The file descriptor sockfd must refer to a socket. If the socket is of
type SOCK_DGRAM then the serv_addr address 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 another
socket. The other socket is specified by serv_addr, which is an address
(of length addrlen) in the communications space of the socket. Each
communications space interprets the serv_addr parameter in its own way.
Generally, connection-based protocol sockets may successfully connect
only 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.
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
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.
The passed address didn't have the correct address family
in its sa_family field.
No more free local ports or insufficient entries in the
routing cache. For PF_INET see the
net.ipv4.ip_local_port_range sysctl in ip(7) on how to
increase the number of local ports.
The socket is non-blocking and a previous connection
attempt has not yet been completed.
The file descriptor is not a valid index in the descriptor
No one listening on the remote address.
The socket structure address is outside the user's address
The socket is non-blocking 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 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 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 connections. 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 BSD 4.2). SVr4
documents the additional general error codes EADDRNOTAVAIL,
EINVAL, EAFNOSUPPORT, EALREADY, EINTR,
EPROTOTYPE, and ENOSR. It also documents many additional error
conditions not described here.