path: root/man2/connect.2

.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
.\" Portions extracted from /usr/include/sys/socket.h, which does not have
.\" any authorship information in it.  It is probably available under the GPL.
.\"
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.\"
.\"
.\" Other portions are from the 6.9 (Berkeley) 3/10/91 man page:
.\"
.\" Copyright (c) 1983 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" SPDX-License-Identifier: BSD-4-Clause-UC
.\"
.\" 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 2023-03-30 "Linux man-pages 6.05.01"
.SH NAME
connect \- initiate a connection on a socket
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.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
Some protocol sockets (e.g., UNIX domain stream sockets)
may successfully
.BR connect ()
only once.
.PP
Some protocol sockets
(e.g., datagram sockets in the UNIX and Internet domains)
may use
.BR connect ()
multiple times to change their association.
.PP
Some protocol sockets
(e.g., TCP sockets as well as datagram sockets in the UNIX and
Internet domains)
may dissolve the association by connecting to an address with the
.I sa_family
member of
.I sockaddr
set to
.BR AF_UNSPEC ;
thereafter, the socket can be connected to another address.
.RB ( AF_UNSPEC
is supported since Linux 2.2.)
.SH RETURN VALUE
If the connection or binding succeeds, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.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 EACCES
It can also be returned if an SELinux policy denied a connection (for
example, if there is a policy saying that an HTTP proxy can only
connect to ports associated with HTTP servers, and the proxy tries to
connect to a different port).
.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
For nonblocking UNIX domain sockets, the socket is nonblocking, and the
connection cannot be completed immediately.
For other socket families, there are 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.
(UNIX domain sockets failed with
.B EAGAIN
instead.)
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 STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, 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
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 EXAMPLES
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),
.BR selinux (8)