connect(2) System Calls Manual connect(2)
connect - Connects two sockets
int connect ( int socket, const struct sockaddr *address, socklen_t address_len );
[XNS4.0] The definition of the connect() function in XNS4.0 uses a size_t data type instead of a socklen_t data type as specified in
XNS5.0 (the previous definition).
[Tru64 UNIX] The following definition of the connect() function does not conform to current standards and is supported only for backward
compatibility (see standards(5)):
int connect ( int socket, struct sockaddr *address, int address_len );
Interfaces documented on this reference page conform to industry standards as follows:
Refer to the standards(5) reference page for more information about industry standards and associated tags.
Specifies a file descriptor for the socket. Points to a sockaddr structure, the format of which is determined by the domain and by the
behavior requested for the socket. The sockaddr structure is an overlay for a sockaddr_in or sockaddr_un structure, depending on which of
the supported address families is active.
[Tru64 UNIX] If the compile-time option _SOCKADDR_LEN is defined before the sys/socket.h header file is included, the sockaddr
structure takes 4.4BSD behavior, with a field for specifying the length of the socket address. Otherwise, the default 4.3BSD sock-
addr structure is used, with the length of the socket address assumed to be 14 bytes or less.
If _SOCKADDR_LEN is defined, the 4.3BSD sockaddr structure is defined with the name osockaddr. Specifies the length of the sockaddr
structure pointed to by the address parameter.
The connect() function requests a connection between two sockets. The kernel sets up the communications links between the sockets; both
sockets must use the same address format and protocol.
The connect() function performs a different action for each of the following types of initiating sockets: If the initiating socket is
SOCK_DGRAM, the connect() function establishes the peer address. The peer address identifies the socket where all datagrams are sent on
subsequent send() functions. It also identifies the socket from where datagrams can be received; datagrams from other peer addresses are
not delivered. If address is a null address for the protocol, the socket's peer address is reset.
No connections are made by this connect function. If the initiating socket is SOCK_STREAM, the connect() function attempts to make
a connection to the socket specified by the address parameter. Each communication space interprets the address parameter differ-
If the function fails for a connection mode socket, applications should use the close() function to deallocate the socket and descriptor.
If attempting to reinitiate the connection, applications should create a new socket.
If the connection cannot be established immediately, one of the following occurs: If the socket file descriptor is marked non-blocking, the
connect() function fails and sets errno to [EINPROGRESS]. However, the connection request is not aborted; the connection will be estab-
lished asynchronously. If you make a call to connect() for the same socket before the connection is established, the function fails and
sets errno to [EALREADY]. If the socket file descriptor is marked blocking, the connect() function blocks for an unspecified amount of
time until the connection is established. If the timeout interval expires before the connection is established, the function fails and
aborts the connection attempt. While blocked, if connect() is interrupted by a signal, it fails and sets errno to [EINTR]. However, the
connection request is not aborted; the connection is established asynchronously.
Upon successful completion, the connect() function returns a value of 0 (zero). Otherwise, a value of -1 is returned and errno is set to
indicate the error.
If the connect() function fails, errno may be set to one of the following values: Search permission is denied for a component of the path
prefix; or write access to the named socket is denied.
[XNS4.0] This error applies to AF_UNIX sockets only. The specified address is already in use. The specified address is not avail-
able from the local machine. The addresses in the specified address family cannot be used with this socket. A connection request
is already in progress for the specified socket. The socket parameter is not valid. The attempt to connect was rejected. The
remote host reset the connection request. The address parameter is not in a readable part of the user address space. The specified
host is not reachable. O_NONBLOCK is set for the file descriptor for the socket and the connection cannot be immediately estab-
lished; the connection will be established asynchronously. The connect() function was interrupted by a signal while waiting for the
connection to be established. The connection establishment may continue asynchronously. The value of the address_len parameter is
invalid for the specified address family; or the sa_family field in the socket address structure is invalid for the protocol. For
an AF_UNIX socket, an I/O error occurred while reading from or writing to the file system. The socket is already connected. For an
AF_UNIX socket, too many symbolic links were encountered in translating the pathname in address. A component of the pathname
exceeded NAME_MAX characters, or an entire pathname exceeded PATH_MAX characters. The local network connection is not operational.
No route to the network or host is present. Insufficient resources are available in the system to complete the call. For an
AF_UNIX socket, a component of the pathname does not name an existing file or the pathname is an empty string. The available
STREAMS resources were insufficient for the operation to complete. For an AF_UNIX socket, a component of the path prefix of the
pathname in address is not a directory. The socket parameter refers to a file, not a socket. The socket is listening and cannot be
connected. The specified address has a different type than the socket bound to the specified peer address. The establishment of a
connection timed out before a connection was made. [Tru64 UNIX] The socket is marked nonblocking, so the connection cannot be
immediately completed. The application program can select the socket for writing during the connection process.
Functions: accept(2), bind(2), socket(2), getsockname(2), select(2), send(2).
Standards: standards(5). delim off