Case when blocking recv() returns less than requested bytes

Question

The recv() library function man page mention that:

It returns the number of bytes received. It normally returns any data available, up to the reque

Answer 1

The behavior is determined by two things. The recv low water mark and whether or not you pass the MSG_WAITALL flag. If you pass this flag the call will block until the requested number of bytes are received, even if the server crashes. Other wise it returns as soon as at least SO_RCVLOWAT bytes are available in the socket's receive buffer.

SO_RCVLOWAT

Sets the minimum number of bytes to process for socket input operations. The default value for SO_RCVLOWAT is 1. If SO_RCVLOWAT is set to a larger value, blocking receive calls normally wait until they have received the smaller of the low water mark value or the requested amount. (They may return less than the low water mark if an error occurs, a signal is caught, or the type of data next in the receive queue is different than that returned, e.g. out of band data). This option takes an int value. Note that not all implementations allow this option to be set.