socket() - create an endpoint for communication
int socket(int af, int type, int protocol);
The socket() system call creates an endpoint for communication and
returns a descriptor. The socket descriptor returned is used in all
subsequent socket-related system calls.
The af parameter specifies an address family to be used to interpret
addresses in later operations that specify the socket. These address
families are defined in the include files <<<<sys/socket.h>>>> and
<<<<x25/ccittproto.h>>>>. The only currently supported address families
AF_INET (DARPA Internet addresses)
AF_UNIX (path names on a local node)
AF_CCITT (CCITT X.25 addresses)
AF_VME_LINK (backplane communications on VMEbus)
The type specifies the semantics of communication for the socket.
Currently defined types are:
SOCK_STREAM Sequenced, reliable, two-way-connection-based
SOCK_DGRAM Datagrams (connectionless, unreliable
messages of a fixed, typically small, maximum
length; for AF_INET only).
protocol specifies a particular protocol to be used with the socket.
Normally, only a single protocol exists to support a particular socket
type using a given address family. However, many protocols may exist,
in which case a particular protocol must be specified. The protocol
number to use depends on the communication domain in which
communication is to take place (see services(4) and protocols(4)).
protocol can be specified as zero, which causes the system to choose a
protocol type to use.
Sockets of type SOCK_STREAM are byte streams similar to pipes, except
that they are full-duplex instead of half-duplex. A stream socket
must be in a connected state before any data can be sent or received
on it. A connection to another socket is created with a connect() or
accept() call. Once connected, data can be transferred using some
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variant of the send() and recv() or the read() and write() calls.
When a session is complete, use close() or shutdown() calls to
terminate the connection.
TCP, the communications protocol used to implement SOCK_STREAM for
AF_INET sockets, ensures that data is not lost or duplicated. If a
peer has buffer space for data and the data cannot be successfully
transmitted within a reasonable length of time, the connection is
considered broken and the next recv() call indicates an error with
errno set to [ETIMEDOUT]. If SO_KEEPALIVE is set and the connection
has been idle for two hours, the TCP protocol sends "keepalive"
packets every 75 seconds to determine whether the connection is
active. These transmissions are not visible to users and cannot be
read by a recv() call. If the remote system does not respond within
10 minutes (i.e., after 8 "keepalive" packets have been sent), the
next socket call (e.g., recv()) returns an error with errno set to
[ETIMEDOUT]. A SIGPIPE signal is raised if a process sends on a
broken stream. This causes naive processes that do not handle the
signal to exit. An end-of-file condition (zero bytes read) is
returned if a process tries to read on a broken stream.
SOCK_DGRAM sockets allow sending of messages to correspondents named
in send() calls. It is also possible to receive messages at such a
socket with recv().
The operation of sockets is controlled by socket level options set by
the setsockopt() system call described by the getsockopt(2) manual
entry. These options are defined in the file <<<<sys/socket.h>>>> and
explained in the getsockopt(2) manual entry.
Socket endpoints for communication over an X.25/9000 link can be in
either address family, AF_INET or AF_CCITT. If the socket is in the
AF_INET family, the connection behaves as described above. TCP is
used if the socket type is SOCK_STREAM. UDP is used if the socket
type is SOCK_DGRAM. In both cases, Internet protocol (IP) and the
X.25-to-IP interface module are used.
If the socket is in the AF_CCITT address family, only the SOCK_STREAM
socket type is supported. Refer to the topic "Comparing X.25 Level 3
Access to IP" in the X.25 Programmer's Guide for more details on the
difference between programmatic access to X.25 via IP and X.25 Level
If the socket is in the AF_CCITT family, the connection and all other
operations pass data directly from the application to the X.25 Packet
Level (level 3) without passing through a TCP or UDP protocol.
Connections of the AF_CCITT family cannot use most of the socket level
options described in getsockopt(2). However, AF_CCITT connections can
use many X.25-specific ioctl() calls, described in socketx25(7).
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AF_CCITT and AF_VME_LINK
Only the SOCK_STREAM type is supported.
socket() returns the following values:
n Successful completion. n is a valid file descriptor
referring to the socket.
-1 Failure. errno is set to indicate the error.
If socket() fails, errno is set to one of the following values.
[EAFNOSUPPORT] The specified address family is not
supported in this version of the system.
[EHOSTDOWN] The networking subsystem is not up.
[EINVAL] SOCK_DGRAM sockets are currently not
supported for the AF_UNIX or AF_VME_LINK
[EMFILE] The per-process descriptor table is
[ENFILE] The system's table of open files is
temporarily full and no more socket()
calls can be accepted.
[ENOBUFS] No buffer space is available. The
socket cannot be created.
[ENOMEM] No memory is available. The socket
cannot be created.
[EPROTONOSUPPORT] The specified protocol is not supported.
[EPROTOTYPE] The type of socket and protocol do not
[ESOCKTNOSUPPORT] The specified socket type is not
supported in this address family.
[ETIMEDOUT] Connection timed out.
Currently, the default behavior is the HP-UX BSD Sockets; however, it
might be changed to X/Open Sockets in a future release. At that time,
any HP-UX BSD Sockets behavior that is incompatible with X/Open
Sockets might be obsoleted. Applications that conform to the X/Open
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specification now will avoid migration problems (see
The socket() system call is thread-safe. It has a cancellation point;
and it is async-cancel safe, async-signal safe, and fork-safe.
socket() was developed by HP and the University of California,
accept(2), bind(2), connect(2), getsockname(2), getsockopt(2),
ioctl(2), listen(2), recv(2), select(2), send(2), shutdown(2),
af_ccitt(7F), af_vme_link(7F), socket(7), socketx25(7), tcp(7P),
udp(7P), unix(7P), xopen_networking(7).
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