1socket(2)                     System Calls Manual                    socket(2)
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NAME

6       socket - create an endpoint for communication
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <sys/socket.h>
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14       int socket(int domain, int type, int protocol);
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DESCRIPTION

17       socket()  creates  an endpoint for communication and returns a file de‐
18       scriptor that refers to that endpoint.  The file descriptor returned by
19       a  successful call will be the lowest-numbered file descriptor not cur‐
20       rently open for the process.
21
22       The domain argument specifies a communication domain; this selects  the
23       protocol  family  which will be used for communication.  These families
24       are defined in <sys/socket.h>.  The formats currently understood by the
25       Linux kernel include:
26
27       Name         Purpose                                    Man page
28       AF_UNIX      Local communication                        unix(7)
29       AF_LOCAL     Synonym for AF_UNIX
30       AF_INET      IPv4 Internet protocols                    ip(7)
31       AF_AX25      Amateur radio AX.25 protocol               ax25(4)
32       AF_IPX       IPX - Novell protocols
33       AF_APPLETALK AppleTalk                                  ddp(7)
34       AF_X25       ITU-T X.25 / ISO-8208 protocol             x25(7)
35       AF_INET6     IPv6 Internet protocols                    ipv6(7)
36       AF_DECnet    DECet protocol sockets
37       AF_KEY       Key  management protocol, originally de‐
38                    veloped for usage with IPsec
39       AF_NETLINK   Kernel user interface device               netlink(7)
40       AF_PACKET    Low-level packet interface                 packet(7)
41       AF_RDS       Reliable Datagram Sockets (RDS) protocol   rds(7)
42                                                               rds-rdma(7)
43       AF_PPPOX     Generic PPP transport layer, for setting
44                    up L2 tunnels (L2TP and PPPoE)
45       AF_LLC       Logical  link  control  (IEEE 802.2 LLC)
46                    protocol
47       AF_IB        InfiniBand native addressing
48       AF_MPLS      Multiprotocol Label Switching
49       AF_CAN       Controller Area Network  automotive  bus
50                    protocol
51       AF_TIPC      TIPC, "cluster domain sockets" protocol
52       AF_BLUETOOTH Bluetooth low-level socket protocol
53       AF_ALG       Interface to kernel crypto API
54       AF_VSOCK     VSOCK   (originally  "VMWare  VSockets")   vsock(7)
55                    protocol for hypervisor-guest communica‐
56                    tion
57       AF_KCM       KCM  (kernel connection multiplexer) in‐
58                    terface
59       AF_XDP       XDP (express data path) interface
60
61       Further details of the above address families, as well  as  information
62       on several other address families, can be found in address_families(7).
63
64       The  socket  has  the indicated type, which specifies the communication
65       semantics.  Currently defined types are:
66
67       SOCK_STREAM     Provides sequenced, reliable, two-way, connection-based
68                       byte  streams.  An out-of-band data transmission mecha‐
69                       nism may be supported.
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71       SOCK_DGRAM      Supports datagrams (connectionless, unreliable messages
72                       of a fixed maximum length).
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74       SOCK_SEQPACKET  Provides  a  sequenced,  reliable,  two-way connection-
75                       based data transmission path  for  datagrams  of  fixed
76                       maximum  length;  a consumer is required to read an en‐
77                       tire packet with each input system call.
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79       SOCK_RAW        Provides raw network protocol access.
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81       SOCK_RDM        Provides a reliable datagram layer that does not  guar‐
82                       antee ordering.
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84       SOCK_PACKET     Obsolete  and  should  not be used in new programs; see
85                       packet(7).
86
87       Some socket types may not be implemented by all protocol families.
88
89       Since Linux 2.6.27, the type argument serves a second purpose: in addi‐
90       tion  to specifying a socket type, it may include the bitwise OR of any
91       of the following values, to modify the behavior of socket():
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93       SOCK_NONBLOCK   Set the O_NONBLOCK file status flag on  the  open  file
94                       description  (see  open(2)) referred to by the new file
95                       descriptor.  Using this flag saves extra calls  to  fc‐
96                       ntl(2) to achieve the same result.
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98       SOCK_CLOEXEC    Set the close-on-exec (FD_CLOEXEC) flag on the new file
99                       descriptor.  See the description of the O_CLOEXEC  flag
100                       in open(2) for reasons why this may be useful.
101
102       The  protocol  specifies  a  particular  protocol  to  be used with the
103       socket.  Normally only a single protocol exists to support a particular
104       socket  type within a given protocol family, in which case protocol can
105       be specified as 0.  However, it is possible that many protocols may ex‐
106       ist, in which case a particular protocol must be specified in this man‐
107       ner.  The protocol number to use is specific to the “communication  do‐
108       main”  in  which communication is to take place; see protocols(5).  See
109       getprotoent(3) on how to map protocol name strings to protocol numbers.
110
111       Sockets of type SOCK_STREAM are full-duplex byte streams.  They do  not
112       preserve  record  boundaries.   A  stream socket must be in a connected
113       state before any data may be sent or received on it.  A  connection  to
114       another socket is created with a connect(2) call.  Once connected, data
115       may be transferred using read(2) and write(2) calls or some variant  of
116       the  send(2)  and  recv(2)  calls.  When a session has been completed a
117       close(2) may be performed.  Out-of-band data may also be transmitted as
118       described in send(2) and received as described in recv(2).
119
120       The  communications protocols which implement a SOCK_STREAM ensure that
121       data is not lost or duplicated.  If a piece of data for which the  peer
122       protocol  has  buffer space cannot be successfully transmitted within a
123       reasonable length of time, then the  connection  is  considered  to  be
124       dead.   When  SO_KEEPALIVE is enabled on the socket the protocol checks
125       in a protocol-specific manner if the other end is still alive.  A  SIG‐
126       PIPE  signal  is  raised  if  a  process  sends or receives on a broken
127       stream; this causes naive processes, which do not handle the signal, to
128       exit.    SOCK_SEQPACKET   sockets  employ  the  same  system  calls  as
129       SOCK_STREAM sockets.  The only difference is that  read(2)  calls  will
130       return only the amount of data requested, and any data remaining in the
131       arriving packet will be discarded.  Also all message boundaries in  in‐
132       coming datagrams are preserved.
133
134       SOCK_DGRAM  and  SOCK_RAW  sockets allow sending of datagrams to corre‐
135       spondents named in sendto(2) calls.  Datagrams are  generally  received
136       with  recvfrom(2),  which  returns the next datagram along with the ad‐
137       dress of its sender.
138
139       SOCK_PACKET is an obsolete socket type to receive raw packets  directly
140       from the device driver.  Use packet(7) instead.
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142       An  fcntl(2)  F_SETOWN  operation  can  be used to specify a process or
143       process group to receive a SIGURG signal when the out-of-band data  ar‐
144       rives  or SIGPIPE signal when a SOCK_STREAM connection breaks unexpect‐
145       edly.  This operation may also be used to set the  process  or  process
146       group that receives the I/O and asynchronous notification of I/O events
147       via SIGIO.  Using F_SETOWN is equivalent to an ioctl(2) call  with  the
148       FIOSETOWN or SIOCSPGRP argument.
149
150       When  the  network  signals  an  error condition to the protocol module
151       (e.g., using an ICMP message for IP) the pending error flag is set  for
152       the  socket.   The  next operation on this socket will return the error
153       code of the pending error.  For some protocols it is possible to enable
154       a per-socket error queue to retrieve detailed information about the er‐
155       ror; see IP_RECVERR in ip(7).
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157       The operation of sockets is controlled by socket level options.   These
158       options are defined in <sys/socket.h>.  The functions setsockopt(2) and
159       getsockopt(2) are used to set and get options.
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RETURN VALUE

