ssize_t for Visual Studio

The POSIX C type ssize_t is available on Unix-like systems in <sys/types.h>. Windows Visual Studio BaseTsd.h has SSIZE_T.

However, ssize_t is POSIX, but not C standard. It’s possible to define a signed size type “ssize_t” using “ptrdiff_t” for “ssize_t” in C and C++. Using ptrdiff_t instead of ssize_t is the practice of major projects like Emacs.

size_t bit width is guaranteed by C and C++ standards to have bit width not less than 16.

ptrdiff_t bit width is guaranteed by C standard to have bit width not less than 16, and C++ standard to have bit width not less than 17.

This example shows how to use ssize_t across computing platforms.

Related: C++ size_type property vs size_t