[英]Non blocking read and blocking write on Posix PIPE
I'm writing Client-Server application using posix FIFO to communicate. 我正在编写使用posix FIFO进行通信的Client-Server应用程序。 Both client and server are single thread and single process application.
客户端和服务器都是单线程和单进程应用程序。
The server is design to handle multiple clients. 服务器设计用于处理多个客户端。 Each client has own pair of named pipe, one to send messages from Server to this Client and the other to send messages from Client to the Server.
每个客户端都有自己的一对命名管道,一个用于从服务器向此客户端发送消息,另一个用于从客户端向服务器发送消息。
The idea is quite simple, Server loops through all Client to Server pipes and check if there is something to read there. 这个想法很简单,Server循环遍历所有客户端到服务器管道,并检查是否有东西可以在那里阅读。
My first implemenentation was sth like: 我的第一个实现是......
/* SERVER */
int desc = open(pipeName, O_RDONLY | O_NDELAY); //1
assert(desc > 0); //just simplyfing error handling
int read = read(desc, buffer, BUFSIZE); //2
if(read > 0)
do_stuff();
close(desc); //3
/* CLIENT */
int desc = open(pipeName, O_WRONLY) //4
assert(desc > 0); //just simplyfing error handling
int read = write(desc, buffer, BUFSIZE) //5
close(desc); //6
As far as I know this code is invalid. 据我所知,这段代码无效。
There are race conditions and for example an order of calls like: 1, 2, 4, 3, 5, 6 - will probably cause SIGPIPE. 有竞争条件,例如:1,2,3,3,5,6的调用顺序 - 可能会导致SIGPIPE。
The problem is that nonblocking read open is always successful even if there is no writer on the other side of PIPE. 问题是即使PIPE的另一侧没有写入器,非阻塞读取打开也总是成功的。 That means that if Client will block on open() then server do nonblocking open (which will unlock Client) and then read() which will return 0 because at the moment nothing will be in PIPE and close() afterwards, then when control will go back to client which want to do write() on opened PIPE it will cause SIGPIPE as the reader is no longer available (server already closed the pipe).
这意味着如果客户端将阻塞open()然后服务器执行非阻塞打开(这将解锁客户端)然后读取()将返回0,因为此时PIPE中没有任何内容,之后将关闭(),然后当控制将回到想要在打开的PIPE上执行write()的客户端,它将导致SIGPIPE,因为读取器不再可用(服务器已经关闭了管道)。
For now i see two solutions: 现在我看到两个解决方案:
Here are my questions: 这是我的问题:
Thanks for every answer. 谢谢你的每一个答案。
I'm having a lot of trouble understanding your question. 我在理解你的问题时遇到了很多麻烦。 Things
事情
Client will block on open() <== nothing blocks on open 客户端将阻止open()<==打开时没有任何块
Use select or epoll to determine which file descriptors are readable and writeable. 使用select或epoll确定哪些文件描述符是可读写的。
Then, just call read on those. 然后,只需调用那些读取。
You will be notified that the client closed the pipe as a read event you'll need to handle and not write if it is closed. 您将收到通知,客户端将管道关闭为您需要处理的读取事件,如果关闭则不会写入。
http://linux.die.net/man/4/epoll http://linux.die.net/man/4/epoll
https://banu.com/blog/2/how-to-use-epoll-a-complete-example-in-c/ https://banu.com/blog/2/how-to-use-epoll-a-complete-example-in-c/
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