realpath.c

#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
    if (argc != 2) {
        fprintf(stderr, "Usage: %s path\n", argv[0]);
        return 1;
    }

    char path[PATH_MAX] = { 0 };
    puts(realpath(argv[1], path));

    return 0;
}

For mac OS.