ACCEPT

Section: Linux Programmer's Manual (2)
Updated: 23 Oct 1998
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NAME

accept, accept_secure - accept a connection on a socket

SYNOPSIS

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

int accept(int s, struct sockaddr *addr, int *addrlen);

int accept_secure(int s, struct sockaddr *addr, int *addrlen, security_id_t *sid);

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 function 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 socket returned by accept 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. 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.

Accept_secure takes an additional result parameter, sid, that is filled in with the security identifier of the peer socket.

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 DECNet, 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. Currently only DECNet has these semantics on Linux.

NOTES

If you want accept to never block the listening socket needs to have the non blocking flag set. Assuming that there is always a connection waiting after select returned true is not reliable, because the connection might be removed by an asynchronous network error between the select/poll returning and the accept call. The application would hang then if the listen socket is not non blocking.

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.

ERROR HANDLING

Linux accept passes already-pending network errors on the new socket as an error code from accept. This behaviour differs from other BSD socket implementations. For reliable operation the application should detect the network errors defined for the protocol after accept and treat them like EAGAIN by retrying. In case of TCP/IP these are ENETDOWN, EPROTO, ENOPROTOOPT, EHOSTDOWN, ENONET, EHOSTUNREACH, EOPNOTSUPP, and ENETUNREACH.

ERRORS

EBADF: The descriptor is invalid.
ENOTSOCK: The descriptor references a file, not a socket.
EOPNOTSUPP: The referenced socket is not of type SOCK_STREAM.
EFAULT: The addr parameter is not in a writable part of the user address space.
EAGAIN: The socket is marked non-blocking and no connections are present to be accepted.
EPERM: Firewall rules forbid connection.
ENOBUFS, ENOMEM: Not enough free memory.

In addition, network errors for the new socket and as defined for the protocol may be returned.

CONFORMING TO

SVr4, 4.4BSD (the accept function first appeared in BSD 4.2).

This document was created by man2html, using the manual pages.
Time: 17:08:26 GMT, December 18, 2000