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Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. 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Raymond .\" Modified 1998, 1999 by Andi Kleen .\" Modified 2002-07-17 by Michael Kerrisk .\" Modified 2004-06-17 by Michael Kerrisk .\" .TH SOCKET 2 2020-06-09 "Linux" "Linux Programmer's Manual" .SH NAME socket \- create an endpoint for communication .SH SYNOPSIS .BR "#include " " /* See NOTES */" .br .B #include .PP .BI "int socket(int " domain ", int " type ", int " protocol ); .SH DESCRIPTION .BR socket () creates an endpoint for communication and returns a file descriptor that refers to that endpoint. The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process. .PP The .I domain argument specifies a communication domain; this selects the protocol family which will be used for communication. These families are defined in .IR . The formats currently understood by the Linux kernel include: .TS tab(:); l1 lw40 l. Name:Purpose:Man page T{ .BR AF_UNIX T}:T{ Local communication T}:T{ .BR unix (7) T} T{ .B AF_LOCAL T}:T{ Synonym for .B AF_UNIX T}:T{ T} T{ .B AF_INET T}:IPv4 Internet protocols:T{ .BR ip (7) T} T{ .B AF_AX25 T}:T{ Amateur radio AX.25 protocol T}:T{ .\" Part of ax25-tools .BR ax25 (4) T} T{ .B AF_IPX T}:IPX \- Novell protocols: T{ .B AF_APPLETALK T}:AppleTalk:T{ .BR ddp (7) T} T{ .B AF_X25 T}:ITU-T X.25 / ISO-8208 protocol:T{ .BR x25 (7) T} T{ .B AF_INET6 T}:IPv6 Internet protocols:T{ .BR ipv6 (7) T} T{ .B AF_DECnet T}:T{ DECet protocol sockets T} T{ .B AF_KEY T}:T{ Key management protocol, originally developed for usage with IPsec T} T{ .B AF_NETLINK T}:T{ Kernel user interface device T}:T{ .BR netlink (7) T} T{ .B AF_PACKET T}:T{ Low-level packet interface T}:T{ .BR packet (7) T} T{ .B AF_RDS T}:T{ .\" commit: 639b321b4d8f4e412bfbb2a4a19bfebc1e68ace4 Reliable Datagram Sockets (RDS) protocol T}:T{ .\" rds-tools: https://github.com/oracle/rds-tools/blob/master/rds.7 .\" rds-tools: https://github.com/oracle/rds-tools/blob/master/rds-rdma.7 .BR rds (7) .br .BR rds\-rdma (7) T} T{ .B AF_PPPOX T}:T{ Generic PPP transport layer, for setting up L2 tunnels (L2TP and PPPoE) T} T{ .B AF_LLC T}:T{ .\" linux-history commit: 34beb106cde7da233d4df35dd3d6cf4fee937caa Logical link control (IEEE 802.2 LLC) protocol T} T{ .B AF_IB T}:T{ .\" commits: 8d36eb01da5d371f..ce117ffac2e93334 InfiniBand native addressing T} T{ .B AF_MPLS T}:T{ .\" commits: 0189197f441602acdca3f97750d392a895b778fd Multiprotocol Label Switching T} T{ .B AF_CAN T}:T{ .\" commits: 8dbde28d9711475a..5423dd67bd0108a1 Controller Area Network automotive bus protocol T} T{ .B AF_TIPC T}:T{ .\" commits: b97bf3fd8f6a16966d4f18983b2c40993ff937d4 TIPC, "cluster domain sockets" protocol T} T{ .B AF_BLUETOOTH T}:T{ .\" commits: 8d36eb01da5d371f..ce117ffac2e93334 Bluetooth low-level socket protocol T} T{ .B AF_ALG T}:T{ .\" commit: 03c8efc1ffeb6b82a22c1af8dd908af349563314 Interface to kernel crypto API T} T{ .B AF_VSOCK T}:T{ .\" commit: d021c344051af91f42c5ba9fdedc176740cbd238 VSOCK (originally "VMWare VSockets") protocol for hypervisor-guest communication T}:T{ .BR vsock (7) T} T{ .B AF_KCM T}:T{ .\" commit: 03c8efc1ffeb6b82a22c1af8dd908af349563314 KCM (kernel connection multiplexer) interface T} T{ .B AF_XDP T}:T{ .