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SOCKET(2)                  Linux Programmer's ManualSOCKET(2)NAME       socket - create an endpoint for communicationSYNOPSIS       #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 communicationunix(7)       AF_LOCAL     Synonym for AF_UNIX       AF_INET      IPv4 Internet protocolsip(7)       AF_AX25      Amateur radio AX.25 protocolax25(4)       AF_IPX       IPX - Novell protocols       AF_APPLETALK AppleTalkddp(7)       AF_X25       ITU-T X.25 / ISO-8208 protocolx25(7)       AF_INET6     IPv6 Internet protocolsipv6(7)       AF_DECnet    DECet protocol sockets       AF_KEY       Key  management protocol, originally de-                    veloped for usage with IPsec       AF_NETLINK   Kernel user interface devicenetlink(7)       AF_PACKET    Low-level packet interfacepacket(7)       AF_RDS       Reliable Datagram Sockets (RDS) protocolrds(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 multiplexor) 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 inaddress_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; seepacket(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  (seeopen(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                       inopen(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; seeprotocols(5).  Seegetprotoent(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 aconnect(2) call.  Once connected, data       may be transferred usingread(2) andwrite(2) calls or some variant  of       thesend(2)  andrecv(2)  calls.  When a session has been completed aclose(2) may be performed.  Out-of-band data may also be transmitted as       described insend(2) and received as described inrecv(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 thatread(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 insendto(2) calls.  Datagrams are  generally  received       withrecvfrom(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.  Usepacket(7) instead.       Anfcntl(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 anioctl(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 inip(7).       The operation of sockets is controlled by socket level options.   These       options are defined in <sys/socket.h>.  The functionssetsockopt(2) andgetsockopt(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.EXAMPLE       An example of the use of socket() is shown ingetaddrinfo(3).SEE ALSOaccept(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 5.05 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                             2019-03-06SOCKET(2)