NAME
accept
, accept4
,
paccept
—
accept a connection on a
socket
LIBRARY
library “libc”
SYNOPSIS
#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);
int
paccept
(int
s, struct sockaddr *
restrict addr, socklen_t
* restrict addrlen, const
sigset_t * restrict sigmask,
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). Theaccept
()
argument extracts the first connection request on the queue of pending
connections, creates a new socket with the same properties of
s and allocates a new file descriptor for the socket. If
no pending connections are present on the queue, and the socket is not marked
as non-blocking, 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, 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 parameter that
is filled in with the address of the connecting entity, as known to the
communications layer. The exact format of the addr
parameter is determined by the domain in which the communication is
occurring. The addrlen is a value-result parameter; 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) or
poll(2) a socket for the purposes of doing an
accept
()
by selecting or polling it for read.
For certain protocols which require an explicit
confirmation, such as ISO or DATAKIT,
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.
One can obtain user connection request data without confirming the connection by issuing a recvmsg(2) call with an msg_iovlen of 0 and a non-zero msg_controllen, or by issuing a getsockopt(2) request. Similarly, one can provide user connection rejection information by issuing a sendmsg(2) call with providing only the control information, or by calling setsockopt(2).
The
accept4
()
function is equivalent to paccept with sigmask
NULL
.
The
paccept
()
function behaves exactly like accept
(), but it also
allows to set the following flags on the returned file
descriptor:
SOCK_CLOEXEC
- Set the close on exec property.
SOCK_NONBLOCK
- Sets non-blocking I/O.
SOCK_NOSIGPIPE
- Return
EPIPE
instead of raisingSIGPIPE
.
It can also temporarily replace the signal mask of
the calling thread if sigmask is a
non-NULL
pointer, then the
paccept
()
function shall replace the signal mask of the caller by the set of signals
pointed to by sigmask before waiting for a connection,
and shall restore the signal mask of the calling thread before
returning.
RETURN VALUES
The accept
() and
paccept
() calls return -1 on error. If they succeed,
they return a non-negative integer that is a descriptor for the accepted
socket.
COMPATIBILITY
The accept
() implementation makes the new
file descriptor inherit file flags (like O_NONBLOCK
)
from the listening socket. It's a traditional behaviour for BSD derivative
systems. On the other hand, there are implementations which don't do so.
Linux is an example of such implementations. Portable programs should not
rely on either of the behaviours. The
accept4
() function is compatible with the
Linux implementation.
ERRORS
The accept
() function will fail if:
- [
EAGAIN
] - The socket is marked non-blocking and no connections are present to be accepted.
- [
EBADF
] - The descriptor is invalid.
- [
ECONNABORTED
] - A connection has been aborted.
- [
EFAULT
] - The addr parameter is not in a writable part of the user address space.
- [
EINTR
] - The
accept
() call has been interrupted by a signal. - [
EINVAL
] - The socket has not been set up to accept connections (using bind(2) and listen(2)).
- [
EMFILE
] - The per-process descriptor table is full.
- [
ENFILE
] - The system file table is full.
- [
ENOTSOCK
] - The descriptor references a file, not a socket.
- [
EOPNOTSUPP
] - The referenced socket is not of type
SOCK_STREAM
.
SEE ALSO
bind(2), connect(2), listen(2), poll(2), select(2), socket(2)
HISTORY
The accept
() function appeared in
4.2BSD. The accept4
()
function matches Linux semantics and appeared in NetBSD
8.0. The paccept
() function is inspired from
Linux and appeared in NetBSD 6.0.