Section (2) connect
connect — initiate a connection on a socket
#include <sys/types.h> /* See NOTES */ #include <sys/socket.h>
|const struct sockaddr *addr,|
connect() system call
connects the socket referred to by the file descriptor
sockfd to the address
specifies the size of
addr. The format of the address
addr is determined
by the address space of the socket
sockfd; see socket(2) for further
If the socket
sockfd is of type
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_SEQPACKET, this call attempts to make
a connection to the socket that is bound to the address
Generally, connection-based protocol sockets may
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
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.
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 path_resolution(7).)
- 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
sockfdhad 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_zsingle_quotesz_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 been completed.
sockfdis not a valid open file descriptor.
connect() on a stream socket found no one listening on the remote address.
The socket structure address is outside the user_zsingle_quotesz_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_ERRORoption at level
SOL_SOCKETto determine whether
connect() completed successfully (
SO_ERRORis zero) or unsuccessfully (
SO_ERRORis 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
sockfddoes 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 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.
For background on the socklen_t type, see accept(2).
connect() 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(3).
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Copyright 1993 Rickard E. Faith (faithcs.unc.edu)
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Modified 1997-01-31 by Eric S. Raymond <esrthyrsus.com>
Modified 1998, 1999 by Andi Kleen
Modified 2004-06-23 by Michael Kerrisk <mtk.manpagesgmail.com>