非阻塞写入和阻塞接收

Question

I am trying to write a parallel stream for my C proxy, where data is simultaneously written to a parallel log while forwarding it to the destination server. Essentially I am looking to have 2 threads where:

1. Thread does a non blocking write to a buffer
2. 2nd Thread does a blocking read from the buffer.

Naturally, the writer thread can exceed the reader thread. Hence whenever, a buffer overflow does happen, we raise and flag it as an error.

Let's say that Thread 1(the writer), is 5 times as fast as Thread 2( the reader). I want Thread 1 not to slowdown, and write at 5x the speed of the reader (ie the buffer should not be locked away by the reader etc.). This will obviously lead to a buffer overflow since thread 1 is writing much faster than thread 2 is reading( I want to capture buffer overflow as well)

My understanding is sockets/file descriptors are either blocking or non blocking, cannot have write operations as non blocking and read as blocking. I am guessing this is to avoid buffer overflows. However, I am fine with buffer overflows.

Can anyone help in devising how such parallel threads can be created where the producer is non blocking, and the receiver is blocking?

Answer 1

You can use select() to block until the socket is readable. Leave it in non-blocking mode. If send() returns -1 and errno is set to EAGAIN or EWOULDBLOCK then you will need to use select() to tell you when the socket has become writable again. Only do that when send() reports EAGAIN / EWOULDBLOCK , as sockets are almost always writable.