ACCEPT(2)

NAME
accept - accept a connection on a socket

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int
accept(int s, struct sockaddr *addr, socklen_t *addrlen);

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). The accept() call extracts the first connection request on the queue of pending connections, creates a new socket with the same properties as s, and allocates a new file descriptor for the socket. 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. 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. 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.

One can obtain user connection request data without confirming the connection 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 providing only the control information, or by calling setsockopt(2).

IMPLEMENTATION NOTES

In the non-threaded library accept() is implemented as the accept syscall.

In the threaded library, the accept syscall is assembled to _thread_sys_accept() and accept() is implemented as a function which locks s for read and write, then calls _thread_sys_accept(). If the call to _thread_sys_accept() would block, a context switch is performed. Before returning, accept() unlocks s.

RETURN VALUES
The call returns -1 on error. If it succeeds, it returns a non-negative integer that is a descriptor for the accepted socket.

ERRORS
The accept() 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.

SEE ALSO
bind(2), connect(2), getpeername(2), listen(2), select(2), socket(2)

HISTORY
The accept() function appeared in 4.2BSD.