OpenWrt – Rev 4

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/*
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License,
 * version 2 as published by the Free Software Foundation.
 *
 * (C) John Crispin <blogic@openwrt.org>
 */

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdint.h>
#include <unistd.h>
#include <errno.h>

int main(int argc, char **argv)
{
        uint32_t t = 0, sum = 0x55aa55aa;
        int fd;

        if ((argc != 2) || ((fd = open(argv[1], O_RDWR)) == -1)) {
                fprintf(stderr, "Usage: %s input_file\n", *argv);
                return -EINVAL;
        }

        lseek(fd, -4, SEEK_END);
        write(fd, &t, 4);
        lseek(fd, 0, SEEK_SET);

        while (read(fd, &t, 4) > 0)
                sum -= t;

        lseek(fd, -4, SEEK_END);
        write(fd, &sum, 4);
        close(fd);

        return 0;
}