CONNECT(2) Linux Manual Page

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Name

connect - initiate a connection on a socket

Synopsis

#include <sys/types.h>
#include <sys/socket.h>

int connect(int sockfd, const struct sockaddr *serv_addr, socklen_t addrlen);

Description

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.

Return Value

If the connection or binding succeeds, zero is returned. On error, -1 is returned, and errno is set appropriately.

Errors

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 table.
EFAULT: The socket structure address is outside the user's address space.
ENOTSOCK: The file descriptor is not associated with a socket.
EISCONN: The socket is already connected.
ECONNREFUSED: No one listening on the remote address.
ETIMEDOUT: 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.
ENETUNREACH: Network is unreachable.
EADDRINUSE: Local address is already in use.
EINPROGRESS: 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).
EALREADY: 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.
EAFNOSUPPORT: The passed address didn't have the correct address family in its sa_family field.
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.

Conforming to

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.

Note

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 accept(2) .

Bugs

Unconnecting a socket by calling connect with a AF_UNSPEC address is not yet implemented.