FreeBSD manual
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ACCEPT(2) FreeBSD System Calls Manual ACCEPT(2)
NAME
accept, accept4 - accept a connection on a socket
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#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);
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). The accept() system call extracts the first connection
request on the queue of pending connections, creates a new socket, and
allocates a new file descriptor for the socket which inherits the state
of the O_NONBLOCK and O_ASYNC properties and the destination of SIGIO and
SIGURG signals from the original socket s.
The accept4() system call is similar, but the O_NONBLOCK property of the
new socket is instead determined by the SOCK_NONBLOCK flag in the flags
argument, the O_ASYNC property is cleared, the signal destination is
cleared and the close-on-exec flag on the new file descriptor can be set
via the SOCK_CLOEXEC flag in the flags argument.
If no pending connections are present on the queue, and the original
socket is not marked as non-blocking, accept() blocks the caller until a
connection is present. If the original 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 argument that is filled-in with the address
of the connecting entity, as known to the communications layer. The
exact format of the addr argument is determined by the domain in which
the communication is occurring. A null pointer may be specified for addr
if the address information is not desired; in this case, addrlen is not
used and should also be null. Otherwise, the addrlen argument is a
value-result argument; 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) a socket for the purposes of doing an
accept() by selecting it for read.
For certain protocols which require an explicit confirmation, such as ISO
or DATAKIT, accept() can be thought of as merely dequeueing the next
connection request and not implying confirmation. Confirmation can be
and O_ASYNC properties and the signal destination being inherited, but
should set them explicitly using fcntl(2); accept4() sets these
properties consistently, but may not be fully portable across UNIX
platforms.
RETURN VALUES
These calls return -1 on error. If they succeed, they return a non-
negative integer that is a descriptor for the accepted socket.
ERRORS
The accept() and accept4() system calls will fail if:
[EBADF] The descriptor is invalid.
[EINTR] The accept() operation was interrupted.
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
[ENOTSOCK] The descriptor references a file, not a socket.
[EINVAL] listen(2) has not been called on the socket
descriptor.
[EFAULT] The addr argument is not in a writable part of the
user address space.
[EWOULDBLOCK] or [EAGAIN]
The socket is marked non-blocking and no connections
are present to be accepted.
[ECONNABORTED] A connection arrived, but it was closed while waiting
on the listen queue.
The accept4() system call will also fail if:
[EINVAL] The flags argument is invalid.
SEE ALSO
bind(2), connect(2), getpeername(2), getsockname(2), listen(2),
select(2), socket(2), accept_filter(9)
HISTORY
The accept() system call appeared in 4.2BSD.
The accept4() system call appeared in FreeBSD 10.0.
FreeBSD 14.0-RELEASE-p11 October 9, 2014 FreeBSD 14.0-RELEASE-p11