When to use the POLLOUT event of the poll C function?

Question

I wrote a small TCP servers with socket() + POLLIN poll() + recv() + send(), but I don\'t know when to use <

Answer 1

From nginx source, I found that:

If there is some data to send out, nginx tries to send it with a syscall (maybe writev). However, if nginx can not send total data at one time, it will set POLLOUT on pollfd, if using poll event, to wait for a writable event. When getting a writable event, nginx will send the left data.

It is easy to reproduce this case when nginx tries to response large static file