Given a list of network interfaces, report back which interfaces are missing

missing_interfaces.c

#include <stdio.h>
#include <sysexits.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <net/if.h>

int
main(int argc, char *argv[]) {
    int i;
    int result;
    if (argc == 1) {
        fprintf(stderr, "usage: %s [ifname ...]\n", argv[0]);
        fprintf(stderr, "returns 0 if all ifname exist\n"
            "or prints each ifname that doesn't exist\n");
        return EX_USAGE;
    } else {
        result = EX_OK;
        for (i=1; i < argc; i++)
            if (if_nametoindex(argv[i]) == 0) {
                printf("%s\n", argv[i]);
                result = 1;
            }
        return result;
    }
}