240 lines
6.1 KiB
Groff
240 lines
6.1 KiB
Groff
.\" Copyright (c) 1983, 1990, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)accept.2 8.2 (Berkeley) 12/11/93
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.\" $FreeBSD$
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.\"
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.Dd October 9, 2014
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.Dt ACCEPT 2
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.Os
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.Sh NAME
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.Nm accept ,
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.Nm accept4
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.Nd accept a connection on a socket
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/socket.h
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.Ft int
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.Fn accept "int s" "struct sockaddr * restrict addr" "socklen_t * restrict addrlen"
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.Ft int
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.Fn accept4 "int s" "struct sockaddr * restrict addr" "socklen_t * restrict addrlen" "int flags"
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.Sh DESCRIPTION
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The argument
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.Fa s
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is a socket that has been created with
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.Xr socket 2 ,
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bound to an address with
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.Xr bind 2 ,
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and is listening for connections after a
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.Xr listen 2 .
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The
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.Fn accept
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system call extracts the first connection request on the
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queue of pending connections, creates a new socket,
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and allocates a new file descriptor for the socket which
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inherits the state of the
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.Dv O_NONBLOCK
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and
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.Dv O_ASYNC
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properties and the destination of
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.Dv SIGIO
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and
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.Dv SIGURG
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signals from the original socket
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.Fa s .
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.Pp
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The
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.Fn accept4
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system call is similar,
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but the
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.Dv O_NONBLOCK
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property of the new socket is instead determined by the
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.Dv SOCK_NONBLOCK
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flag in the
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.Fa flags
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argument,
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the
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.Dv O_ASYNC
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property is cleared,
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the signal destination is cleared
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and the close-on-exec flag on the new file descriptor can be set via the
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.Dv SOCK_CLOEXEC
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flag in the
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.Fa flags
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argument.
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.Pp
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If no pending connections are
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present on the queue, and the original socket
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is not marked as non-blocking,
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.Fn accept
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blocks the caller until a connection is present.
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If the original socket
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is marked non-blocking and no pending
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connections are present on the queue,
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.Fn accept
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returns an error as described below.
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The accepted socket
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may not be used
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to accept more connections.
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The original socket
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.Fa s
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remains open.
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.Pp
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The argument
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.Fa addr
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is a result argument that is filled-in with
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the address of the connecting entity,
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as known to the communications layer.
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The exact format of the
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.Fa addr
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argument is determined by the domain in which the communication
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is occurring.
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A null pointer may be specified for
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.Fa addr
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if the address information is not desired;
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in this case,
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.Fa addrlen
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is not used and should also be null.
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Otherwise, the
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.Fa addrlen
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argument
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is a value-result argument; it should initially contain the
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amount of space pointed to by
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.Fa addr ;
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on return it will contain the actual length (in bytes) of the
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address returned.
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This call
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is used with connection-based socket types, currently with
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.Dv SOCK_STREAM .
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.Pp
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It is possible to
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.Xr select 2
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a socket for the purposes of doing an
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.Fn accept
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by selecting it for read.
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.Pp
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For certain protocols which require an explicit confirmation,
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such as
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.Tn ISO
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or
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.Tn DATAKIT ,
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.Fn accept
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can be thought of
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as merely dequeueing the next connection
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request and not implying confirmation.
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Confirmation can be implied by a normal read or write on the new
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file descriptor, and rejection can be implied by closing the
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new socket.
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.Pp
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For some applications, performance may be enhanced by using an
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.Xr accept_filter 9
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to pre-process incoming connections.
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.Pp
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When using
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.Fn accept ,
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portable programs should not rely on the
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.Dv O_NONBLOCK
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and
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.Dv O_ASYNC
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properties and the signal destination being inherited,
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but should set them explicitly using
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.Xr fcntl 2 ;
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.Fn accept4
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sets these properties consistently,
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but may not be fully portable across
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.Ux
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platforms.
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.Sh RETURN VALUES
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These calls return \-1 on error.
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If they succeed, they return a non-negative
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integer that is a descriptor for the accepted socket.
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.Sh ERRORS
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The
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.Fn accept
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and
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.Fn accept4
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system calls will fail if:
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.Bl -tag -width Er
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.It Bq Er EBADF
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The descriptor is invalid.
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.It Bq Er EINTR
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The
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.Fn accept
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operation was interrupted.
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.It Bq Er EMFILE
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The per-process descriptor table is full.
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.It Bq Er ENFILE
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The system file table is full.
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.It Bq Er ENOTSOCK
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The descriptor references a file, not a socket.
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.It Bq Er EINVAL
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.Xr listen 2
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has not been called on the socket descriptor.
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.It Bq Er EFAULT
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The
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.Fa addr
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argument is not in a writable part of the
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user address space.
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.It Bo Er EWOULDBLOCK Bc or Bq Er EAGAIN
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The socket is marked non-blocking and no connections
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are present to be accepted.
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.It Bq Er ECONNABORTED
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A connection arrived, but it was closed while waiting
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on the listen queue.
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.El
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.Pp
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The
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.Fn accept4
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system call will also fail if:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The
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.Fa flags
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argument is invalid.
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.El
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.Sh SEE ALSO
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.Xr bind 2 ,
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.Xr connect 2 ,
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.Xr getpeername 2 ,
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.Xr getsockname 2 ,
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.Xr listen 2 ,
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.Xr select 2 ,
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.Xr socket 2 ,
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.Xr accept_filter 9
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.Sh HISTORY
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The
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.Fn accept
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system call appeared in
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.Bx 4.2 .
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.Pp
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The
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.Fn accept4
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system call appeared in
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.Fx 10.0 .
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