# socket

## NAME



           socket - create an endpoint for communication
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/socket.h>**
    
           **int socket(int** _domain_**, int** _type_**, int** _protocol_**);**
    


## DESCRIPTION



           **socket**() creates an endpoint for communication and returns a file
           descriptor that refers to that endpoint.  The file descriptor
           returned by a successful call will be the lowest-numbered file
           descriptor not currently 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/IEC 8208 protocol         x25(7)
           **AF_INET6** IPv6 Internet protocols                    ipv6(7)
           **AF_DECnet** DECet protocol sockets
           **AF_KEY** Key management protocol, originally
                        developed 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
                        communication
           **AF_KCM** KCM (kernel connection multiplexer)
                        interface
           **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 mechanism
                  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 entire packet
                  with each input system call.
    
           **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.
    
           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 open file
                  description (see open(2)) referred to by the new file
                  descriptor.  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.  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 **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
           arriving 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 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 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.
    


## RETURN VALUE



           On success, a file descriptor for the new socket is returned.  On
           error, -1 is returned, and _errno_ is set to indicate the error.
    


## 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** 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.
    


## STANDARDS



           POSIX.1-2008.
    
           **SOCK_NONBLOCK** and **SOCK_CLOEXEC** are Linux-specific.
    


## EXAMPLES



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


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