ACCEPT(2) BSD System Calls Manual ACCEPT(2)NAMEaccept — accept a connection on a socket
accept(int socket, struct sockaddr *restrict address,
socklen_t *restrict address_len);
The argument socket 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). accept() extracts the first connection request on the queue
of pending connections, creates a new socket with the same properties of
socket, and allocates a new file descriptor for the socket. If no pend‐
ing 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 socket, remains open.
The argument address 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 address parameter is determined by the domain in
which the communication is occurring. The address_len is a value-result
parameter; it should initially contain the amount of space pointed to by
address; 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 dequeuing the next con‐
nection request and not implying confirmation. Confirmation can be
implied by a normal read or write on the new file descriptor, and rejec‐
tion can be implied by closing the new socket.
One can obtain user connection request data without confirming the con‐
nection by issuing a recvmsg(2) call with an msg_iovlen of 0 and a non-
zero msg_controllen, or by issuing a getsockopt(2) request. Similarly,
one can provide user connection rejection information by issuing a
sendmsg(2) call with providing only the control information, or by call‐
The call returns -1 on error and the global variable errno is set to
indicate the 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] socket is not a valid file descriptor.
[ECONNABORTED] The connection to socket has been aborted.
[EFAULT] The address parameter is not in a writable part of the
user address space.
[EINTR] The accept() system call was terminated by a signal.
[EINVAL] socket is unwilling to accept connections.
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
[ENOMEM] Insufficient memory was available to complete the
[ENOTSOCK] socket references a file type other than a socket.
[EOPNOTSUPP] socket is not of type SOCK_STREAM and thus does not
[EWOULDBLOCK] socket is marked as non-blocking and no connections
are present to be accepted.
The include file <sys/types.h> is necessary.
SEE ALSObind(2), connect(2), listen(2), select(2), socket(2), compat(5)HISTORY
The accept() function appeared in 4.2BSD.
4.2 Berkeley Distribution December 11, 1993 4.2 Berkeley Distribution