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通过命名管道进行异步通信

 3 years ago
source link: https://blog.lilydjwg.me/2011/10/14/asynchronized-ipc-through-named-pipe.30205.html
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通过命名管道进行异步通信

本文来自依云's Blog，转载请注明。

需求是这样子的：一个程序要提供一个IPC接口，接收异步的命令。这个接口应该尽量简单，能像/proc下的文件那样通过写入数据来通信，所以我选中了命名管道。读取命名管道很简单，像普通文件那样打开然后读取就可以了。但这样做的问题是，在没有写者的时候open会阻塞。man 2 open下找到了两个标志位：O_ASYNC和O_NONBLOCK。我被排在前面的O_ASYNC骗了，它只是读写时使用信号进行异步操作，open依旧阻塞。继续向后翻，才看到O_NONBLOCK，还特意注明了Neither the open() nor any subsequent operations on the file descriptor which is returned will cause the calling process to wait.

试了试，发现open并不像读写时那样在将阻塞时返回EWOULDBLOCK错误，而是返回了一个可用的文件描述符。既然文件描述符都有了，接下来自然毫无悬念地select了。完整的演示代码如下：

nonblock_fifo
#!/usr/bin/env python3
# vim:fileencoding=utf-8
import os
import time
import select
fd = os.open('test', os.O_NONBLOCK | os.O_RDONLY)
while True:
if not select.select([fd], [], [], 1)[0]:
print('waiting...')
else:
got = os.read(fd, 1024).decode().rstrip()
if not got:
os.close(fd)
fd = os.open('test', os.O_NONBLOCK | os.O_RDONLY)
else:
print('got', got)

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