Is write() safe to be called from multiple threads simultaneously?

Question

Assuming I have opened dev/poll as mDevPoll, is it safe for me to call code like this

struct pollfd tmp_pfd;
tmp_pfd.fd = fd;
tmp_pfd.events = P         


        

Answer 1

Solaris 10 claims to be POSIX compliant. The write() function is not among the handful of system interfaces that POSIX permits to be non-thread-safe, so we can conclude that that on Solaris 10, it is safe in a general sense to call write() simultaneously from two or more threads.

POSIX also designates write() among those functions whose effects are atomic relative to each other when they operate on regular files or symbolic links. Specifically, it says that

If two threads each call one of these functions, each call shall either see all of the specified effects of the other call, or none of them.

If your writes were directed to a regular file then that would be sufficient to conclude that your proposed multi-thread actions are safe, in the sense that they would not interfere with one another, and the data written in one call would not be commingled with that written by a different call in any thread. Unfortunately, /dev/poll is not a regular file, so that does not apply directly to you.

You should also be aware that write() is not in general required to transfer the full number of bytes specified in a single call. For general purposes, one must therefore be prepared to transfer the desired bytes over multiple calls, by using a loop. Solaris may provide applicable guarantees beyond those expressed by POSIX, perhaps specific to the destination device, but absent such guarantees it is conceivable that one of your threads performs a partial write, and the next write is performed by a different thread. That very likely would not produce the results you want or expect.