A little program that converts a cidr ip/size into a base/mask pair for tcp wrappers

cidr.c

#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdarg.h>
#include <string.h>
#include <libgen.h>
#include <unistd.h>
#include <stdlib.h>

char *pgm = "cidr";

void
die(char *why, ...)
{
    if (why) {
	    va_list args;
	    va_start(args,why);
	    fprintf(stderr, "%s: ", pgm);
	    vfprintf(stderr, why, args);
	    va_end(args);
    }
    else
	    fprintf(stderr, "usage: %s ip/width\n", pgm);
    exit(1);
}


int
main(argc, argv)
char **argv;
{
    char *p;
    int size;
    struct in_addr check;
    in_addr_t addr;
    in_addr_t netmask;

    pgm = basename(argv[0]);

    if ( argc != 2 ) 
	    die(0);

    if ( (p = strchr(argv[1], '/')) == 0 )
	    die(0);

    *p++ = 0;

    if ( (size = atoi(p)) < 1 || size > 32 )
	    die("width needs to be between 1 & 32");

    if ( inet_aton(argv[1], &check) == 0 )
	    die("incomprehensible IP address %s", argv[1]);

    /* sanity checking by inet_aton() call above */
    addr = ntohl(inet_addr(argv[1]));

    netmask = (-1) << (32-size);

    addr &= netmask;

    check.s_addr = htonl(addr);
    printf("%s/", inet_ntoa(check));
    check.s_addr = htonl(netmask);
    printf("%s\n", inet_ntoa(check));
    return 0;
}