SOCKET(2) Linux Programmer's Manual SOCKET(2)
NAME
 socket - create an endpoint for communication
SYNOPSIS
 #include <sys/types.h> /* See NOTES */
 #include <sys/socket.h>
 int socket(int domain, int type, int protocol);
DESCRIPTION
 socket() creates an endpoint for communication and returns a file de-
 scriptor that refers to that endpoint. The file descriptor returned by
 a successful call will be the lowest-numbered file descriptor not cur-
 rently open for the process.
 The domain argument specifies a communication domain; this selects the
 protocol family which will be used for communication. These families
 are defined in <sys/socket.h>. The formats currently understood by the
 Linux kernel include:
 Name Purpose Man page
 AF_UNIX Local communication unix(7)
 AF_LOCAL Synonym for AF_UNIX
 AF_INET IPv4 Internet protocols ip(7)
 AF_AX25 Amateur radio AX.25 protocol ax25(4)
 AF_IPX IPX - Novell protocols
 AF_APPLETALK AppleTalk ddp(7)
 AF_X25 ITU-T X.25 / ISO-8208 protocol x25(7)
 AF_INET6 IPv6 Internet protocols ipv6(7)
 AF_DECnet DECet protocol sockets
 AF_KEY Key management protocol, originally de-
 veloped for usage with IPsec
 AF_NETLINK Kernel user interface device netlink(7)
 AF_PACKET Low-level packet interface packet(7)
 AF_RDS Reliable Datagram Sockets (RDS) protocol rds(7)
 rds-rdma(7)
 AF_PPPOX Generic PPP transport layer, for setting
 up L2 tunnels (L2TP and PPPoE)
 AF_LLC Logical link control (IEEE 802.2 LLC)
 protocol
 AF_IB InfiniBand native addressing
 AF_MPLS Multiprotocol Label Switching
 AF_CAN Controller Area Network automotive bus
 protocol
 AF_TIPC TIPC, "cluster domain sockets" protocol
 AF_BLUETOOTH Bluetooth low-level socket protocol
 AF_ALG Interface to kernel crypto API
 AF_VSOCK VSOCK (originally "VMWare VSockets") vsock(7)
 protocol for hypervisor-guest communica-
 tion
 AF_KCM KCM (kernel connection multiplexer) in-
 terface
 AF_XDP XDP (express data path) interface
 Further details of the above address families, as well as information
 on several other address families, can be found in address_families(7).
 The socket has the indicated type, which specifies the communication
 semantics. Currently defined types are:
 SOCK_STREAM Provides sequenced, reliable, two-way, connection-based
 byte streams. An out-of-band data transmission mecha-
 nism may be supported.
 SOCK_DGRAM Supports datagrams (connectionless, unreliable messages
 of a fixed maximum length).
 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 en-
 tire packet with each input system call.
 SOCK_RAW Provides raw network protocol access.
 SOCK_RDM Provides a reliable datagram layer that does not guar-
 antee ordering.
 SOCK_PACKET Obsolete and should not be used in new programs; see
 packet(7).
 Some socket types may not be implemented by all protocol families.
 Since Linux 2.6.27, the type argument serves a second purpose: in addi-
 tion to specifying a socket type, it may include the bitwise OR of any
 of the following values, to modify the behavior of socket():
 SOCK_NONBLOCK Set the O_NONBLOCK file status flag on the open file
 description (see open(2)) referred to by the new file
 descriptor. Using this flag saves extra calls to fc-
 ntl(2) to achieve the same result.
 SOCK_CLOEXEC Set the close-on-exec (FD_CLOEXEC) flag on the new file
 descriptor. See the description of the O_CLOEXEC flag
 in open(2) for reasons why this may be useful.
 The 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 protocol can
 be specified as 0. However, it is possible that many protocols may ex-
 ist, in which case a particular protocol must be specified in this man-
 ner. The protocol number to use is specific to the "communication do-
 main" in which communication is to take place; see protocols(5). See
 getprotoent(3) on how to map protocol name strings to protocol numbers.
