connect

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.TH CONNECT 2 2008-12-03 "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>
.sp
.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.

If the socket
.I sockfd
is of type
.B 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 EAFNOSUPPORT
The passed address didn't have the correct address family in its
.I sa_family
field.
.TP
.B EAGAIN
No more free local ports or insufficient entries in the routing cache.
For
.B AF_INET
see the description of
.I /proc/sys/net/ipv4/ip_local_port_range
.BR ip (7)
for information on how to increase the number of local ports.
.TP
.B EALREADY
The socket is nonblocking and a previous connection attempt has not yet
been completed.
.TP
.B EBADF
The file descriptor is not a valid index in the descriptor table.
.TP
.B ECONNREFUSED
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 is not associated with a 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"
SVr4, 4.4BSD, (the
.BR connect ()
function first appeared in 4.2BSD), POSIX.1-2001.
.\" 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.1-2001 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.

The third argument of
.BR connect ()
is in reality an
.I int
(and this is what 4.x BSD and libc4 and libc5 have).
Some POSIX confusion resulted in the present
.IR socklen_t ,
also used by glibc.
See also
.BR accept (2).
.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 3.27 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.