CONNECT(2) Linux Programmer's Manual CONNECT(2)
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 argument specifies the size of addr. The for-
mat 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; connec-
tionless 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
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 directo-
ries in the path prefix. (See also path_resolution(7).)
The user tried to connect to a broadcast address without having the socket broad-
cast flag enabled or the connection request failed because of a local firewall
Local address is already in use.
The passed address didn't have the correct address family in its sa_family field.
Non-existent interface was requested or the requested address was not local.
The socket is nonblocking and a previous connection attempt has not yet been com-
EBADF The file descriptor is not a valid index in the descriptor table.
No-one listening on the remote address.
EFAULT The socket structure address is outside the user's address space.
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 suc-
cessfully (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).
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 con-
nections. 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)
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Linux 2008-12-03 CONNECT(2)