Try to acquire lock on a given resource (file) on fs using the flock

fcntl_lock.c

#include<stdlib.h>
#include<unistd.h>
#include<errno.h>
#include<fcntl.h>

int main(int argc, char **argv)
{

int fd;
struct flock fl;

if (argc <= 1) {
    printf("Provide at least an argument (file name)\n");
    exit(-1);
}

fd = open(argv[1], O_RDWR);
if (fd == -1) {
    /* Handle error */;
    printf("File doesn't exists\n");
    exit(1);
}
/* Make a non-blocking request to place a write lock
 * on bytes 100-109 of testfile
 * */


fl.l_type = F_WRLCK;
fl.l_whence = SEEK_SET;
fl.l_start = 100;
fl.l_len = 10;


if (fcntl(fd, F_SETLK, &fl) == -1) {
    if (errno == EACCES || errno == EAGAIN) {
        printf("Already locked by another process\n");

/* We can't get the lock at the moment */


    } else {

        if (errno == EIO || errno == EBADF)
            /* Handle unexpected error */;
            printf("I/O Error in locking file [CODE: %d]\n", errno);
    }
} else { /* Lock was granted... */


    /* Perform I/O on bytes 100 to 109 of file */

    printf("Lock on file was granted\n");
    /* Unlock the locked bytes */


        fl.l_type = F_UNLCK;
        fl.l_whence = SEEK_SET;
        fl.l_start = 100;
        fl.l_len = 10;
        if (fcntl(fd, F_SETLK, &fl) == -1)
            /* Handle error */;
}
exit(EXIT_SUCCESS);
}