What does poll() do with a timeout of 0?

Question

I\'m looking at the poll() man page, and it tells me the behavior of poll() when positive and negative values are passed in for the timeout parameter.

Answer 1

It will return immediately:

If timeout is greater than zero, it specifies a maximum interval (in milliseconds) to wait for any file descriptor to become ready. If timeout is zero, then poll() will return without blocking. If the value of timeout is -1, the poll blocks indefinitely.

, as of Mac OS X 10.5;

Maximum interval to wait for the poll to complete, in milliseconds. If this value is 0, poll() will return immediately. If this value is INFTIM (-1), poll() will block indefinitely until a condition is found.

, as of OpenBSD 3.8

Answer 2

As I see it, waiting for a timeout means "having" a timeout. This way I would expect that poll() actually checks the file descriptors, and then waits if no one is ready to a timeout of 0 milliseconds (no wait at all). But the case is that it will just signal if a fd is available.

I also checked linux source code and to my knowledge, this is the way it works: first calculates the "future" waiting point, then checks the file descriptors, then if none available, waits for the timeout specified time.

Regards,

Answer 3

From the Ubuntu man pages:

The timeout argument specifies an upper limit on the time for which poll() will block, in milliseconds. Specifying a negative value in timeout means an infinite timeout.

Because there is no special case for 0, I would assume that poll() will block for 0 milliseconds.