 Sockets of type SOCK_STREAM are full-duplex byte streams. They do not
 preserve record boundaries. A stream socket must be in a connected
 state before any data may be sent or received on it. A connection to
 another socket is created with a connect(2) call. Once connected, data
 may be transferred using read(2) and write(2) calls or some variant of
 the send(2) and recv(2) calls. When a session has been completed a
 close(2) may be performed. Out-of-band data may also be transmitted as
 described in send(2) and received as described in recv(2).
 The communications protocols which implement a 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 SO_KEEPALIVE is enabled on the socket the protocol checks
 in a protocol-specific manner if the other end is still alive. A SIG-
 PIPE 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. SOCK_SEQPACKET sockets employ the same system calls as
 SOCK_STREAM sockets. The only difference is that 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 in-
 coming datagrams are preserved.
 SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to corre-
 spondents named in sendto(2) calls. Datagrams are generally received
 with recvfrom(2), which returns the next datagram along with the ad-
 dress of its sender.
 SOCK_PACKET is an obsolete socket type to receive raw packets directly
 from the device driver. Use packet(7) instead.
 An fcntl(2) F_SETOWN operation can be used to specify a process or
 process group to receive a SIGURG signal when the out-of-band data ar-
 rives or SIGPIPE signal when a SOCK_STREAM connection breaks unexpect-
 edly. 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 SIGIO. Using F_SETOWN is equivalent to an ioctl(2) call with the
 FIOSETOWN or SIOCSPGRP argument.
 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 er-
 ror; see IP_RECVERR in ip(7).
 The operation of sockets is controlled by socket level options. These
 options are defined in <sys/socket.h>. The functions setsockopt(2) and
 getsockopt(2) are used to set and get options.
RETURN VALUE
 On success, a file descriptor for the new socket is returned. On er-
 ror, -1 is returned, and errno is set appropriately.
ERRORS
 EACCES Permission to create a socket of the specified type and/or pro-
 tocol is denied.
 EAFNOSUPPORT
 The implementation does not support the specified address fam-
 ily.
 EINVAL Unknown protocol, or protocol family not available.
 EINVAL Invalid flags in type.
 EMFILE The per-process limit on the number of open file descriptors has
 been reached.
 ENFILE The system-wide limit on the total number of open files has been
 reached.
 ENOBUFS or ENOMEM
 Insufficient memory is available. The socket cannot be created
 until sufficient resources are freed.
 EPROTONOSUPPORT
 The protocol type or the specified protocol is not supported
 within this domain.
 Other errors may be generated by the underlying protocol modules.
CONFORMING TO
 POSIX.1-2001, POSIX.1-2008, 4.4BSD.
 The SOCK_NONBLOCK and SOCK_CLOEXEC flags are Linux-specific.
 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).
NOTES
 POSIX.1 does not require the inclusion of <sys/types.h>, 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.
 The manifest constants used under 4.x BSD for protocol families are
 PF_UNIX, PF_INET, and so on, while AF_UNIX, 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 sub-
 sequent standards use AF_* everywhere.
EXAMPLES
 An example of the use of socket() is shown in getaddrinfo(3).
SEE ALSO
 accept(2), bind(2), close(2), connect(2), fcntl(2), getpeername(2),
 getsockname(2), getsockopt(2), ioctl(2), listen(2), read(2), recv(2),
 select(2), send(2), shutdown(2), socketpair(2), write(2), getpro-
 toent(3), address_families(7), ip(7), socket(7), tcp(7), udp(7),
 unix(7)
 "An Introductory 4.3BSD Interprocess Communication Tutorial" and "BSD
 Interprocess Communication Tutorial", reprinted in UNIX Programmer's
 Supplementary Documents Volume 1.
COLOPHON
 This page is part of release 5.10 of the Linux 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/.
Linux 2020年06月09日 SOCKET(2)