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Raymond .\" Modified 1998, 1999 by Andi Kleen .\" Modified 2004-06-23 by Michael Kerrisk .\" .TH CONNECT 2 2016-12-12 "Linux" "Linux Programmer's Manual" .SH NAME connect \- initiate a connection on a socket .SH SYNOPSIS .nf .BR "#include " " /* See NOTES */" .br .B #include .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 .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 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, (the .BR connect () function 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 , 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. For background on the .I socklen_t type, see .BR accept (2). 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 4.10 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/.