162       On success, a file descriptor for the new socket is returned.   On  er‐
163       ror, -1 is returned, and errno is set to indicate the error.
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ERRORS

166       EACCES Permission  to create a socket of the specified type and/or pro‐
167              tocol is denied.
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169       EAFNOSUPPORT
170              The implementation does not support the specified  address  fam‐
171              ily.
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173       EINVAL Unknown protocol, or protocol family not available.
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175       EINVAL Invalid flags in type.
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177       EMFILE The per-process limit on the number of open file descriptors has
178              been reached.
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180       ENFILE The system-wide limit on the total number of open files has been
181              reached.
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183       ENOBUFS or ENOMEM
184              Insufficient  memory is available.  The socket cannot be created
185              until sufficient resources are freed.
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187       EPROTONOSUPPORT
188              The protocol type or the specified  protocol  is  not  supported
189              within this domain.
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191       Other errors may be generated by the underlying protocol modules.
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STANDARDS

194       POSIX.1-2008.
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196       SOCK_NONBLOCK and SOCK_CLOEXEC are Linux-specific.
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HISTORY

199       POSIX.1-2001, 4.4BSD.
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201       socket()  appeared in 4.2BSD.  It is generally portable to/from non-BSD
202       systems supporting clones of the BSD socket layer  (including  System V
203       variants).
204
205       The  manifest  constants  used  under 4.x BSD for protocol families are
206       PF_UNIX, PF_INET, and so on, while AF_UNIX, AF_INET, and so on are used
207       for address families.  However, already the BSD man page promises: "The
208       protocol family generally is the same as the address family", and  sub‐
209       sequent standards use AF_* everywhere.
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EXAMPLES

212       An example of the use of socket() is shown in getaddrinfo(3).
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SEE ALSO

215       accept(2),  bind(2),  close(2),  connect(2),  fcntl(2), getpeername(2),
216       getsockname(2), getsockopt(2), ioctl(2), listen(2),  read(2),  recv(2),
217       select(2),   send(2),  shutdown(2),  socketpair(2),  write(2),  getpro‐
218       toent(3),  address_families(7),  ip(7),  socket(7),   tcp(7),   udp(7),
219       unix(7)
220
221       “An  Introductory  4.3BSD Interprocess Communication Tutorial” and “BSD
222       Interprocess Communication Tutorial”, reprinted  in  UNIX  Programmer's
223       Supplementary Documents Volume 1.
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227Linux man-pages 6.05              2023-03-30                         socket(2)
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