How to cleanly interrupt a thread blocking on a recv call?

Shutdown the socket for input from another thread. That will cause the reading thread to receive an EOS, which should cause it to close the socket and terminate if it is correctly written.

So you have at least these possibilities:

(1) pthread_kill will blow the thread out of recv with errno == EINTR and you can clean up and exit the thread on your own. Some people think this is nasty. Depends, really.

(2) Make your client socket(s) non-blocking and use select to wait on input for a specific period of time before checking if a switch used between the threads has been set to indicated they should shut down.

(3) In combo with (2) have each thread share a pipe with the master thread. Add it to the select. If it becomes readable and contains a shutdonw request, the thread shuts itself down.

(4) Look into the pthread_cancel mechanism if none of the above (or variations thereof) do not meet your needs.