Method: File#flock

Defined in:

file.c

#flock(locking_constant) ⇒ 0, false

Locks or unlocks a file according to locking_constant (a logical or of the values in the table below). Returns false if File::LOCK_NB is specified and the operation would otherwise have blocked. Not available on all platforms.

Locking constants (in class File):

LOCK_EX   | Exclusive lock. Only one process may hold an
          | exclusive lock for a given file at a time.
----------+------------------------------------------------
LOCK_NB   | Don't block when locking. May be combined
          | with other lock options using logical or.
----------+------------------------------------------------
LOCK_SH   | Shared lock. Multiple processes may each hold a
          | shared lock for a given file at the same time.
----------+------------------------------------------------
LOCK_UN   | Unlock.

Example:

# update a counter using write lock
# don't use "w" because it truncates the file before lock.
File.open("counter", File::RDWR|File::CREAT, 0644) {|f|
  f.flock(File::LOCK_EX)
  value = f.read.to_i + 1
  f.rewind
  f.write("#{value}\n")
  f.flush
  f.truncate(f.pos)
}

# read the counter using read lock
File.open("counter", "r") {|f|
  f.flock(File::LOCK_SH)
  p f.read
}

Returns:

• (0, false)

# File 'file.c', line 5132

static VALUE
rb_file_flock(VALUE obj, VALUE operation)
{
    rb_io_t *fptr;
    int op[2], op1;
    struct timeval time;

    op[1] = op1 = NUM2INT(operation);
    GetOpenFile(obj, fptr);
    op[0] = fptr->fd;

    if (fptr->mode & FMODE_WRITABLE) {
	rb_io_flush_raw(obj, 0);
    }
    while ((int)rb_thread_io_blocking_region(rb_thread_flock, op, fptr->fd) < 0) {
	int e = errno;
	switch (e) {
	  case EAGAIN:
	  case EACCES:
#if defined(EWOULDBLOCK) && EWOULDBLOCK != EAGAIN
	  case EWOULDBLOCK:
#endif
	    if (op1 & LOCK_NB) return Qfalse;

	    time.tv_sec = 0;
	    time.tv_usec = 100 * 1000;	/* 0.1 sec */
	    rb_thread_wait_for(time);
	    rb_io_check_closed(fptr);
	    continue;

	  case EINTR:
#if defined(ERESTART)
	  case ERESTART:
#endif
	    break;

	  default:
	    rb_syserr_fail_path(e, fptr->pathv);
	}
    }
    return INT2FIX(0);
}