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CONNECT(2) Linux Programmer's Manual CONNECT(2)
connect - initiate a connection on a socket
#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int connect(int sockfd, const struct sockaddr *addr,
socklen_t addrlen);
The connect() system call connects the socket referred to by the file
descriptor sockfd to the address specified by addr. The addrlen argument
specifies the size of addr. The format of the address in addr is determined
by the address space of the socket sockfd; see socket(2) for further details.
If the socket sockfd is of type SOCK_DGRAM then 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 SOCK_STREAM or SOCK_SEQPACKET, this call
attempts to make a connection to the socket that is bound to the address
specified by addr.
Generally, connection-based protocol sockets may successfully connect() only
once; connectionless protocol sockets may use connect() multiple times to
change their association. Connectionless sockets may dissolve the association
by connecting to an address with the sa_family member of sockaddr set to
AF_UNSPEC (supported on Linux since kernel 2.2).
If the connection or binding succeeds, zero is returned. On error, -1 is
returned, and errno is set appropriately.
The following are general socket errors only. There may be other domain-
specific error codes.
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
path_resolution(7).)
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.
EADDRINUSE
Local address is already in use.
EAFNOSUPPORT
The passed address didn't have the correct address family in its
sa_family field.
EAGAIN No more free local ports or insufficient entries in the routing cache.
For AF_INET see the description of
/proc/sys/net/ipv4/ip_local_port_range ip(7) for information on how to
increase the number of local ports.
EALREADY
The socket is nonblocking and a previous connection attempt has not yet
been completed.
EBADF The file descriptor is not a valid index in the descriptor table.
ECONNREFUSED
No-one listening on the remote address.
EFAULT The socket structure address is outside the user's address space.
EINPROGRESS
The socket is nonblocking and the connection cannot be completed
immediately. It is possible to select(2) or poll(2) for completion by
selecting the socket for writing. After select(2) indicates
writability, use getsockopt(2) to read the SO_ERROR option at level
SOL_SOCKET to determine whether connect() completed successfully
(SO_ERROR is zero) or unsuccessfully (SO_ERROR is one of the usual
error codes listed here, explaining the reason for the failure).
EINTR The system call was interrupted by a signal that was caught; see
signal(7).
EISCONN
The socket is already connected.
ENETUNREACH
Network is unreachable.
ENOTSOCK
The file descriptor is not associated with a socket.
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.
SVr4, 4.4BSD, (the connect() function first appeared in 4.2BSD), POSIX.1-2001.
POSIX.1-2001 does not require the inclusion of <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 connect() is in reality an int (and this is what 4.x BSD
and libc4 and libc5 have). Some POSIX confusion resulted in the present
socklen_t, also used by glibc. See also accept(2).
An example of the use of connect() is shown in getaddrinfo(3).
accept(2), bind(2), getsockname(2), listen(2), socket(2), path_resolution(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-12-03 CONNECT(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface