SOCKET(2) Linux Programmer's Manual SOCKET(2)
NAME
socket - create an endpoint for communication
socket函数用来申请一个通信端口。
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 descriptor.
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 currently understood formats include:
domain defination
Name Purpose Man page
AF_UNIX, AF_LOCAL Local communication unix(7)
AF_INET IPv4 Internet protocols ip(7)
AF_INET6 IPv6 Internet protocols ipv6(7)
AF_IPX IPX - Novell protocols
AF_NETLINK Kernel user interface device netlink(7)
AF_X25 ITU-T X.25 / ISO-8208 protocol x25(7)
AF_AX25 Amateur radio AX.25 protocol
AF_ATMPVC Access to raw ATM PVCs
AF_APPLETALK Appletalk ddp(7)
AF_PACKET Low level packet interface packet(7)
- PF_UNIX、PF_LOCAL、AF_UNIX、AF_LOCAL：这些是Unix进程通信协议。
- PF_INET、AF_INET：IPv4网络协议。
- PF_INET6、AF_INT6：IPv6网络协议。
- PF_IPX、AF_IPX：IPX-Novell协议。
- PF_NETLINK、AF_NETLINK：核心用户接口装置。
- PF_X25、AF_X25、ITU-T X.25: ISO-8208协议。
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 mechanism may be supported.
提供面向连接的稳定数据传输，即TCP协议。
SOCK_DGRAM Supports datagrams (connectionless, unreliable messages of a fixed maximum length).
使用不连续不可靠的数据包连接。
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. 提供连续可靠的数据包连接。
SOCK_RAW Provides raw network protocol access.
SOCK_RDM Provides a reliable datagram layer that does not guarantee 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; for example, SOCK_SEQPACKET is not implemented for AF_INET.
Since Linux 2.6.27, the 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 socket():
SOCK_NONBLOCK Set the O_NONBLOCK file status flag on the new open file description. Using this flag saves extra calls to fcntl(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 exist, in which case a particular protocol must be specified in this manner. The protocol number to use is specific to the “communication domain” 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, similar to pipes. 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 connec?
tion 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 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. 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 arriv?
ing packet will be discarded. Also all message boundaries in incoming datagrams are preserved.
SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to correspondents named in sendto(2) calls. Datagrams are generally received with recvfrom(2), which returns the next datagram along with the address 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 arrives or SIGPIPE signal when a 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 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 a 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 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, respectively.
RETURN VALUE
On success, a file descriptor for the new socket is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS
EACCES Permission to create a socket of the specified type and/or protocol is denied.
EAFNOSUPPORT
The implementation does not support the specified address family.
EINVAL Unknown protocol, or protocol family not available.
EINVAL Invalid flags in type.
EMFILE Process file table overflow.
ENFILE The system 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
4.4BSD, POSIX.1-2001.
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.1-2001 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 subsequent standards use AF_* everywhere.
EXAMPLE
An example of the use of socket() is shown in getaddrinfo(3).
