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 system call extracts the first connection request on the queue of pending connections, creates a new socket, and allocates a new file descriptor for the socket which inherits the state of the O_NONBLOCK property from the original socket s.
If no pending connections are present on the queue, and the original socket is not marked as non-blocking, accept blocks the caller until a connection is present. If the original 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 argument that is filled-in with the address of the connecting entity, as known to the communications layer. The exact format of the addr argument is determined by the domain in which the communication is occurring. A null pointer may be specified for addr if the address information is not desired; in this case, addrlen is not used and should also be null. Otherwise, the addrlen argument is a value-result argument; 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) a socket for the purposes of doing an accept by selecting it for read.
For certain protocols which require an explicit confirmation, such as ISO or DATAKIT, accept can be thought of as merely dequeueing 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.
For some applications, performance may be enhanced by using an accept_filter(9) to pre-process incoming connections.
The call returns -1 on error. If it succeeds, it returns a non-negative integer that is a descriptor for the accepted socket.
The accept system call will fail if: