233 lines
6.1 KiB
Groff
233 lines
6.1 KiB
Groff
.\" $OpenBSD: accept.2,v 1.30 2022/09/11 06:38:11 jmc Exp $
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.\" $NetBSD: accept.2,v 1.7 1996/01/31 20:14:42 mycroft Exp $
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.\"
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.\" 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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.\" 3. 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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.\"
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.Dd $Mdocdate: September 11 2022 $
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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 SYNOPSIS
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.In sys/socket.h
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.Ft int
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.Fn accept "int s" "struct sockaddr *addr" "socklen_t *addrlen"
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.Ft int
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.Fn accept4 "int s" "struct sockaddr *addr" "socklen_t *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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call extracts the first connection request on the queue of pending
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connections, creates a new socket with the same non-blocking I/O mode as
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.Fa s ,
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and allocates a new file descriptor for the socket with the
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close-on-exec flag clear.
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.Pp
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The
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.Fn accept4
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system call is similar, however the non-blocking I/O mode of the
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new socket is 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 and the close-on-exec flag on the new file descriptor is
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determined by 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 present on the queue,
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and the socket 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 socket 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 may not be used 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 parameter that is filled in with the address of the connecting
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entity 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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parameter is determined by the domain in which the communication
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is occurring.
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The structure
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.Vt sockaddr_storage
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exists for greater portability.
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It is large enough to hold any of the types that may be returned in the
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.Fa addr
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parameter.
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.Pp
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The
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.Fa addrlen
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is a value-result parameter; 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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If
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.Fa addrlen
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does not point to enough space to hold the entire socket address, the
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result will be truncated to the initial value of
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.Fa addrlen
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(in bytes).
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This call 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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or
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.Xr poll 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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.Sh RETURN VALUES
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If successful,
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.Fn accept
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and
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.Fn accept4
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return a non-negative integer, the accepted socket file descriptor.
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Otherwise, a value of \-1 is returned and
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.Va errno
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is set to indicate the error.
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.Sh EXAMPLES
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The following code uses struct
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.Vt sockaddr_storage
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to allocate enough space for the returned address:
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.Bd -literal -offset indent
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#include <sys/types.h>
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#include <sys/socket.h>
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struct sockaddr_storage addr;
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socklen_t len = sizeof(addr);
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int retcode;
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retcode = accept(s, (struct sockaddr *)&addr, &len);
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if (retcode == -1)
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err(1, "accept");
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.Ed
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.Sh ERRORS
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.Fn accept
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and
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.Fn accept4
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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 ENOTSOCK
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The descriptor doesn't reference a socket.
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.It Bq Er EOPNOTSUPP
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The referenced socket is not of type
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.Dv SOCK_STREAM .
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.It Bq Er EINTR
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A signal was caught before a connection arrived.
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.It Bq Er EINVAL
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The referenced socket is not listening for connections (that is,
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.Xr listen 2
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has not yet been called).
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.It Bq Er EFAULT
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The
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.Fa addr
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or
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.Fa addrlen
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parameter is not in a valid part of the process address space.
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.It Bq Er EWOULDBLOCK
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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 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 ECONNABORTED
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A connection has been aborted.
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.El
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.Pp
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In addition,
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.Fn accept4
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will fail if
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.Bl -tag -width Er
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.It Bq Er EINVAL
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.Fa flags
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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 listen 2 ,
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.Xr poll 2 ,
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.Xr select 2 ,
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.Xr socket 2
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.Sh STANDARDS
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The
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.Fn accept
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function conforms to
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.St -p1003.1-2008 .
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The
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.Fn accept4
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function is expected to conform to a future revision of that standard.
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.Sh HISTORY
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The
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.Fn accept
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system call first appeared in
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.Bx 4.1c
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and
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.Fn accept4
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in
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.Ox 5.7 .
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.Sh CAVEATS
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When
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.Er EMFILE
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or
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.Er ENFILE
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is returned,
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new connections are neither dequeued nor discarded.
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Thus considerable care is required in
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.Xr select 2
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and
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.Xr poll 2
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loops.
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