accept, accept4 — accept a connection on a socket
#include <sys/socket.h>
int
accept( |
int sockfd, |
struct sockaddr *restrict addr, | |
socklen_t *restrict addrlen) ; |
#define _GNU_SOURCE /* See feature_test_macros(7) */ #include <sys/socket.h>
int
accept4( |
int sockfd, |
struct sockaddr *restrict addr, | |
socklen_t *restrict addrlen, | |
int flags) ; |
The accept
() system call is
used with connection-based socket types (SOCK_STREAM
, SOCK_SEQPACKET
). It extracts the first
connection request on the queue of pending connections for
the listening socket, sockfd
, creates a new connected
socket, and returns a new file descriptor referring to that
socket. The newly created socket is not in the listening
state. The original socket sockfd
is unaffected by this
call.
The argument sockfd
is a socket that has
been created with socket(2), bound to a local
address with bind(2), and is listening
for connections after a listen(2).
The argument addr
is a pointer to a sockaddr
structure. This structure is filled in with the address of
the peer socket, as known to the communications layer. The
exact format of the address returned addr
is determined by the
socket's address family (see socket(2) and the
respective protocol man pages). When addr
is NULL, nothing is filled
in; in this case, addrlen
is not used, and should
also be NULL.
The addrlen
argument is a value-result argument: the caller must
initialize it to contain the size (in bytes) of the structure
pointed to by addr
;
on return it will contain the actual size of the peer
address.
The returned address is truncated if the buffer provided
is too small; in this case, addrlen
will return a value
greater than was supplied to the call.
If no pending connections are present on the queue, and
the socket is not marked as nonblocking, accept
() blocks the caller until a
connection is present. If the socket is marked nonblocking
and no pending connections are present on the queue,
accept
() fails with the error
EAGAIN or EWOULDBLOCK.
In order to be notified of incoming connections on a
socket, you can use select(2), poll(2), or epoll(7). A readable event
will be delivered when a new connection is attempted and you
may then call accept
() to get a
socket for that connection. Alternatively, you can set the
socket to deliver SIGIO
when
activity occurs on a socket; see socket(7) for details.
If flags
is 0,
then accept4
() is the same as
accept
(). The following values
can be bitwise ORed in flags
to obtain different
behavior:
SOCK_NONBLOCK
Set the O_NONBLOCK
file status flag on the open file description (see
open(2)) referred to
by the new file descriptor. Using this flag saves extra
calls to fcntl(2) to achieve
the same result.
SOCK_CLOEXEC
Set the close-on-exec (FD_CLOEXEC
) flag on the new file
descriptor. See the description of the O_CLOEXEC
flag in open(2) for reasons
why this may be useful.
On success, these system calls return a file descriptor
for the accepted socket (a nonnegative integer). On error,
−1 is returned, errno
is
set to indicate the error, and addrlen
is left unchanged.
Linux accept
() (and
accept4
()) passes
already-pending network errors on the new socket as an
error code from accept
().
This behavior differs from other BSD socket
implementations. For reliable operation the application
should detect the network errors defined for the protocol
after accept
() and treat them
like EAGAIN by retrying. In
the case of TCP/IP, these are ENETDOWN, EPROTO, ENOPROTOOPT, EHOSTDOWN, ENONET, EHOSTUNREACH, EOPNOTSUPP, and ENETUNREACH.
The socket is marked nonblocking and no connections are present to be accepted. POSIX.1-2001 and POSIX.1-2008 allow either error to be returned for this case, and do not require these constants to have the same value, so a portable application should check for both possibilities.
sockfd
is
not an open file descriptor.
A connection has been aborted.
The addr
argument is not in a writable part of the user address
space.
The system call was interrupted by a signal that was caught before a valid connection arrived; see signal(7).
Socket is not listening for connections, or
addrlen
is
invalid (e.g., is negative).
(accept4
()) invalid
value in flags
.
The per-process limit on the number of open file descriptors has been reached.
The system-wide limit on the total number of open files has been reached.
Not enough free memory. This often means that the memory allocation is limited by the socket buffer limits, not by the system memory.
The file descriptor sockfd
does not refer to
a socket.
The referenced socket is not of type SOCK_STREAM
.
Protocol error.
In addition, Linux accept
()
may fail if:
Firewall rules forbid connection.
In addition, network errors for the new socket and as
defined for the protocol may be returned. Various Linux
kernels can return other errors such as ENOSR, ESOCKTNOSUPPORT, EPROTONOSUPPORT, ETIMEDOUT. The value ERESTARTSYS
may be seen during a trace.
The accept4
() system call is
available starting with Linux 2.6.28; support in glibc is
available starting with version 2.10.
accept
(): POSIX.1-2001,
POSIX.1-2008, SVr4, 4.4BSD (accept
() first appeared in 4.2BSD).
accept4
() is a nonstandard
Linux extension.
On Linux, the new socket returned by accept
() does not
inherit file status flags
such as O_NONBLOCK
and
O_ASYNC
from the listening
socket. This behavior differs from the canonical BSD sockets
implementation. Portable programs should not rely on
inheritance or noninheritance of file status flags and always
explicitly set all required flags on the socket returned from
accept
().
There may not always be a connection waiting after a
SIGIO
is delivered or select(2), poll(2), or epoll(7) return a
readability event because the connection might have been
removed by an asynchronous network error or another thread
before accept
() is called. If
this happens, then the call will block waiting for the next
connection to arrive. To ensure that accept
() never blocks, the passed socket
sockfd
needs to have
the O_NONBLOCK
flag set (see
socket(7)).
For certain protocols which require an explicit
confirmation, such as DECnet, accept
() can be thought of as merely
dequeuing 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. Currently, only DECnet has
these semantics on Linux.
This page is part of release 5.11 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright (c) 1983, 1990, 1991 The Regents of the University of California. All rights reserved. %%%LICENSE_START(BSD_4_CLAUSE_UCB) 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. All advertising materials mentioning features or use of this software must display the following acknowledgement: This product includes software developed by the University of California, Berkeley and its contributors. 4. 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. %%%LICENSE_END Modified 1993-07-24 by Rik Faith <faithcs.unc.edu> Modified 1996-10-21 by Eric S. Raymond <esrthyrsus.com> Modified 1998-2000 by Andi Kleen to match Linux 2.2 reality Modified 2002-04-23 by Roger Luethi <rlhellgate.ch> Modified 2004-06-17 by Michael Kerrisk <mtk.manpagesgmail.com> 2008-12-04, mtk, Add documentation of accept4() |