Will read() ever block after select()?

Question

I\'m reading a stream of data through TCP/IP socket. The stream load is very uneven. Sometimes large bulks of data arrive every second, sometimes no data come for an hour. I

Answer 1

You can use recv() which doesn't block by default( if the flag MSG_WAITALL is not specified)

Answer 2

No, read() will read up to specified size and will return the actual bytes read, which can be less.

Answer 3

There is O_NONBLOCK which can be set by fcntl/F_SETFL and should result in non-blocking read.

Answer 4

Actually it should not block (that is what select() is for!), but in fact, it might, exceptionally. Normally, read() should return up to the maximum number of bytes that you've specified, which possibly includes zero bytes (this is actually a valid thing to happen!), but it should never block after previously having reported readiness.

Nevertheless, see the Linux select man page:

Under Linux, select() may report a socket file descriptor as "ready for reading", while nevertheless a subsequent read blocks. This could for example happen when data has arrived but upon examination has wrong checksum and is discarded. There may be other circumstances in which a file descriptor is spuriously reported as ready. Thus it may be safer to use O_NONBLOCK on sockets that should not block.

Answer 5

A blocking file descriptor will block on read() until there is something to read - could be one byte or your entire request. A non-blocking descriptor won't block on read() if there is nothing to read. Select() is not read(). It basically puts the process to sleep and monitors the file descriptor(s), including non-blocking descriptors. When there is activity on one of the descriptors (or the timeout period expires) select returns and you can read your data, or do something else in the case of the timeout.

So you have two separate issues. (1) You want to "take some actions" when there is no data. That's the select timeout. (2) Once you have data (notified by select) you don't want to block on a read. That's the non-blocking mode. When you get EAGAIN on the non-blocking read you loop back to the select and/or "take some actions" and loop back to select.