Linux man pages : connect (2)
CONNECT(2) Linux Programmer's Manual CONNECT(2)
connect - initiate a connection on a socket
int connect(int sockfd, const struct sockaddr *serv_addr, socklen_t
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 data-
grams are received. If the socket is of type SOCK_STREAM or SOCK_SEQ-
PACKET, 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 communica-
tions 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.
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.
EBADF The file descriptor is not a valid index in the descriptor ta-
EFAULT The socket structure address is outside the user's address
The file descriptor is not associated with a socket.
The socket is already connected.
No one listening on the remote address.
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.
Network is unreachable.
Local address is already in use.
The socket is non-blocking and the connection cannot be com-
pleted 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 com-
pleted successfully (SO_ERROR is zero) or unsuccessfully
(SO_ERROR is one of the usual error codes listed here, explain-
ing the reason for the failure).
The socket is non-blocking and a previous connection attempt has
not yet been completed.
EAGAIN 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 passed address didn't have the correct address family in its
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.
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 docu-
ments many additional error conditions not described here.
The third argument of connect is in reality an int (and this is what
BSD 4.* and libc4 and libc5 have). Some POSIX confusion resulted in
the present socklen_t. The draft standard has not been adopted yet,
but glibc2 already follows it and also has socklen_t. See also
Unconnecting a socket by calling connect with a AF_UNSPEC address is
not yet implemented.
accept(2), bind(2), listen(2), socket(2), getsockname(2)
Linux 2.2 1998-10-03 CONNECT(2)