Pass separators to C programs (Apache 2.0)

prog.c

#include <stddef.h>
#include <stdio.h>
#include <string.h>

#ifdef _WIN32
    #define SEP "\r\n"
#else
    #define SEP "\n"
#endif

int main(int argc, char *argv[])
{
    if (argc < 2) {
        return 0;
    }

    size_t index = 0;
    char *sep;
    if (strcmp(argv[1], "-s") == 0) {
        if (argc < 3) {
            fprintf(stderr, "No valid separator\n");
            return 1;
        }
        sep = argv[2];
        index = 3;
    }
    else {
        sep = SEP;
        index = 1;
    }

    for (size_t i = index; i < argc; i++) {
        if (i < argc -1) {
            printf("%s%s", argv[i], sep);
        }
        else {
            printf("%s", argv[i]);
        }
    }

    printf("%s", SEP);

    return 0;
}