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.

Debian                                           October 9, 2014                                       ACCEPT(2)