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 firewall rule.
- 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
- The passed address didn't have the correct address family
in its sa_family field.
- Insufficient entries in the routing cache.
- The socket is nonblocking and a previous connection attempt
has not yet been completed.
- sockfd is not a valid open file descriptor.
- No-one listening on the remote address.
- 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 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).
- 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 sockfd does not refer to a
- 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, (the connect
() function first
appeared in 4.2BSD).
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
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