c - What is the difference between ssize_t and ptrdiff_t?

Question

The C standard (ISO/IEC 9899:2011 or 9899:1999) defines a type ptrdiff_t in <stddef.h>.

The POSIX standard (ISO/IEC 9945; IEEE Std 1003.1-2008) defines a type ssize_t in <sys/types.h>.

• What is the difference between these types (or why were both deemed necessary)?
• Is there an implementation where the underlying base type for ssize_t is not the same as for ptrdiff_t?

Answer 1

Is there an implementation where the underlying base type for ssize_t is not the same as for ptrdiff_t?

x86-16 with the large memory model. Pointers are far (32-bit), but individual objects are limited to one segment (so size_t is allowed to be 16-bit).