ACCEPT(2)                 FreeBSD System Calls Manual                ACCEPT(2)

NAME
     accept, accept4 - accept a connection on a socket

LIBRARY
     Standard C Library (libc, -lc)

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

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

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

DESCRIPTION
     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.

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

ERRORS
     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.

     [EWOULDBLOCK] or [EAGAIN]
                        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.

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

HISTORY
     The accept() system call appeared in 4.2BSD.

     The accept4() system call appeared in FreeBSD 10.0.

FreeBSD 13.1-RELEASE-p6         October 9, 2014        FreeBSD 13.1-RELEASE-p6