Alternative to ssize_t on POSIX-unconformant systems

Question

I'm writing a program involving network I/O, so send and recv are used, which are POSIX functions. They return a ssize_t, which is POSIX-specific too.
The wrappers look like this ATM:

ssize_t sock_send(int sock, const void* msg, size_t len) {
    return send(sock, msg, len, 0);
}

Even though I'm heavily depending on POSIX in my current implementation, I want to make the interface stick closely to the standard because I am planning on writing a Windows implementation later, where POSIX isn't necessarily available (dammit, Windows!).

What would be a good substitution for ssize_t as specified by the C11 standard? Perhaps ptrdiff_t?
Or how should I approach this issue otherwise?

Answer 1

If the type ssize_t is not defined, you can just define it yourself. It is supposed to be a signed type with the same size as size_t. Technically, the type ptrdiff_t should not be smaller than size_t, but it could be larger to accommodate for the larger range.

Here is a portable way to define it:

#include <limits.h>
#include <stddef.h>
#include <inttypes.h>
#include <stdint.h>

#if SIZE_MAX == UINT_MAX
typedef int ssize_t;        /* common 32 bit case */
#define SSIZE_MIN  INT_MIN
#define SSIZE_MAX  INT_MAX
#elif SIZE_MAX == ULONG_MAX
typedef long ssize_t;       /* linux 64 bits */
#define SSIZE_MIN  LONG_MIN
#define SSIZE_MAX  LONG_MAX
#elif SIZE_MAX == ULLONG_MAX
typedef long long ssize_t;  /* windows 64 bits */
#define SSIZE_MIN  LLONG_MIN
#define SSIZE_MAX  LLONG_MAX
#elif SIZE_MAX == USHRT_MAX
typedef short ssize_t;      /* is this even possible? */
#define SSIZE_MIN  SHRT_MIN
#define SSIZE_MAX  SHRT_MAX
#elif SIZE_MAX == UINTMAX_MAX
typedef uintmax_t ssize_t;  /* last resort, chux suggestion */
#define SSIZE_MIN  INTMAX_MIN
#define SSIZE_MAX  INTMAX_MAX
#else
#error platform has exotic SIZE_MAX
#endif