write(s, buf, sizeof buf); read(s, buf, sizeof buf);
send(s, buf, sizeof buf, flags); recv(s, buf, sizeof buf, flags);
send(3SOCKET) and recv(3SOCKET) are very similar to read(2) and write(2), but the flags argument is important. The flags argument, which is defined in sys/socket.h, can be specified as a nonzero value if one or more of the following is required:
Send and receive out-of-band data
Look at data without reading
Send data without routing packets
Out-of-band data is specific to stream sockets. When MSG_PEEK is specified with a recv(3SOCKET) call, any data present is returned to the user, but treated as still unread. The next read(2) or recv(3SOCKET) call on the socket returns the same data. The option to send data without routing packets applied to the outgoing packets is currently used only by the routing table management process.