\" commit: c0c77d8fb787cfe0c3fca689c2a30d1dad4eaba7 XDP (express data path) interface T} .TE .PP Further details of the above address families, as well as information on several other address families, can be found in .BR address_families (7). .PP The socket has the indicated .IR type , which specifies the communication semantics. Currently defined types are: .TP 16 .B SOCK_STREAM Provides sequenced, reliable, two-way, connection-based byte streams. An out-of-band data transmission mechanism may be supported. .TP .B SOCK_DGRAM Supports datagrams (connectionless, unreliable messages of a fixed maximum length). .TP .B SOCK_SEQPACKET Provides a sequenced, reliable, two-way connection-based data transmission path for datagrams of fixed maximum length; a consumer is required to read an entire packet with each input system call. .TP .B SOCK_RAW Provides raw network protocol access. .TP .B SOCK_RDM Provides a reliable datagram layer that does not guarantee ordering. .TP .B SOCK_PACKET Obsolete and should not be used in new programs; see .BR packet (7). .PP Some socket types may not be implemented by all protocol families. .PP Since Linux 2.6.27, the .I type argument serves a second purpose: in addition to specifying a socket type, it may include the bitwise OR of any of the following values, to modify the behavior of .BR socket (): .TP 16 .B SOCK_NONBLOCK Set the .BR O_NONBLOCK file status flag on the open file description (see .BR open (2)) referred to by the new file descriptor. Using this flag saves extra calls to .BR fcntl (2) to achieve the same result. .TP .B SOCK_CLOEXEC Set the close-on-exec .RB ( FD_CLOEXEC ) flag on the new file descriptor. See the description of the .B O_CLOEXEC flag in .BR open (2) for reasons why this may be useful. .PP The .I protocol specifies a particular protocol to be used with the socket. Normally only a single protocol exists to support a particular socket type within a given protocol family, in which case .I protocol can be specified as 0. However, it is possible that many protocols may exist, in which case a particular protocol must be specified in this manner. The protocol number to use is specific to the \*(lqcommunication domain\*(rq in which communication is to take place; see .BR protocols (5). See .BR getprotoent (3) on how to map protocol name strings to protocol numbers. .PP Sockets of type .B SOCK_STREAM are full-duplex byte streams. They do not preserve record boundaries. A stream socket must be in a .I connected state before any data may be sent or received on it. A connection to another socket is created with a .BR connect (2) call. Once connected, data may be transferred using .BR read (2) and .BR write (2) calls or some variant of the .BR send (2) and .BR recv (2) calls. When a session has been completed a .BR close (2) may be performed. Out-of-band data may also be transmitted as described in .BR send (2) and received as described in .BR recv (2). .PP The communications protocols which implement a .B SOCK_STREAM ensure that data is not lost or duplicated. If a piece of data for which the peer protocol has buffer space cannot be successfully transmitted within a reasonable length of time, then the connection is considered to be dead. When .B SO_KEEPALIVE is enabled on the socket the protocol checks in a protocol-specific manner if the other end is still alive. A .B SIGPIPE signal is raised if a process sends or receives on a broken stream; this causes naive processes, which do not handle the signal, to exit. .B SOCK_SEQPACKET sockets employ the same system calls as .B SOCK_STREAM sockets. The only difference is that .BR read (2) calls will return only the amount of data requested, and any data remaining in the arriving packet will be discarded. Also all message boundaries in incoming datagrams are preserved. .PP .B SOCK_DGRAM and .B SOCK_RAW sockets allow sending of datagrams to correspondents named in .BR sendto (2) calls. Datagrams are generally received with .BR recvfrom (2), which returns the next datagram along with the address of its sender. .PP .B SOCK_PACKET is an obsolete socket type to receive raw packets