[Refresher] POSIX.1 Advisory File Locking

The sample code here demonstrates the following techniques:

• The use of POSIX fcntl() to perform file locking
• The realization of mutual exclusion via file locking

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

#define LOCK_FILENAME  "/tmp/lock_filename"

int main(int argc, char *argv[])
{
  int fd;
  struct flock flock_lock, flock_unlock;

  /*--------------------------------*/
  /* Initialization                 */
  /*--------------------------------*/

  /* Create the lock file if it's not there yet */
  if ((fd = open(LOCK_FILENAME, O_CREAT | O_WRONLY, (mode_t) S_IWUSR)) < 0) {
    if (errno == EEXIST) {
      if ((fd = open(LOCK_FILENAME, O_RDWR)) < 0) {
        return EXIT_FAILURE;
      }
    }
    else {
      return EXIT_FAILURE;
    }
  }

  /* Miscellaneous */
  flock_lock.l_type = F_WRLCK;
  flock_lock.l_whence = SEEK_SET;
  flock_lock.l_start = 0;
  flock_lock.l_len = 0;
  flock_unlock.l_type = F_UNLCK;
  flock_unlock.l_whence = SEEK_SET;
  flock_unlock.l_start = 0;
  flock_unlock.l_len = 0;

  /*--------------------------------*/
  /* Mutual Exclusion               */
  /*--------------------------------*/

  /* Lock the file */
  while (fcntl(fd, F_SETLKW, &flock_lock) < 0) {
    // errno is set appropriately
    if (errno == EINTR)
      continue;
    else
      return EXIT_FAILURE;
  }

  /* Critical section! */
  puts("Critical section");

  /* Unlock the file */
  if (fcntl(fd, F_SETLKW, &flock_unlock) < 0) {
    // errno is set appropriately
    return EXIT_FAILURE;
  }

  // End of mutual exclusion

  return EXIT_SUCCESS;
}

Notes:

• When any file descriptor for a file is closed, all locks that the process holds on that file are also released.
• Locks are automatically released when a process exits.
• Locks are not inherited by fork()-ed children.