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
to the address specified by addr
argument specifies the size of addr
. The format of the
address in addr
is determined by the address space of the socket
; see socket
(2) for further details.
If the socket sockfd
is of type SOCK_DGRAM
, then addr
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
, 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
times to change their association. Connectionless sockets may dissolve the
association by connecting to an address with the sa_family
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
- 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
- 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 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
/proc/sys/net/ipv4/ip_local_port_range in ip(7).
- The passed address didn't have the correct address family in its
- For nonblocking UNIX domain sockets, the socket is nonblocking, and the
connection cannot be completed immediately. For other socket families,
there are insufficient entries in the routing cache.
- The socket is nonblocking and a previous connection attempt has not yet
- sockfd is not a valid open file descriptor.
- A connect() on a stream socket found no one listening on the remote
- The socket structure address is outside the user's address space.
- The socket is nonblocking and the connection cannot be completed
immediately. (UNIX domain sockets failed with EAGAIN instead.) 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
- The system call was interrupted by a signal that was caught; see
- The socket is already connected.
- Network is unreachable.
- The file descriptor sockfd does not refer to 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.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD, (connect()
first appeared in
POSIX.1 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.
For background on the socklen_t
type, see accept
() 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 connect
() is shown in getaddrinfo