CONNECT(2) System Calls Manual CONNECT(2)

connect — initiate a connection on a socket

Standard C Library (libc, -lc)

#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. Each communications space interprets the name parameter in its own way. Generally, stream sockets may successfully () 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 a () call is interrupted by a signal, it will return with errno set to EINTR and the connection attempt will proceed as if the socket was non-blocking. Subsequent calls to connect() will set errno to EALREADY.

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 connect() call fails if:

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s is not a valid descriptor.
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s is a descriptor for a file, not a socket.
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The socket address length passed was outside the allowable range.
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The specified address is not available on this machine.
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Addresses in the specified address family cannot be used with this socket.
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The socket is already connected.
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Connection establishment timed out without establishing a connection.
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The attempt to connect was forcefully rejected.
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The network isn't reachable from this host.
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The address is already in use.
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The name parameter specifies an area outside the process address space.
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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 or polling the socket for writing. The success or failure of the connect operation may be determined by using getsockopt(2) to read the socket error status with the SO_ERROR option at the SOL_SOCKET level. The returned socket error status is zero on success, or one of the error codes listed here on failure.
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Either the socket is non-blocking mode or a previous call to connect() was interrupted by a signal, and the connection attempt has not yet been completed.
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The connection attempt was interrupted by a signal.

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.

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A component of the path prefix is not a directory.
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A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
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The named socket does not exist.
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Search permission is denied for a component of the path prefix, or write access to the named socket is denied.
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Too many symbolic links were encountered in translating the pathname.

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

The connect() function call appeared in 4.2BSD.

March 27, 2021 NetBSD 11.0