CODEWITHSUNDEEP
cmultithreadingpipeposix
Alexander Ushakov
Alexander Ushakov

Reputation: 5397

How to check if non-blocking anonymous pipe has data without removing it

I have a POSIX thread which reads from the non-blocking anonymous pipe (marked with O_NONBLOCK flag). When thread is stopping (because of errors for example) I want to check if there is something left in pipe (in its internal buffer). If pipe has data - run new thread with the same read descriptor (it is shared between threads) so the new thread can continue reading from pipe. If pipe is empty - close pipe and do nothing.

So I need to check if pipe is empty without removing data from pipe (as regular read will do). Is there any way to do it?

P.S. I think setting count = 0 in read(int fd, void *buf, size_t count); may help but the documentation sais that it is some kind of undefined behavior:

If count is zero, read() may detect the errors described below. In the absence of any errors, or if read() does not check for errors, a read() with a count of 0 returns zero and has no other effects.

Upvotes: 2

Views: 308

Answers (1)

Snild Dolkow
Snild Dolkow

Reputation: 6866

I believe you want poll or select, called with a zero timeout.

Short description from the select() docs:

   select() and pselect() allow a program to monitor multiple file
   descriptors, waiting until one or more of the file descriptors become
   "ready" for some class of I/O operation (e.g., input possible).

...and the poll() docs:

   poll() performs a similar task to select(2): it waits for one of a
   set of file descriptors to become ready to perform I/O.

Upvotes: 2

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