SEE ALSO
accept(2), bind(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), getprotoent(3), 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 3.54 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
1 // /usr/include/bits/socket.h 2 3 /* Protocol families. */ 4 #define PF_UNSPEC 0 /* Unspecified. */ 5 #define PF_LOCAL 1 /* Local to host (pipes and file-domain). */ 6 #define PF_UNIX PF_LOCAL /* POSIX name for PF_LOCAL. */ 7 #define PF_FILE PF_LOCAL /* Another non-standard name for PF_LOCAL. */ 8 #define PF_INET 2 /* IP protocol family. */ 9 #define PF_AX25 3 /* Amateur Radio AX.25. */ 10 #define PF_IPX 4 /* Novell Internet Protocol. */ 11 #define PF_APPLETALK 5 /* Appletalk DDP. */ 12 #define PF_NETROM 6 /* Amateur radio NetROM. */ 13 #define PF_BRIDGE 7 /* Multiprotocol bridge. */ 14 #define PF_ATMPVC 8 /* ATM PVCs. */ 15 #define PF_X25 9 /* Reserved for X.25 project. */ 16 #define PF_INET6 10 /* IP version 6. */ 17 #define PF_ROSE 11 /* Amateur Radio X.25 PLP. */ 18 #define PF_DECnet 12 /* Reserved for DECnet project. */ 19 #define PF_NETBEUI 13 /* Reserved for 802.2LLC project. */ 20 #define PF_SECURITY 14 /* Security callback pseudo AF. */ 21 #define PF_KEY 15 /* PF_KEY key management API. */ 22 #define PF_NETLINK 16 23 #define PF_ROUTE PF_NETLINK /* Alias to emulate 4.4BSD. */ 24 #define PF_PACKET 17 /* Packet family. */ 25 #define PF_ASH 18 /* Ash. */ 26 #define PF_ECONET 19 /* Acorn Econet. */ 27 #define PF_ATMSVC 20 /* ATM SVCs. */ 28 #define PF_RDS 21 /* RDS sockets. */ 29 #define PF_SNA 22 /* Linux SNA Project */ 30 #define PF_IRDA 23 /* IRDA sockets. */ 31 #define PF_PPPOX 24 /* PPPoX sockets. */ 32 #define PF_WANPIPE 25 /* Wanpipe API sockets. */ 33 #define PF_LLC 26 /* Linux LLC. */ 34 #define PF_CAN 29 /* Controller Area Network. */ 35 #define PF_TIPC 30 /* TIPC sockets. */ 36 #define PF_BLUETOOTH 31 /* Bluetooth sockets. */ 37 #define PF_IUCV 32 /* IUCV sockets. */ 38 #define PF_RXRPC 33 /* RxRPC sockets. */ 39 #define PF_ISDN 34 /* mISDN sockets. */ 40 #define PF_PHONET 35 /* Phonet sockets. */ 41 #define PF_IEEE802154 36 /* IEEE 802.15.4 sockets. */ 42 #define PF_CAIF 37 /* CAIF sockets. */ 43 #define PF_ALG 38 /* Algorithm sockets. */ 44 #define PF_NFC 39 /* NFC sockets. */ 45 #define PF_VSOCK 40 /* vSockets. */ 46 #define PF_MAX 41 /* For now.. */ 47 48 49 /* Address families. */ 50 #define AF_UNSPEC PF_UNSPEC 51 #define AF_LOCAL PF_LOCAL 52 #define AF_UNIX PF_UNIX 53 #define AF_FILE PF_FILE 54 #define AF_INET PF_INET 55 #define AF_AX25 PF_AX25 56 #define AF_IPX PF_IPX 57 #define AF_APPLETALK PF_APPLETALK 58 #define AF_NETROM PF_NETROM 59 #define AF_BRIDGE PF_BRIDGE 60 #define AF_ATMPVC PF_ATMPVC 61 #define AF_X25 PF_X25 62 #define AF_INET6 PF_INET6 63 #define AF_ROSE PF_ROSE 64 #define AF_DECnet PF_DECnet 65 #define AF_NETBEUI PF_NETBEUI 66 #define AF_SECURITY PF_SECURITY 67 #define AF_KEY PF_KEY 68 #define AF_NETLINK PF_NETLINK 69 #define AF_ROUTE PF_ROUTE 70 #define AF_PACKET PF_PACKET 71 #define AF_ASH PF_ASH 72 #define AF_ECONET PF_ECONET 73 #define AF_ATMSVC PF_ATMSVC 74 #define AF_RDS PF_RDS 75 #define AF_SNA PF_SNA 76 #define AF_IRDA PF_IRDA 77 #define AF_PPPOX PF_PPPOX 78 #define AF_WANPIPE PF_WANPIPE 79 #define AF_LLC PF_LLC 80 #define AF_CAN PF_CAN 81 #define AF_TIPC PF_TIPC 82 #define AF_BLUETOOTH PF_BLUETOOTH 83 #define AF_IUCV PF_IUCV 84 #define AF_RXRPC PF_RXRPC 85 #define AF_ISDN PF_ISDN 86 #define AF_PHONET PF_PHONET 87 #define AF_IEEE802154 PF_IEEE802154 88 #define AF_CAIF PF_CAIF 89 #define AF_ALG PF_ALG 90 #define AF_NFC PF_NFC 91 #define AF_VSOCK PF_VSOCK 92 #define AF_MAX PF_MAX