#include <unistd.h>#include <stdlib.h>#include <stdio.h>#include <fcntl.h>const char *test_file = "/tmp/test_lock";int main() {    int file_desc;    int byte_count;    char *byte_to_write = "A";    struct flock region_1;    struct flock region_2;    int res;        /* open a file descriptor */    file_desc = open(test_file, O_RDWR | O_CREAT, 0666);    if (!file_desc) {        fprintf(stderr, "Unable to open %s for read/write\n", test_file);        exit(EXIT_FAILURE);    }        /* put some data in the file */    for(byte_count = 0; byte_count < 100; byte_count++) {        (void)write(file_desc, byte_to_write, 1);    }        /* setup region 1, a shared lock, from bytes 10 -> 30 */    region_1.l_type = F_RDLCK;    region_1.l_whence = SEEK_SET;    region_1.l_start = 10;    region_1.l_len = 20;             /* setup region 2, an exclusive lock, from bytes 40 -> 50 */    region_2.l_type = F_WRLCK;    region_2.l_whence = SEEK_SET;    region_2.l_start = 40;    region_2.l_len = 10;        /* now lock the file */    printf("Process %d locking file\n", getpid());    res = fcntl(file_desc, F_SETLK, &region_1);    if (res == -1) fprintf(stderr, "Failed to lock region 1\n");    res = fcntl(file_desc, F_SETLK, &region_2);    if (res == -1) fprintf(stderr, "Failed to lock region 2\n");            /* and wait for a while */    sleep(60);    printf("Process %d closing file\n", getpid());        close(file_desc);    exit(EXIT_SUCCESS);}