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     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.

     [EWOULDBLOCK]      The socket is marked non-blocking and no connections are present to be

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

     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),


     The accept() system call appeared in 4.2BSD.

     The accept4() system call appeared in FreeBSD 10.0.