Are parallel calls to send/recv on the same socket valid?

POSIX defines send/recv as atomic operations, so assuming you're talking about POSIX send/recv then yes, you can call them simultaneously from multiple threads and things will work.

This doesn't necessarily mean that they'll be executed in parallel -- in the case of multiple sends, the second will likely block until the first completes. You probably won't notice this much, as a send completes once its put its data into the socket buffer.

If you're using SOCK_STREAM sockets, trying to do things a parallel is less likely to be useful as send/recv might send or receive only part of a message, which means things could get split up.

Blocking send/recv on SOCK_STREAM sockets only block until they send or recv at least 1 byte, so the difference between blocking and non-blocking is not useful.

The socket descriptor belongs to the process, not to a particular thread. Hence, it is possible to send/receive to/from the same socket in different threads, the OS will handle the synchronization.

However, if the order of sending/receiving is semantically significant, you yourself (respectively your code) have to ensure proper sequencing between the operations in the different threads - as is always the case with threads.