larger objects, such as a byte or a word. You cannot rely on what size of object is accessed in order to read or write the bitfield; it may even vary for a given bitfield according to the precise usage.

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

• GNU CC comes with shell scripts to fix certain known problems in system header files. They install corrected copies of various header files in a special directory where only GNU CC will normally look for them. The scripts adapt to various systems by searching all the system header files for the problem cases that we know about.

  If new system header files are installed, nothing automatically arranges to update the corrected header files. You will have to reinstall GNU CC to fix the new header files. More specifically, go to the build directory and delete the files `stmp-fixinc' and `stmp-headers', and the subdirectory include; then do `make install' again.

• On 68000 systems, you can get paradoxical results if you test the precise values of floating point numbers. For example, you can find that a floating point value which is not a NaN is not equal to itself. This results from the fact that the the floating point registers hold a few more bits of precision than fit in a double in memory. Compiled code moves values between memory and floating point registers at its convenience, and moving them into memory truncates them.

  You can partially avoid this problem by using the `-ffloat-store' option (see Optimize Options).

• On the MIPS, variable argument functions using `varargs.h' cannot have a floating point value for the first argument. The reason for this is that in the absence of a prototype in scope, if the first argument is a floating point, it is passed in a floating point register, rather than an integer register.

  If the code is rewritten to use the ANSI standard `stdarg.h' method of variable arguments, and the prototype is in scope at the time of the call, everything will work fine.