What can cause a “Resource temporarily unavailable” on sock send() command

"Resource temporarily unavailable" is the error message corresponding to EAGAIN, which means that the operation would have blocked but nonblocking operation was requested. For send(), that could be due to any of:

• explicitly marking the file descriptor as nonblocking with fcntl(); or
• passing the MSG_DONTWAIT flag to send(); or
• setting a send timeout with the SO_SNDTIMEO socket option.