C: Which character should be used for ptrdiff_t in printf?

Which character should be used for ptrdiff_t in printf?

Does C standard clearly explains how to print ptrdiff_t in printf? I haven't found any one.

int a = 1;
int b = 2;

int* pa = &a;
int* pb = &b;

ptrdiff_t diff = b - a;

printf("diff = %?", diff); // % what?

It's %td. See here.

C11 draft explains the length modifier for ptrdiff_t in 7.21.6.1 7 "The fprintf function"

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.

Use "%td" as in the following: Credit: @trojanfoe

ptrdiff_t diff = b - a;
printf("diff = %td", diff);

If the compiler does not support "%td", cast to a signed type - the longer, the better. Then insure the alternative format and argument match.

// Note the cast
printf("diff = %lld", (long long) diff); // or
printf("diff = %ld", (long) diff);

Ref format specifiers

Use %td and if your compiler does not support it, you should try %ld (also cast the input to long).