signed size_t/unsigned ptrdiff_t?

Originally Posted by C99 draft, fprintf

z Specifies that a following d, i, o, u, x, or X conversion specifier applies to a size_t or the corresponding signed integer type argument; or that a following n conversion specifier applies to a pointer to a signed integer type corresponding to size_t argument.

t Specifies that a following d, i, o, u, x, or X conversion specifier applies to a ptrdiff_t or the corresponding unsigned integer type argument; or that a following n conversion specifier applies to a pointer to a ptrdiff_t argument.

What does "corresponding signed/unsigned type" mean? I tried to make a signed size_t and an unsigned ptrdiff_t type, but GCC wouldn't have it.

Code:

$ cat file.c
#include <stddef.h> 

int main ()
{
        signed size_t x;
        unsigned ptrdiff_t y;

        return 0;
}
$ gcc -std=c99 -pedantic file.c
file.c: In function 'main':
file.c:5: warning: ISO C forbids nested functions
file.c:5: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'x'
file.c:5: error: 'x' undeclared (first use in this function)
file.c:5: error: (Each undeclared identifier is reported only once
file.c:5: error: for each function it appears in.)
file.c:6: warning: ISO C forbids nested functions
file.c:6: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'y'
file.c:6: error: 'y' undeclared (first use in this function)
$

size_t is defined as an unsigned type, and ptrdiff_t is defined as a signed type, period. They are almost certainly typedef's of existing types, so the corresponding signed/unsigned type would be ... whatever the corresponding type is (long? long long?) But if you do %zd with a size_t variable, it will be printed as signed, since that's what d does, and if you do %tx with a ptrdiff_t variable, it will be printed as unsigned, since that's what x does.

I think what they mean by "... or the corresponding signed integer type argument..." is that the format string may have a number of such type specifiers, each applying (AKA corresponding) to a respective argument.

fprintf(fp, "%d %u", abc, def)

Two conversion specifiers, each corresponds to their respective arguments. The first corresponds to 'abc', the second to 'def' and so on.

Thanks all.