PHP flock() - what's under the hood?

Question

After wrestling with PHP source for a half an hour, I gave up. :P The question is - what system call does the PHP flock() function call boil down to on a Gentoo Linux system? I'm having some issues with it (like block-for-30-seconds-in-every-one-of-20-loop-iterations kind of issues) and I would like to know why that is so.

Answer 1

Unless I'm misunderstanding what you're asking, PHP's flock() is a call to the Unix system function flock() on Gentoo. They have identical semantics.

Answer 2

Are you using it on a networked or mounted drive? I wouldn't be surprised if what you are experiencing is deadlock, and some of the comments in the documentation talk about that.

The documentation for flock.

Answer 3

// example: $stream = fopen(FILE, 'rb') or die('???');
$md = stream_get_meta_data($stream);
echo $md['wrapper_type'];

flock($stream);

if this prints plainfile then the call to the php function flock() is handled by php_stdiop_set_option(...) which calls flock(). Depending on whether PHP was compiled with HAVE_FLOCK or not this may be the system call flock() or a function defined in flock_compat.c which utilizes fcntl(). On my gentoo system PHP was compiled with HAVE_FLOCK.

main/streams/plain_wrapper.c @ static int php_stdiop_set_option(...):

    case PHP_STREAM_OPTION_LOCKING:
      if (fd == -1) {
       return -1;
      }

      if ((zend_uintptr_t) ptrparam == PHP_STREAM_LOCK_SUPPORTED) {
       return 0;
      }

      if (!flock(fd, value)) {
       data->lock_flag = value;
       return 0;
      } else {
       return -1;
      }
      break;

Answer 4

http://www.opengroup.org/onlinepubs/009695399/functions/fcntl.html

/etc/standard/flock_compat.c [line 66]    
ret = fcntl(fd, operation & LOCK_NB ? F_SETLK : F_SETLKW, &flck);