The argument
s is a socket that has been created with
socket(2), bound to an address with
bind(2), and is listening for connections after a
listen(2). The
accept() argument extracts the first connection request on the queue of pending connections, creates a new socket with the same properties of
s and allocates a new file descriptor for the socket. If no pending connections are present on the queue, and the socket is not marked as non-blocking,
accept() blocks the caller until a connection is present. If the socket is marked non-blocking and no pending connections are present on the queue,
accept() returns an error as described below. The accepted socket may not be used to accept more connections. The original socket
s remains open.
The argument
addr is a result parameter that is filled in with the address of the connecting entity, as known to the communications layer. The exact format of the
addr parameter is determined by the domain in which the communication is occurring. The
addrlen is a value-result parameter; it should initially contain the amount of space pointed to by
addr; on return it will contain the actual length (in bytes) of the address returned. This call is used with connection-based socket types, currently with
SOCK_STREAM.
It is possible to
select(2) or
poll(2) a socket for the purposes of doing an
accept() by selecting or polling it for read.
For certain protocols which require an explicit confirmation, such as ISO or DATAKIT,
accept() can be thought of as merely dequeuing the next connection request and not implying confirmation. Confirmation can be implied by a normal read or write on the new file descriptor, and rejection can be implied by closing the new socket.
One can obtain user connection request data without confirming the connection by issuing a
recvmsg(2) call with an
msg_iovlen of 0 and a non-zero
msg_controllen, or by issuing a
getsockopt(2) request. Similarly, one can provide user connection rejection information by issuing a
sendmsg(2) call with providing only the control information, or by calling
setsockopt(2).