libc/sys/accept.2

.\" Copyright (c) 1983, 1990, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)accept.2	8.2 (Berkeley) 12/11/93
.\" $FreeBSD: src/lib/libc/sys/accept.2,v 1.10.2.11 2002/05/09 02:24:40 silby Exp $
.\" $DragonFly: src/lib/libc/sys/accept.2,v 1.4 2006/06/25 10:55:51 corecode Exp $
.\"
.Dd October 29, 2015
.Dt ACCEPT 2
.Os
.Sh NAME
.Nm accept ,
.Nm accept4
.Nd accept a connection on a socket
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.Ft int
.Fn accept "int s" "struct sockaddr *addr" "socklen_t *addrlen"
.Ft int
.Fn accept4 "int s" "struct sockaddr * restrict addr" "socklen_t * restrict addrlen" "int flags"
.Sh DESCRIPTION
The argument
.Fa s
is a socket that has been created with
.Xr socket 2 ,
bound to an address with
.Xr bind 2 ,
and is listening for connections after a
.Xr listen 2 .
The
.Fn 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
.Dv O_NONBLOCK
and
.Dv O_ASYNC
properties,
socket buffer settings,
socket options,
and the destination of
.Dv SIGIO
and
.Dv SIGURG
signals from the original socket
.Fa s .
.Pp
The
.Fn accept4
system call is similar,
but the
.Dv O_NONBLOCK
property of the new socket is instead determined by the
.Dv SOCK_NONBLOCK
flag in the
.Fa flags
argument,
the
.Dv 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
.Dv SOCK_CLOEXEC
flag in the
.Fa flags
argument.
.Pp
If no pending connections are
present on the queue, and the socket is not marked
as non-blocking,
.Fn accept
blocks the caller until a connection is present.
If the socket is marked non-blocking and no pending
connections are present on the queue,
.Fn accept
returns an error as described below.
The accepted socket
may not be used
to accept more connections.  The original socket
.Fa s
remains open.
.Pp
The argument
.Fa addr
is a result parameter that is filled-in with
the address of the connecting entity,
as known to the communications layer.
The exact format of the
.Fa addr
parameter is determined by the domain in which the communication
is occurring.
To ensure that the returned address fits,
.Fa *addr
should have a size of at least
.Ft sizeof(struct sockaddr_storage) .
The
.Fa addrlen
is a value-result parameter; it should initially contain the
amount of space pointed to by
.Fa addr ;
on return it will contain the actual length (in bytes) of the
address returned.
These system calls
are used with connection-based socket types, currently with
.Dv SOCK_STREAM
and
.Dv SOCK_SEQPACKET .
.Pp
It is possible to
.Xr select 2
a socket for the purposes of doing an
.Fn accept
by selecting it for read.
.Pp
For certain protocols which require an explicit confirmation,
such as
.Tn ISO
or
.Tn DATAKIT ,
.Fn 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.
.Pp
For some applications, performance may be enhanced by using an
.Xr accept_filter 9
to pre-process incoming connections.
.Sh RETURN VALUES
These calls returns \-1 on error.
If they succeed,
they return a non-negative integer
that is a descriptor for the accepted socket.
.Sh ERRORS
The
.Fn accept
and
.Fn accept4
system calls will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The descriptor is invalid.
.It Bq Er EINTR
The
.Fn accept
operation was interrupted.
.It Bq Er EMFILE
The per-process descriptor table is full.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er ENOTSOCK
The descriptor references a file, not a socket.
.It Bq Er EINVAL
.Xr listen 2
has not been called on the socket descriptor.
.It Bq Er EFAULT
The
.Fa addr
parameter is not in a writable part of the
user address space.
.It Bq Er EWOULDBLOCK
The socket is marked non-blocking and no connections
are present to be accepted.
.It Bq Er ECONNABORTED
A connection arrived, but it was closed while waiting
on the listen queue.
.El
.Pp
The
.Fn accept4
system call will also fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa flags
argument is invalid.
.El
.Sh SEE ALSO
.Xr bind 2 ,
.Xr connect 2 ,
.Xr getpeername 2 ,
.Xr listen 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr accept_filter 9
.Sh HISTORY
The
.Fn accept
system call appeared in
.Bx 4.2 .
.Pp
The
.Fn accept4
system call appeared in
.Dx 4.3 .