Section: Linux Programmer's Manual (2)
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connect - initiate a connection on a socket
#include <sys/types.h> /* See NOTES */
int connect(int sockfd, const struct sockaddr *addr,
system call connects the socket referred to by the file descriptor
to the address specified by
argument specifies the size of
The format of the address in
is determined by the address space of the socket
for further details.
If the socket
is of type
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
this call attempts to make a connection to the socket that is bound
to the address specified by
Generally, connection-based protocol sockets may successfully
only once; connectionless protocol sockets may use
multiple times to change their association.
Connectionless sockets may
dissolve the association by connecting to an address with the
(supported on Linux since kernel 2.2).
If the connection or binding succeeds, zero is returned.
On error, -1 is returned, and
is set appropriately.
The following are general socket errors only.
There may be other domain-specific error codes.
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.
- 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
Local address is already in use.
(Internet domain sockets)
The socket referred to by
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
The passed address didn't have the correct address family in its
Insufficient entries in the routing cache.
The socket is nonblocking and a previous connection attempt has not yet
The file descriptor is not a valid index in the descriptor table.
No-one listening on the remote address.
The socket structure address is outside the user's address space.
The socket is nonblocking and the connection cannot be completed
It is possible to
for completion by selecting the socket for writing.
indicates writability, use
to read the
option at level
to determine whether
is zero) or unsuccessfully
is one of the usual error codes listed here,
explaining the reason for the failure).
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.
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.
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
function first appeared in 4.2BSD), POSIX.1-2001.
POSIX.1-2001 does not require the inclusion of
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
is in reality an
(and this is what 4.x BSD and libc4 and libc5 have).
Some POSIX confusion resulted in the present
also used by glibc.
fails, consider the state of the socket as unspecified.
Portable applications should close the socket and create a new one for
An example of the use of
is shown in
This page is part of release 3.74 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
- RETURN VALUE
- CONFORMING TO
- SEE ALSO