NAME
accept, accept4
— accept a connection on a
socket
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/types.h>
#include <sys/socket.h>
int
accept(int
s, struct sockaddr
*addr, socklen_t
*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). Theaccept()
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, socket buffer settings, socket
options, 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 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. To ensure that the returned address fits,
*addr should have a size of at least
sizeof(struct sockaddr_storage). 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. These system calls are used with
connection-based socket types, currently with
SOCK_STREAM and
SOCK_SEQPACKET.
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.
RETURN VALUES
These calls returns -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 parameter is not in a writable part of the user address space.
- [
EWOULDBLOCK] - 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), listen(2), select(2), socket(2), accept_filter(9)
HISTORY
The accept() system call appeared in
4.2BSD.
The accept4() system call appeared in
DragonFly 4.3.