ACCEPT(2)                    BSD System Calls Manual                   ACCEPT(2)

     accept, accept4 — accept a connection on a socket

     Standard C Library (libc, -lc)

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

     accept(int s, struct sockaddr * restrict addr,
         socklen_t * restrict addrlen);

     accept4(int s, struct sockaddr * restrict addr,
         socklen_t * restrict addrlen, int flags);

     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 and O_ASYNC properties and the destination of SIGIO and SIGURG
     signals from the original socket s.

     The accept4() system call is similar, but the O_NONBLOCK property of the
     new socket is instead determined by the SOCK_NONBLOCK flag in the flags
     argument, the O_ASYNC property is cleared, the signal destination is
     cleared and the close-on-exec flag on the new file descriptor can be set
     via the SOCK_CLOEXEC flag in the flags argument.

     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.

     When using accept(), portable programs should not rely on the O_NONBLOCK
     and O_ASYNC properties and the signal destination being inherited, but
     should set them explicitly using fcntl(2); accept4() sets these properties
     consistently, but may not be fully portable across UNIX platforms.

     These calls return -1 on error.  If they succeed, they return a non-
     negative integer that is a descriptor for the accepted socket.

     The accept() and accept4() system calls will fail if:

     [EBADF]            The descriptor is invalid.

     [EINTR]            The accept() operation was interrupted.

     [EMFILE]           The per-process descriptor table is full.

     [ENFILE]           The system file table is full.

     [ENOTSOCK]         The descriptor references a file, not a socket.

     [EINVAL]           listen(2) has not been called on the socket descriptor.

     [EFAULT]           The addr argument is not in a writable part of the user
                        address space.

                        The socket is marked non-blocking and no connections are
                        present to be accepted.

     [ECONNABORTED]     A connection arrived, but it was closed while waiting on
                        the listen queue.

     The accept4() system call will also fail if:

     [EINVAL]           The flags argument is invalid.

     bind(2), connect(2), getpeername(2), getsockname(2), listen(2), select(2),
     socket(2), accept_filter(9)

     The accept() system call appeared in 4.2BSD.

     The accept4() system call appeared in FreeBSD 10.0.

BSD                              October 9, 2014                             BSD