accept — accept a connection on a socket
Standard C Library (libc, -lc)
accept(int s, struct sockaddr * restrict addr,
socklen_t * restrict addrlen);
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.
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.
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).
The call returns -1 on error. If it succeeds, it returns a non-negative
integer that is a descriptor for the accepted socket.
The accept() system call 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
[EINVAL] The addrlen argument is negative.
[EFAULT] The addr argument 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.
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.