Moving to AMD64 bit and porting issues

[Ian Lance Taylor]

"Ernest L. Williams Jr." <ernesto@ornl.gov> writes:

> Well, then what we are experiencing:
> 0 is an int and most likely 32 bits, 
> NULL is a pointer and 64 bits in the 64-bit architecture.
> 
> 
> This then must be the problem, which makes using 0 to represent a NULL
> pointer not portable?  Or would this be considered a bug with GCC for a
> 64-bit environment?

You can reliably use 0 as a null pointer constant with one exception.
When calling a varargs function which expects a pointer as an unnamed
argument, and when pointers are 64-bits but int is 32-bits, then you
must either use NULL or you must cast 0 to a pointer type.

For example:

char *
foo (int i, ...) 
{
  va_list va;
  va_start (va, i);
  char *p = va_arg (va, char *);
  va_end (va);
  return p;
}

char *bar () { return foo (1, 0); }

This will do the wrong thing if pointers are 64-bits.  You must do
this instead:

char *bar () { return foo (1, (char *) 0); }

This is not a bug in gcc.

Ian