Typically read(2) (i.e. sysread) will block until at least some data is available, if the descriptor is not set to non-blocking. Once the first byte is available, as many bytes, as are available, will be returned.

In particular, on many systems socket buffers are fairly small (32K is typical). On such systems a sysread from a socket will never return more than 32K bytes, regardless of the size of the corresponding write to the socket.

On my XP box this concept seems to be mixed: a sysread of 8MB (with a corresponding write on the client socket of the same size) terminates with 1K read, if the read is pending when the write is issued, but brings in the full 8MB, if the write is pending when the sysread is done.

AFAIK, read will wait for the specified size, unless the underlying handle is set to non-blocking.