非阻塞flock函数的返回值和$wouldblock参数之间的区别？

我试图理解非阻塞flock http://php.net//manual/en/function.flock.php和 willblock 论点

$fp = fopen('/tmp/lock.txt', 'r+'); 
if(flock($fp, LOCK_EX | LOCK_NB, $wouldblock)) {
  echo 'Lock obtained';
}
else{
  echo 'Unable to obtain lock';
}
fclose($fp);

文档中有关于 willblock 的说明：

如果锁会阻塞（EWOULDBLOCK errno 条件），则可选的第三个参数设置为 1。

在测试环境中重现并发情况，如果另一个进程获得了锁，flock 函数将立即返回 FALSE（非阻塞）

那么我为什么要关心$会阻止value 如果非阻塞模式下的flock函数的返回值allready告诉我无法获得锁？

我无法理解flock函数返回FALSE和$wouldblock参数设置为1之间的区别，以及$wouldblock参数的用途。

这是因为flock()可能失败不仅是因为在其他地方已经获得了锁。 在这种情况下，它不会阻塞等待锁被释放，但会立即返回 false。换句话说，与LOCK_NB如果flock返回false并且wouldblock=1那么这意味着它试图获得锁但它已经在其他地方获得了。但如果蜂拥而至LOCK_NB返回 false 且 willblock=0 那么这意味着发生了非常糟糕的事情，flock 甚至没有考虑等待获得锁，因为这是完全不可能的。

查看这段代码（这也是一个要点 https://gist.github.com/arvenil/a1d273f5287ce8e1f4df):

<?php
// Let's create /tmp/ninja-lock1.txt ...
$fp0 = fopen('/tmp/ninja-lock1.txt', 'c');
// ... and close it imiedietly
fclose($fp0);

// File handler $fp0 was closed so flock()
// is unable to use it to gain lock.
// It will fail with wouldblock set to 0
// as it doesn't make sense to wait on unusable file handle.
//
// BTW flock() throws in such case warning "x is not a valid stream resource".
// Just for the purpose of clear output from this example
// I've suppressed it with @ - don't use @ in production
$flockResult = @flock($fp0, LOCK_EX | LOCK_NB, $wouldblock);
printf("flock=%b; wouldblock=%d (Acquire lock: %s)\n", $flockResult, $wouldblock, "failure, broken file handle");

// Two handlers for /tmp/ninja-lock2.txt
// to show flock() blocking result.
$fp1 = fopen('/tmp/ninja-lock2.txt', 'c');
$fp2 = fopen('/tmp/ninja-lock2.txt', 'c'); 

// Nobody is locking on /tmp/ninja-lock2.txt,
// so it will acquire lock and wouldblock will be 0
$flockResult = flock($fp1, LOCK_EX | LOCK_NB, $wouldblock);
printf("flock=%b; wouldblock=%d (Acquire lock: %s)\n", $flockResult, $wouldblock, "success");

// File is locked on $fp1 handle so flock won't acquire lock
// and wouldblock will be 1
$flockResult = flock($fp2, LOCK_EX | LOCK_NB, $wouldblock);
printf("flock=%b; wouldblock=%d (Acquire lock: %s)\n", $flockResult, $wouldblock, "failure, already acquired somewhere else");

// Result:
// $ php flock.php 
// flock=0; wouldblock=0 (Acquire lock: failure, broken file handle)
// flock=1; wouldblock=0 (Acquire lock: success)
// flock=0; wouldblock=1 (Acquire lock: failure, already acquired somewhere else)
?>

另外，为了消除未来读者的任何困惑，值得注意的是，检查EWOULDBLOCK只对flock()有意义LOCK_NB标志，就像在阻塞模式下一样，它要么成功并阻塞，要么失败但不阻塞。

您可以通过查看来确认这一点群的 php 源代码 https://github.com/php/php-src/blob/af6c11c5f060870d052a2b765dc634d9e47d0f18/ext/standard/flock_compat.c#L48:

PHPAPI int php_flock(int fd, int operation)
#if HAVE_STRUCT_FLOCK /* {{{ */
{
    struct flock flck;
    int ret;

    flck.l_start = flck.l_len = 0;
    flck.l_whence = SEEK_SET;

    if (operation & LOCK_SH)
        flck.l_type = F_RDLCK;
    else if (operation & LOCK_EX)
        flck.l_type = F_WRLCK;
    else if (operation & LOCK_UN)
        flck.l_type = F_UNLCK;
    else {
        errno = EINVAL;
        return -1;
    }

    ret = fcntl(fd, operation & LOCK_NB ? F_SETLK : F_SETLKW, &flck);

    if (operation & LOCK_NB && ret == -1 && 
            (errno == EACCES || errno == EAGAIN))
        errno = EWOULDBLOCK;

    if (ret != -1) ret = 0;

    return ret;
}

EWOULDBLOCK仅当operation & LOCK_NB && ret == -1 && (errno == EACCES || errno == EAGAIN).

如果您对实施更感兴趣，您还可以阅读fcntl 的手册页 http://www.linuxmanpages.com/man2/fcntl.2.php，大部分是关于F_SETLK and F_SETLKW:

F_SETLK

对 l_whence 指定的字节获取锁（当 l_type 为 F_RDLCK 或 F_WRLCK 时）或释放锁（当 l_type 为 F_UNLCK 时）， 锁的 l_start 和 l_len 字段。如果冲突的锁被持有 另一个进程，此调用返回 -1 并将 errno 设置为 EACCES 或 再次。

F_SETLKW

与 F_SETLK 一样，但如果文件上持有冲突锁，则等待该锁被释放。如果捕获到信号 等待，然后调用被中断并且（在信号处理程序之后 已返回）立即返回（返回值 -1 并设置 errno 至 EINTR）。