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.\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>

.\" Modified 1998, 1999 by Andi Kleen

.\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>

.\"

.TH CONNECT 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

connect \- initiate a connection on a socket

.SH SYNOPSIS

.nf

.BR "#include <sys/types.h>" "          /* See NOTES */"

.br

.B #include <sys/socket.h>

.PP

.BI "int connect(int " sockfd ", const struct sockaddr *" addr ,

.BI "            socklen_t " addrlen );

.fi

.SH DESCRIPTION

The

.BR connect ()

system call connects the socket referred to by the file descriptor

.I sockfd

to the address specified by

.IR addr .

The

.I addrlen

argument specifies the size of

.IR addr .

The format of the address in

.I addr

is determined by the address space of the socket

.IR sockfd ;

see

.BR socket (2)

for further details.

.PP

If the socket

.I sockfd

is of type

.BR SOCK_DGRAM ,

then

.I addr

is the address to which datagrams are sent by default, and the only

address from which datagrams are received.

If the socket is of type

.B SOCK_STREAM

or

.BR SOCK_SEQPACKET ,

this call attempts to make a connection to the socket that is bound

to the address specified by

.IR addr .

.PP

Generally, connection-based protocol sockets may successfully

.BR connect ()

only once; connectionless protocol sockets may use

.BR connect ()

multiple times to change their association.

Connectionless sockets may

dissolve the association by connecting to an address with the

.I sa_family

member of

.I sockaddr

set to

.BR AF_UNSPEC

(supported on Linux since kernel 2.2).

.SH RETURN VALUE

If the connection or binding succeeds, zero is returned.

On error, \-1 is returned, and

.I errno

is set appropriately.

.SH ERRORS

The following are general socket errors only.

There may be other domain-specific error codes.

.TP

.B EACCES

For UNIX domain sockets, which are identified by pathname:

Write permission is denied on the socket file,

or search permission is denied for one of the directories

in the path prefix.

(See also

.BR path_resolution (7).)

.TP

.BR EACCES ", " EPERM

The user tried to connect to a broadcast address without having the socket

broadcast flag enabled or the connection request failed because of a local

firewall rule.

.TP

.B EADDRINUSE

Local address is already in use.

.TP

.B EADDRNOTAVAIL

(Internet domain sockets)

The socket referred to by

.I sockfd

had not previously been bound to an address and,

upon attempting to bind it to an ephemeral port,

it was determined that all port numbers in the ephemeral port range

are currently in use.

See the discussion of

.I /proc/sys/net/ipv4/ip_local_port_range

in

.BR ip (7).

.TP

.B EAFNOSUPPORT

The passed address didn't have the correct address family in its

.I sa_family

field.

.TP

.B EAGAIN

Insufficient entries in the routing cache.

.TP

.B EALREADY

The socket is nonblocking and a previous connection attempt has not yet

been completed.

.TP

.B EBADF

.I sockfd

is not a valid open file descriptor.

.TP

.B ECONNREFUSED

A

.BR connect ()

on a stream socket found no one listening on the remote address.

.TP

.B EFAULT

The socket structure address is outside the user's address space.

.TP

.B EINPROGRESS

The socket is nonblocking and the connection cannot be completed

immediately.

It is possible to

.BR select (2)

or

.BR poll (2)

for completion by selecting the socket for writing.

After

.BR select (2)

indicates writability, use

.BR getsockopt (2)

to read the

.B SO_ERROR

option at level

.B SOL_SOCKET

to determine whether

.BR connect ()

completed successfully

.RB ( SO_ERROR

is zero) or unsuccessfully

.RB ( SO_ERROR

is one of the usual error codes listed here,

explaining the reason for the failure).

.TP

.B EINTR

The system call was interrupted by a signal that was caught; see

.BR signal (7).

.\" For TCP, the connection will complete asynchronously.

.\" See http://lkml.org/lkml/2005/7/12/254

.TP

.B EISCONN

The socket is already connected.

.TP

.B ENETUNREACH

Network is unreachable.

.TP

.B ENOTSOCK

The file descriptor

.I sockfd

does not refer to a socket.

.TP

.B EPROTOTYPE

The socket type does not support the requested communications protocol.

This error can occur, for example,

on an attempt to connect a UNIX domain datagram socket to a stream socket.

.TP

.B ETIMEDOUT

Timeout while attempting connection.

The server may be too

busy to accept new connections.

Note that for IP sockets the timeout may

be very long when syncookies are enabled on the server.

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD,

.RB (connect ()

first appeared in 4.2BSD).

.\" SVr4 documents the additional

.\" general error codes

.\" .BR EADDRNOTAVAIL ,

.\" .BR EINVAL ,

.\" .BR EAFNOSUPPORT ,

.\" .BR EALREADY ,

.\" .BR EINTR ,

.\" .BR EPROTOTYPE ,

.\" and

.\" .BR ENOSR .

.\" It also

.\" documents many additional error conditions not described here.

.SH NOTES

POSIX.1 does not require the inclusion of

.IR <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.

.PP

For background on the

.I socklen_t

type, see

.BR accept (2).

.PP

If

.BR connect ()

fails, consider the state of the socket as unspecified.

Portable applications should close the socket and create a new one for

reconnecting.

.SH EXAMPLE

An example of the use of

.BR connect ()

is shown in

.BR getaddrinfo (3).

.SH SEE ALSO

.BR accept (2),

.BR bind (2),

.BR getsockname (2),

.BR listen (2),

.BR socket (2),

.BR path_resolution (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.