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CONNECT(2) System Calls Manual CONNECT(2)

connect — initiate a connection on a socket

#include <sys/socket.h>

int
connect(int s, const struct sockaddr *name, socklen_t namelen);

The parameter s is a socket. If it is of type SOCK_DGRAM, this call specifies the peer with which the socket is to be associated; this address is that to which datagrams are to be sent, and the only address from which datagrams are to be received. If the socket is of type SOCK_STREAM, this call attempts to make a connection to another socket. The other socket is specified by name, which is an address in the communications space of the socket. namelen indicates the amount of space pointed to by name, in bytes; the sa_len member of name is ignored. Each communications space interprets the name parameter in its own way. Generally, stream sockets may use () only once; datagram sockets may use connect() multiple times to change their association. Datagram sockets may dissolve the association by connecting to an invalid address, such as a null address.

If the socket is in non-blocking mode and the connection cannot be completed immediately, or if it is interrupted by a signal, () will return an error and the connection attempt will proceed asynchronously. Subsequent calls to connect() will fail with errno set to EALREADY. It is possible to use select(2) or poll(2) to determine when the connect operation has completed by checking the socket for writability. The success or failure of the connection attempt may be determined by using getsockopt(2) to check the socket error status with the SO_ERROR option at the SOL_SOCKET level. If the connection was successful, the error value will be zero. Otherwise, it will be one of the error values listed below.

If the connection or binding succeeds, 0 is returned. Otherwise a -1 is returned, and a more specific error code is stored in errno.

The following code connects to the host described by name and handles the case where connect() is interrupted by a signal.

#include <sys/socket.h>
#include <poll.h>
#include <errno.h>
#include <err.h>

int
connect_wait(int s)
{
	struct pollfd pfd[1];
	int error = 0;
	socklen_t len = sizeof(error);

	pfd[0].fd = s;
	pfd[0].events = POLLOUT;

	if (poll(pfd, 1, -1) == -1)
		return -1;
	if (getsockopt(s, SOL_SOCKET, SO_ERROR, &error, &len) < 0)
		return -1;
	if (error != 0) {
		errno = error;
		return -1;
	}
	return 0;
}

...

int retcode;

...

for (retcode = connect(s, name, namelen);
    retcode != 0 && errno == EINTR;
    retcode = connect_wait(s))
	continue;
if (retcode == -1)
	err(1, "connect");

The connect() call fails if:

[]
s is not a valid descriptor.
[]
s is not a socket.
[]
The specified address is not available on this machine.
[]
Addresses in the specified address family cannot be used with this socket.
[]
The socket is already connected.
[]
Connection establishment timed out without establishing a connection.
[]
A TCP connection with a local broadcast, the all-ones or a multicast address as the peer was attempted.
[]
The attempt to connect was forcefully rejected.
[]
The destination address specified an unreachable host.
[]
The connection attempt was interrupted by a signal. The attempt will continue asynchronously as if the socket was non-blocking.
[]
The network isn't reachable from this host.
[]
The address is already in use.
[]
The name parameter specifies an area outside the process address space.
[]
The socket is non-blocking and the connection cannot be completed immediately.
[]
Either the socket is non-blocking or a previous call to connect() was interrupted by a signal, and the connection attempt has not yet been completed.
[]
A TCP connection on a socket with socket option TCP_MD5SIG was attempted without configuring the security parameters correctly.

The following errors are specific to connecting names in the UNIX-domain. These errors may not apply in future versions of the UNIX IPC domain.

[]
A component of the path prefix is not a directory.
[]
A component of a pathname exceeded NAME_MAX characters, or an entire pathname (including the terminating NUL) exceeded PATH_MAX bytes.
[]
The named socket does not exist.
[]
Search permission is denied for a component of the path prefix.
[]
Write access to the named socket is denied.
[]
Too many symbolic links were encountered in translating the pathname.
[]
The file described by name is of a different type than s. E.g., s may be of type SOCK_STREAM whereas name may refer to a socket of type SOCK_DGRAM.

accept(2), getsockname(2), getsockopt(2), poll(2), select(2), socket(2)

The connect() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

The connect() system call first appeared in 4.1cBSD.

March 2, 2017 OpenBSD-6.2