I have one thread calling recvfrom with a timeout of 100 ms in a loop.
Another thread calls sendto periodically.
Does the sendto function wait until the recvfrom is released by the timeout (or a successful read) or does it send the data during that period of time.
Are the C functions recvfrom and sendto mutually exclusive?
No. They can both be executed by different threads at the same time.
sendto() doesn't wait for recvfrom() to read the data. It would place the data into the socket's buffer and return. Multiple sendto() may block for the previous sendto() to complete. If any error occurred (buffer full, message too big etc) while sending then you can check inspect errno to check the cause of failure. Basically, you don't need to do any synchronization between sendto() and recvfrom() calls from the two threads; they are atomic operations.
No, it does not wait, (at least, it does not wait any longer than necessary to gain thread-safe access to the comms stack).