1SOCKET(2)                  Linux Programmer's Manual                 SOCKET(2)


6       socket - create an endpoint for communication


9       #include <sys/socket.h>
11       int socket(int domain, int type, int protocol);


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


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


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


191       POSIX.1-2001, POSIX.1-2008, 4.4BSD.
193       The SOCK_NONBLOCK and SOCK_CLOEXEC flags are Linux-specific.
195       socket()  appeared in 4.2BSD.  It is generally portable to/from non-BSD
196       systems supporting clones of the BSD socket layer  (including  System V
197       variants).


200       The  manifest  constants  used  under 4.x BSD for protocol families are
201       PF_UNIX, PF_INET, and so on, while AF_UNIX, AF_INET, and so on are used
202       for address families.  However, already the BSD man page promises: "The
203       protocol family generally is the same as the address family", and  sub‐
204       sequent standards use AF_* everywhere.


207       An example of the use of socket() is shown in getaddrinfo(3).


210       accept(2),  bind(2),  close(2),  connect(2),  fcntl(2), getpeername(2),
211       getsockname(2), getsockopt(2), ioctl(2), listen(2),  read(2),  recv(2),
212       select(2),   send(2),  shutdown(2),  socketpair(2),  write(2),  getpro‐
213       toent(3),  address_families(7),  ip(7),  socket(7),   tcp(7),   udp(7),
214       unix(7)
216       “An  Introductory  4.3BSD Interprocess Communication Tutorial” and “BSD
217       Interprocess Communication Tutorial”, reprinted  in  UNIX  Programmer's
218       Supplementary Documents Volume 1.


221       This  page  is  part of release 5.13 of the Linux man-pages project.  A
222       description of the project, information about reporting bugs,  and  the
223       latest     version     of     this    page,    can    be    found    at
224       https://www.kernel.org/doc/man-pages/.
228Linux                             2021-03-22                         SOCKET(2)