Sockets have been an integral part of SunOS releases since 1981. A socket is an endpoint of communication to which a name can be bound. A socket has a type and an associated process. Sockets were designed to implement the client-server model for interprocess communication where:
The interface to network protocols needs to accommodate multiple communication protocols, such as TCP/IP, Xerox internet protocols (XNS), and the UNIX family.
The interface to network protocols needs to accommodate server code that waits for connections and client code that initiates connections.
Operations differ depending on whether communication is connection-oriented or connectionless.
Application programs might want to specify the destination address of the datagrams that are being delivered instead of binding the address with the open(2) call.
Sockets make network protocols available while behaving like UNIX files. Applications create sockets as sockets are needed. Sockets work with the close(2), read(2), write(2), ioctl(2), and fcntl(2) interfaces. The operating system differentiates between the file descriptors for files and the file descriptors for sockets.
The socket interface routines are in a library that must be linked with the application. The library libsocket.so is contained in /usr/lib with the rest of the system service libraries. Use libsocket.so for dynamic linking.
Socket types define the communication properties that are visible to a user. The Internet family sockets provide access to the TCP/IP transport protocols. The Internet family is identified by the value AF_INET6, for sockets that can communicate over both IPv6 and IPv4. The value AF_INET is also supported for source compatibility with old applications and for “raw” access to IPv4.
The SunOS environment supports three types of sockets:
Stream sockets enable processes to communicate
using TCP. A stream socket provides a bidirectional, reliable, sequenced,
and unduplicated flow of data with no record boundaries. After the connection
has been established, data can be read from and written to these sockets as
a byte stream. The socket type is
Datagram sockets enable processes to
use UDP to communicate. A datagram socket supports a bidirectional flow of
messages. A process on a datagram socket can receive messages in a different
order from the sending sequence. A process on a datagram socket can receive
duplicate messages. Record boundaries in the data are preserved. The socket
Raw sockets provide access to ICMP. These
sockets are normally datagram oriented, although their exact characteristics
are dependent on the interface provided by the protocol. Raw sockets are not
for most applications. Raw sockets are provided to support the development
of new communication protocols, or for access to more esoteric facilities
of an existing protocol. Only superuser processes can use raw sockets. The
socket type is
See Selecting Specific Protocols for further information.
The SunOS 5.9 platform provides two sets of socket interfaces. The BSD socket interfaces are provided and, since SunOS™ version 5.7, the XNS 5 (Unix98) socket interfaces are also provided. The XNS 5 interfaces differ slightly from the BSD interfaces.