我试图了解非阻塞flock和willblock参数
$fp = fopen('/tmp/lock.txt', 'r+');
if(flock($fp, LOCK_EX | LOCK_NB, $wouldblock)) {
echo 'Lock obtained';
}
else{
echo 'Unable to obtain lock';
}
fclose($fp);
文档说关于wouldblock:
如果锁定会阻塞(EWOULDBLOCK错误条件),则可选的第三个参数设置为1。
在测试环境中重现并发条件,如果另一个进程已获得锁定,则flock函数将立即返回FALSE(非阻塞)
那么,如果非阻塞模式下的flock函数的返回值已经告诉我无法获得锁定,为什么我要关心 $ wouldblock 值?
我无法区分flock函数返回FALSE和$ wouldblock参数设置为1,以及$ wouldblock参数的用途。
答案 0 :(得分:9)
这是因为flock()
可能会失败,因为锁已经在其他地方获得了。
在这种情况下,它不会阻止等待锁被释放,但它会立即返回false。换句话说,LOCK_NB
如果flock返回false而willblock = 1那么意味着,它试图获得锁定,但它已经在其他地方被获取。但是如果flock LOCK_NB
返回false并且willblock = 0那么这意味着发生了一些非常糟糕的事情并且flock甚至没有考虑等待获得锁定,因为这是完全不可能的。
查看此代码(here is also a gist):
<?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,为了清除未来读者的任何困惑,值得注意的是,检查EWOULDBLOCK
只对含LOCK_NB
标志的flock()有意义,因为在阻止模式下它是&#39;无论是成功还是阻止或失败,都没有阻止。
您可以通过查看php source code for flock:
来确认这一点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
仅在operation & LOCK_NB && ret == -1 &&
(errno == EACCES || errno == EAGAIN)
。
如果您对实施更感兴趣,还可以阅读man page of fcntl,主要是有关F_SETLK
和F_SETLKW
的部分内容:
F_SETLK
获取锁(当l_type为F_RDLCK或F_WRLCK时)或释放锁(当l_type为F_UNLCK时)对l_whence指定的字节, l_start和l_len字段的锁定。如果持有冲突的锁定 另一个进程,此调用返回-1并将errno设置为EACCES或 EAGAIN。
F_SETLKW
对于F_SETLK,但如果文件上存在冲突的锁,则等待释放该锁。如果发出信号的话 等待,然后呼叫被中断(在信号处理程序之后) 已返回)立即返回(返回值为-1且设置了errno 到EINTR)。