2.1 socket Function

Following from the book 《Unix Network Programming volume 1》, I have done a little modification.

 

2.1 socket Function

To perform network I/O, the first thing a process must do is call the socket function, specifying the type of communication protocol desired (TCP using IPv4, UDP using IPv6, Unix domain stream protocol, etc.).

#include

int socket (int family, int type, int protocol);

Returns: non-negative descriptor if OK, -1 on error

family specifies the protocol family and is one of the constants shown in Figure 4.2. This argument is often referred to as domain instead of family. The socket type is one of the constants shown in Figure 4.3. The protocol argument to the socket function should be set to the specific protocol type found in Figure 4.4, or 0 to select the system's default for the given combination of family and type.

Not all combinations of socket family and type are valid. Figure 2.5 shows the valid combinations, along with the actual protocols that are valid for each pair. The boxes marked "Yes" are valid but do not have handy acronyms. The blank boxes are not supported.

Figure 2.5. Combinations of family and type for the socket function.

We note that you may encounter AF_UNIX (the historical Unix name) instead of AF_LOCAL (the POSIX name).

There are other values for the family and type arguments. For example, 4.4BSD supports both AF_NS (the Xerox NS protocols, often called XNS) and AF_ISO (the OSI protocols). Similarly, the type of SOCK_SEQPACKET, a sequenced-packet socket, is implemented by both the Xerox NS protocols and the OSI protocols, TCP is a byte stream protocol, and supports only SOCK_STREAM sockets.

Linux supports a new socket type, SOCK_PACKET, that provides access to the datalink, similar to BPF and DLPI.

The key socket, AF_KEY, is newer than the others. It provides support for cryptographic security. Similar to the way that a routing socket (AF_ROUTE) is an interface to the kernel's routing table, the key socket is an interface into the kernel's key table.

On success, the socket function returns a small non-negative integer value, similar to a file descriptor. We call this a socket descriptor, or a sockfd. To obtain this socket descriptor, all we have specified is a protocol family (IPv4, IPv6, or Unix) and the socket type (stream, datagram, or raw). We have not yet specified either the local protocol address or the foreign protocol address.

AF_xxx Versus PF_xxx

The "AF_" prefix stands for "address family" and the "PF_" prefix stands for "protocol family." Historically, the intent was that a single protocol family might support multiple address families and that the PF_ value was used to create the socket and the AF_ value was used in socket address structures. But in actuality, a protocol family supporting multiple address families has never been supported and the header defines the PF_ value for a given protocol to be equal to the AF_ value for that protocol. While there is no guarantee that this equality between the two will always be true, should anyone change this for existing protocols, lots of existing code would break. To conform to existing coding practice, we use only the AF_ constants in this text, although you may encounter the PF_ value, mainly in calls to socket.

Finally, we note that the POSIX standard specifies that the first argument to socket be a PF_ value, and the AF_ value be used for a socket address structure. But, it then defines only one family value in the addrinfo structure, intended for use in either a call to socket or in a socket address structure!