CONNECT(2)

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 connect() system call connects the socket referred to by the file descriptor sockfd to the address specified by serv_addr. The addrlen argument specifies the size of serv_addr. The format of the address in serv_addr is determined by the address space of the socket sockfd; see socket(2) for further details. If the socket sockfd is of type SOCK_DGRAM then serv_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_STREAM or SOCK_SEQPACKET, this call attempts to make a connection to the socket that is bound to the address specified by serv_addr.

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.

EACCES
    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(2).)

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.

EADDRINUSE
    Local address is already in use.

EAFNOSUPPORT
    The passed address didn't have the correct address family in its sa_family field.

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.

EALREADY
    The socket is non-blocking and a previous connection attempt has not yet been completed.

EBADF
    The file descriptor is not a valid index in the descriptor table.

ECONNREFUSED
    No one listening on the remote address.

EFAULT
    The socket structure address is outside the user's address space.

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(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).

EINTR
    The system call was interrupted by a signal that was caught.

EISCONN
    The socket is already connected.

ENETUNREACH
    Network is unreachable.

ENOTSOCK
    The file descriptor is not associated with a socket.

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.

CONFORMING TO
SVr4, 4.4BSD (the connect() function first appeared in 4.2BSD). 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 4.x BSD and libc4 and libc5 have). Some POSIX confusion resulted in the present socklen_t, also used by glibc. See also accept(2).

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

SEE ALSO
accept(2), bind(2), getsockname(2), listen(2), path_resolution(2), socket(2)