/////////////////////////////////////////////////////////////////////////////
// Creation 11/12/2003                                               
// 
// 
//                            LOCKDEMO.C
//                            ----------
// 
// 
// Sylvain MARECHAL - sylvain.marechal1@libertysurf.fr
/////////////////////////////////////////////////////////////////////////////
// 
//  Show how to lock a file under Unix, using fcntl()
// 
/////////////////////////////////////////////////////////////////////////////
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>

/*
l_type =>
    F_RDLCK = 0
    F_WRLCK = 1
    F_UNLCK = 2
*/

int main(int argc, char *argv[])
{
    	             /* l_type   l_whence  l_start  l_len  l_pid   */
    struct flock fl = { F_WRLCK, SEEK_SET, 0,       0,     0 };
    int fd, fd2, fd3;

    fl.l_pid = getpid();

    if (argc > 1) 
    	    fl.l_type = F_RDLCK;

    if ((fd = open("lockdemo.c", O_RDWR)) == -1) {
    	    perror("open");
    	    exit(1);
    }
    
    if ((fd2 = open("lockdemo.c", O_RDWR)) == -1) {
    	    perror("open");
    	    exit(1);
    }

    printf( "Reading lock information ...\n" );    
    fl.l_pid = 1;
    if (fcntl(fd2, F_GETLK, &fl) == -1) {
    	    perror("fcntl");
    	    exit(1);
    }
    printf("lock information : (l_type=%d, l_whence=%d, l_start=%d, l_len=%d, l_pid=%d)\n", fl.l_type, fl.l_whence, fl.l_start, fl.l_len, fl.l_pid);

    fl.l_type = F_WRLCK;
    printf("Press <RETURN> to try to get lock: ");
    getchar();
    printf("Trying to get lock...");

    if (fcntl(fd, F_SETLKW, &fl) == -1) {
    	    perror("fcntl");
    	    exit(1);
    }

    printf("got lock (l_type=%d, l_whence=%d, l_start=%d, l_len=%d, l_pid=%d)\n", fl.l_type, fl.l_whence, fl.l_start, fl.l_len, fl.l_pid);

    printf( "Reading lock information ...\n" );    
    if ((fd3 = open("lockdemo.c", O_RDWR)) == -1) {
    	    perror("open");
    	    exit(1);
    }
    fl.l_type = F_WRLCK;
    fl.l_pid = 2;
    if (fcntl(fd3, F_GETLK, &fl) == -1) {
    	    perror("fcntl");
    	    exit(1);
    }
    printf("lock information : (l_type=%d, l_whence=%d, l_start=%d, l_len=%d, l_pid=%d)\n", fl.l_type, fl.l_whence, fl.l_start, fl.l_len, fl.l_pid);

    printf("Press <RETURN> to release lock: ");
    getchar();

    fl.l_type = F_UNLCK;  /* set to unlock same region */

    if (fcntl(fd, F_SETLK, &fl) == -1) {
    	    perror("fcntl");
    	    exit(1);
    }

    printf("Unlocked.\n");

    close(fd);
}