Overview of Sockets

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.

Socket Libraries

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

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 four 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 SOCK_STREAM.
Datagram sockets enable processes to use UDP to communicate. A datagram socket supports a bidirectional flow of messages. A process on a datagram socket might receive messages in a different order from the sending sequence. A process on a datagram socket might receive duplicate messages. Messages that are sent over a datagram socket might be dropped. Record boundaries in the data are preserved. The socket type is SOCK_DGRAM.
Raw sockets provide access to ICMP. Raw sockets also provide access to other protocols based on IP that are not directly supported by the networking stack. 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 SOCK_RAW.
SEQ sockets support 1-to-N Stream Control Transmission Protocol (SCTP) connections. More details on SCTP are in Stream Control Transmission Protocol.

See Selecting Specific Protocols for further information.

Interface Sets

The SunOS 5.11 platform provides two sets of socket interfaces. The BSD socket interfaces are provided and, since SunOS version 5.7, the XNS 5 (UNIX03) socket interfaces are also provided. The XNS 5 interfaces differ slightly from the BSD interfaces.

The XNS 5 socket interfaces are documented in the following man pages:

The traditional BSD Socket behavior is documented in the corresponding 3N man pages. In addition, the following new interfaces have been added to section 3N:

See the standards(5) man page for information on building applications that use the XNS 5 (UNIX03) socket interface.

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