| ACCEPT(2) | System Calls Manual | ACCEPT(2) |
accept, accept4
— accept a connection on a socket
Standard C Library (libc, -lc)
#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);
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 implied by a normal read or write
on the new file descriptor, and rejection can be implied by closing the new
socket.
For some applications, performance may be enhanced by using an accept_filter(9) to pre-process incoming connections.
When using
accept(),
portable programs should not rely on the O_NONBLOCK
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.
These calls return -1 on error. If they succeed, they return a non-negative integer that is a descriptor for the accepted socket.
The accept() and
accept4() system calls will fail if:
EBADF]EINTR]accept() operation was interrupted.EMFILE]ENFILE]ENOTSOCK]EINVAL]EFAULT]EWOULDBLOCK] or
[EAGAIN]ECONNABORTED]The accept4() system call will also fail
if:
EINVAL]bind(2), connect(2), getpeername(2), getsockname(2), listen(2), select(2), socket(2), accept_filter(9)
The accept() system call appeared in
4.2BSD.
The accept4() system call appeared in
FreeBSD 10.0.
| October 9, 2014 | Debian |