Use ssize_t on POSIX systems and with MSVC compilers

mysize.h

#include <sys/types.h>

#ifndef ssize_t
#ifdef _WIN32
#include <BaseTsd.h>
#define ssize_t SSIZE_T
#else
#define ssize_t ptrdiff_t
#endif
#endif

ssize_t.c

#include <stdint.h> /* C23: SIZE_WIDTH, PTRDIFF_WIDTH */
#include <stddef.h>
#include <stdio.h>
#include <sys/types.h>

#include "mysize.h"


int main(void){

  size_t s;
  ssize_t t;

#ifdef SIZE_WIDTH
  printf("SIZE_WIDTH: %d  PTRDIFF_WIDTH %d\n", SIZE_WIDTH, PTRDIFF_WIDTH);
#endif
  printf("sizeof(size_t): %zu sizeof(ssize_t): %zu\n", sizeof(s), sizeof(t));

  return 0;
}

ssize_t.cpp

#include <cstdint> // C23: SIZE_WIDTH, PTRDIFF_WIDTH
#include <cstddef> // std::size_t
#include <bit> // std::bit_width
#include <iostream>
#include <sys/types.h>

#include "mysize.h"


int main()
{
    size_t s;
    ssize_t t;

#ifdef SIZE_WIDTH
    std::cout << "SIZE_WIDTH: " << SIZE_WIDTH << " PTRDIFF_WIDTH: " << PTRDIFF_WIDTH << std::endl;
#endif
#ifdef __cpp_lib_int_pow2
    std::cout << "std::bit_width(size_t): " << std::bit_width(SIZE_WIDTH) << " std::bit_width(ssize_t): " << std::bit_width(PTRDIFF_WIDTH) << std::endl;
#endif
    std::cout <<"sizeof(size_t): " << sizeof(s) << " sizeof(ssize_t): " << sizeof(t) << std::endl;
    return 0;
}