directly from the device driver. Use .BR packet (7) instead. .PP An .BR fcntl (2) .B F_SETOWN operation can be used to specify a process or process group to receive a .B SIGURG signal when the out-of-band data arrives or .B SIGPIPE signal when a .B SOCK_STREAM connection breaks unexpectedly. This operation may also be used to set the process or process group that receives the I/O and asynchronous notification of I/O events via .BR SIGIO . Using .B F_SETOWN is equivalent to an .BR ioctl (2) call with the .B FIOSETOWN or .B SIOCSPGRP argument. .PP When the network signals an error condition to the protocol module (e.g., using an ICMP message for IP) the pending error flag is set for the socket. The next operation on this socket will return the error code of the pending error. For some protocols it is possible to enable a per-socket error queue to retrieve detailed information about the error; see .B IP_RECVERR in .BR ip (7). .PP The operation of sockets is controlled by socket level .IR options . These options are defined in .IR . The functions .BR setsockopt (2) and .BR getsockopt (2) are used to set and get options. .SH RETURN VALUE On success, a file descriptor for the new socket is returned. On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS .TP .B EACCES Permission to create a socket of the specified type and/or protocol is denied. .TP .B EAFNOSUPPORT The implementation does not support the specified address family. .TP .B EINVAL Unknown protocol, or protocol family not available. .TP .B EINVAL .\" Since Linux 2.6.27 Invalid flags in .IR type . .TP .B EMFILE The per-process limit on the number of open file descriptors has been reached. .TP .B ENFILE The system-wide limit on the total number of open files has been reached. .TP .BR ENOBUFS " or " ENOMEM Insufficient memory is available. The socket cannot be created until sufficient resources are freed. .TP .B EPROTONOSUPPORT The protocol type or the specified protocol is not supported within this domain. .PP Other errors may be generated by the underlying protocol modules. .SH CONFORMING TO POSIX.1-2001, POSIX.1-2008, 4.4BSD. .PP The .B SOCK_NONBLOCK and .B SOCK_CLOEXEC flags are Linux-specific. .PP .BR socket () appeared in 4.2BSD. It is generally portable to/from non-BSD systems supporting clones of the BSD socket layer (including System\ V variants). .SH NOTES POSIX.1 does not require the inclusion of .IR , and this header file is not required on Linux. However, some historical (BSD) implementations required this header file, and portable applications are probably wise to include it. .PP The manifest constants used under 4.x BSD for protocol families are .BR PF_UNIX , .BR PF_INET , and so on, while .BR AF_UNIX , .BR AF_INET , and so on are used for address families. However, already the BSD man page promises: "The protocol family generally is the same as the address family", and subsequent standards use AF_* everywhere. .SH EXAMPLES An example of the use of .BR socket () is shown in .BR getaddrinfo (3). .SH SEE ALSO .BR accept (2), .BR bind (2), .BR close (2), .BR connect (2), .BR fcntl (2), .BR getpeername (2), .BR getsockname (2), .BR getsockopt (2), .BR ioctl (2), .BR listen (2), .BR read (2), .BR recv (2), .BR select (2), .BR send (2), .BR shutdown (2), .BR socketpair (2), .BR write (2), .BR getprotoent (3), .BR address_families (7), .BR ip (7), .BR socket (7), .BR tcp (7), .BR udp (7), .BR unix (7) .PP \(lqAn Introductory 4.3BSD Interprocess Communication Tutorial\(rq and \(lqBSD Interprocess Communication Tutorial\(rq, reprinted in .I UNIX Programmer's Supplementary Documents Volume 1. .SH COLOPHON This page is part of release 5.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.