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ACCEPT(2) NetBSD System Calls Manual ACCEPT(2)
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by Kimmo Suominen.
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accept, accept4, paccept -- accept a connection on a socket
Standard C Library (libc, -lc)
accept(int s, struct sockaddr * restrict addr,
socklen_t * restrict addrlen);
accept4(int s, struct sockaddr * restrict addr,
socklen_t * restrict addrlen, int flags);
paccept(int s, struct sockaddr * restrict addr,
socklen_t * restrict addrlen, const sigset_t * restrict sigmask,
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). The accept() 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 con-
nection request and not implying confirmation. Confirmation can be
implied by a normal read or write on the new file descriptor, and rejec-
tion can be implied by closing the new socket.
One can obtain user connection request data without confirming the con-
nection 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 call-
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 raising SIGPIPE.
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.
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
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 implemen-
tations. Portable programs should not rely on either of the behaviours.
accept4() function is compatible with the Linux implementation.
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.
bind(2), connect(2), listen(2), poll(2), select(2), socket(2)
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.
NetBSD 9.1 February 8, 2017 NetBSD 9.1