Where is ptrdiff_t defined in C?

It's defined in stddef.h.

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That header defines the integral types size_t, ptrdiff_t, and wchar_t, the functional macro offsetof, and the constant macro NULL.

It is defined by the POSIX standard: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stddef.h.html Where the type is exactly may be implemetation-specific, but interface is stddef.h