Search code examples
cgccvariadic

How does gcc (x64) deal with types/sizes in variadic functions?

A variadic function and main()

#include <stdio.h>
#include <stdarg.h>
int f(long x,...)
{ va_list ap;
  int i=0;
  va_start(ap,x);
  while(x)
  { i++;
    printf("%ld ", x);
    x=va_arg(ap,long);
  }
  va_end(ap);
  printf("\n");
  return i;
}

int main()
{ return f(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,1L<<63,0);
}

On gcc, linux and x64: even though f()'s arguments are not cast to a 64bit long, gcc seems to get it right.

$ gcc t.c && ./a.out
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 -9223372036854775808

How?

Solution

  • Arguments to a variadic function are "promoted" to 64 bit values on linux x64 so there's no need to explicitly cast up to a 64bit value on this platform.