POSIX File locks lockf function


Example

This example demonstrates usage of lockf function (POSIX XSI).

Notes:

  • Only exclusive locks are supported.
  • Can be applied to a byte range, optionally automatically expanding when data is appended in future (controlled by len argument and position set with lseek function).
  • Locks are released on first close by the locking process of any file descriptor for the file, or when process terminates.
  • The interaction between fcntl and lockf locks is unspecified. On Linux, lockf is a wrapper for POSIX record locks.
#include <stdlib.h>  /* for exit() */
#include <stdio.h>   /* for perror() */
#include <unistd.h>  /* for lockf(), lseek() */
#include <fcntl.h>   /* for open() */

int main(int argc, char **argv) {
    /* open file
     * we need O_RDWR for lockf */
    int fd = open(argv[1], O_RDWR);
    if (fd == -1) {
        perror("open");
        exit(EXIT_FAILURE);
    }

    /* set current position to byte 10 */
    if (lseek(fd, 10, SEEK_SET) == -1) {
        perror("lseek");
        exit(EXIT_FAILURE);
    }

    /* acquire exclusive lock for bytes in range [10; 15)
     * F_LOCK specifies blocking mode */
    if (lockf(fd, F_LOCK, 5) == -1) {
        perror("lockf(LOCK)");
        exit(EXIT_FAILURE);
    }

    /* release lock for bytes in range [10; 15) */
    if (lockf(fd, F_ULOCK, 5) == -1) {
        perror("lockf(ULOCK)");
        exit(EXIT_FAILURE);
    }

    return EXIT_SUCCESS;
}