DragonFly On-Line Manual Pages
ACCEPT(2) DragonFly System Calls Manual ACCEPT(2)
accept, accept4 -- accept a connection on a socket
Standard C Library (libc, -lc)
accept(int s, struct sockaddr *addr, socklen_t *addrlen);
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, 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.
These calls returns -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] 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
[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.
bind(2), connect(2), getpeername(2), listen(2), select(2), socket(2),
The accept() system call appeared in 4.2BSD.
The accept4() system call appeared in DragonFly 4.3.
DragonFly 4.3 October 29, 2015 DragonFly 4.3