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CONNECT(2) System Calls Manual CONNECT(2)

connect — initiate a connection on a socket

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

int
connect(int socket, const struct sockaddr *address, socklen_t address_len);

The parameter socket 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 address, which is an address in the communications space of the socket.

Each communications space interprets the address 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 calling disconnectx(2), or by connecting to an invalid address, such as a null address or an address with the address family set to AF_UNSPEC (the error EAFNOSUPPORT will be harmlessly returned).

Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and the global integer variable errno is set to indicate the error.

The connect() system call will fail if:

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The destination address is a broadcast address and the socket option SO_BROADCAST is not set.
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The address is already in use.
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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 non-blocking and a previous connection attempt has not yet been completed.
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socket is not a valid descriptor.
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The attempt to connect was ignored (because the target is not listening for connections) or explicitly rejected.
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The address parameter specifies an area outside the process address space.
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The target host cannot be reached (e.g., down, disconnected).
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The socket is non-blocking and the connection cannot be completed immediately. It is possible to select(2) for completion by selecting the socket for writing.
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Its execution was interrupted by a signal.
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An invalid argument was detected (e.g., address_len is not valid for the address family, the specified address family is invalid).
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The socket is already connected.
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The local network interface is not functioning.
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The network isn't reachable from this host.
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The system call was unable to allocate a needed memory buffer.
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socket is not a file descriptor for a socket.
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Because socket is listening, no connection is allowed.
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address has a different type than the socket that is bound to the specified peer address.
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Connection establishment timed out without establishing a connection.
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Remote host reset the connection request.

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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Search permission is denied for a component of the path prefix.
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Write access to the named socket is denied.
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An I/O error occurred while reading from or writing to the file system.
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Too many symbolic links were encountered in translating the pathname. This is taken to be indicative of a looping symbolic link.
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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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A component of the path prefix is not a directory.

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

The include file <sys/types.h> is necessary.

accept(2), connectx(2), disconnectx(2), getsockname(2), select(2), socket(2), compat(5)

The connect() function call appeared in 4.2BSD.

March 18, 2015 BSD 4.2