lseek sparse files

lseek.c

#define _FILE_OFFSET_BITS 64
#define _GNU_SOURCE 1

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

#ifndef SEEK_DATA
#warning "SEEK_DATA is undeclared and manually defined."
#define SEEK_DATA 3 /* seek to the next data */
#endif
#ifndef SEEK_HOLE
#warning "SEEK_HOLE is undeclared and manually defined."
#define SEEK_HOLE 4 /* seek to the next hole */
#endif

int main(int argc, char **argv)
{
    if (argc < 2) {
        fprintf(stderr, "Usage: %s file [offset:0] [whence:3]\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    int fd = open(argv[1], O_RDONLY);
    if(fd == -1) {
        perror("failed to open the file.");
        exit(EXIT_FAILURE);
    }

    off_t offset = 0;
    if (argc >= 3) {
        offset = atoi(argv[2]);
    }

    int whence = SEEK_DATA;
    if (argc >= 4) {
        whence = atoi(argv[3]);
    }

    off_t result = lseek(fd, offset, whence);
    fprintf(stdout, "%lld\n", (unsigned long long)result);
}