For stream sockets, data is processed as streams of information with no boundaries separating data. For example, if applications A and B are connected with a stream socket and application A sends 1000 bytes, each call to the SEND function can return 1 byte, 10 bytes, or the entire 1000 bytes, with the number of bytes sent returned in the RETCODE call. Therefore, applications using stream sockets should place the READ call and the SEND call in a loop that repeats until all of the data has been sent or received.

PROTO specifies a particular protocol to be used with the socket. In most cases, a single protocol exists to support one type of socket in a domain (not true with raw sockets). If PROTO is set to 0, the system selects the default protocol number for the domain and socket type requested. The PROTO defaults are TCP for stream sockets and UDP for datagram sockets. There is no default for raw sockets.

SOCK_STREAM sockets model duplex byte streams. They provide reliable, flow-controlled connections between peer applications. Stream sockets are either active or passive. Active sockets are used by clients who initiate connection requests with CONNECT. By default, SOCKET creates active sockets. Passive sockets are used by servers to accept connection requests with the CONNECT macro. An active socket is transformed into a passive socket by binding a name to the socket with the BIND macro and by indicating a willingness to accept connections with the LISTEN macro. If a socket is passive, it cannot be used to initiate connection requests.

In the AF_INET or AF_INET6 domain, the BIND macro, applied to a stream socket, lets the application specify the networks from which it is willing to accept connection requests. The application can fully specify the network interface by setting the Internet address field in the address structure to the Internet address of a network interface. Alternatively, the application can set the address in the name structure to zeros to indicate that it wants to receive connection requests from any network.

After a connection has been established between stream sockets, the data transfer macros READ, WRITE, SEND, RECV, SENDTO, and RECVFROM can be used. Usually, the READ-WRITE or SEND-RECV pairs are used for sending data on stream sockets.

SOCK_DGRAM sockets are used to model datagrams. They provide connectionless message exchange without guarantees of reliability. Messages sent have a maximum size. Datagram sockets are not supported in the AF_IUCV domain.

The active or passive concepts for stream sockets do not apply to datagram sockets. Servers must still call BIND to name a socket and to specify from which network interfaces it wants to receive datagrams. Wildcard addressing, as described for stream sockets, also applies to datagram sockets. Because datagram sockets are connectionless, the LISTEN macro has no meaning for them and must not be used.

After an application receives a datagram socket, it can exchange datagrams using the SENDTO and RECVFROM macros. If the application goes one step further by calling CONNECT and fully specifying the name of the peer with which all messages are exchanged, then the other data transfer macros READ, WRITE, SEND, and RECV can be used as well. For more information about placing a socket into the connected state, see CONNECT.

Datagram sockets allow message broadcasting to multiple recipients. Setting the destination address to a broadcast address depends on the network interface (address class and whether subnets are used).

SOCK_RAW sockets supply an interface to lower layer protocols, such as IP. You can use this interface to bypass the transport layer when you need direct access to lower layer protocols. Raw sockets are also used to test new protocols. Raw sockets are not supported in the AF_IUCV domain.

Raw sockets are connectionless and data transfer is the same as for datagram sockets. You can also use the CONNECT macro to specify a peer socket in the same way that is previously described for datagram sockets.

Outgoing datagrams have an IP header prefixed to them. Your program receives incoming datagrams with the IP header intact. You can set and inspect IP options by using the SETSOCKOPT and GETSOCKOPT macros.

Use the CLOSE macro to deallocate sockets.

Regardless of the type of socket (SOCK_STREAM, SOCK_DGRAM or SOCK_RAW), all commands that pass a socket address must be consistent with the address family specified when the socket was opened. If the socket was opened with an address family of AF_INET, then any command for that socket that includes a socket address must use an AF_INET socket address. If the socket was opened with an address family of AF_INET6, then any command for that socket that includes a socket address must use an AF_INET6 socket address.