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ACCEPT(2) System Calls Manual ACCEPT(2)

acceptaccept a connection on a socket

#include <sys/types.h>
#include <sys/socket.h>

int
accept(int s, struct sockaddr *addr, socklen_t *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() call extracts the first connection request on the queue of pending connections, creates a new socket with the same properties of 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 structure sockaddr_storage exists for greater portability. It is large enough to hold any of the types that may be returned in the addr parameter. 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) or poll(2) a socket for the purposes of doing an accept() by selecting it for read.

For certain protocols which require an explicit confirmation, accept() can be thought of as merely dequeuing 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).

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

The following code uses struct sockaddr_storage to allocate enough space for the returned address:

#include <sys/types.h>
#include <sys/socket.h>

struct sockaddr_storage addr;
socklen_t len = sizeof(addr);
int retcode;

retcode = accept(s, (struct sockaddr *)&addr, &len);
if (retcode == -1)
	err(1, "accept");

The accept() will fail if:

[]
The descriptor is invalid.
[]
The descriptor references a file, not a socket.
[]
The referenced socket is not of type SOCK_STREAM.
[]
A signal was caught before a connection arrived.
[]
The referenced socket is not listening for connections (that is, listen(2) has not yet been called).
[]
The addr or addrlen parameter is not in a valid part of the process address space.
[]
The socket is marked non-blocking and no connections are present to be accepted.
[]
The per-process descriptor table is full.
[]
The system file table is full.
[]
A connection has been aborted.

bind(2), connect(2), listen(2), poll(2), select(2), socket(2)

The accept() function appeared in 4.2BSD.

May 31, 2007 OpenBSD-5.1