CONNECT(2) Linux Programmer's Manual CONNECT(2)NAME
connect - initiate a connection on a socket
#include <sys/types.h> /* See NOTES */
int connect(int sockfd, const struct sockaddr *addr,
The connect() system call connects the socket referred to by the file
descriptor sockfd to the address specified by addr. The addrlen argu‐
ment 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
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
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.
Local address is already in use.
The passed address didn't have the correct address family in its
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.
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 ta‐
No-one listening on the remote address.
EFAULT The socket structure address is outside the user's address
The socket is nonblocking and the connection cannot be completed
immediately. It is possible to select(2) or poll(2) for comple‐
tion by selecting the socket for writing. After select(2) indi‐
cates 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
The socket is already connected.
Network is unreachable.
The file descriptor is not associated with a socket.
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 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).
SEE ALSOaccept(2), bind(2), getsockname(2), listen(2), socket(2), path_resolu‐
This page is part of release 3.54 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)