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.\"     $OpenBSD: accept.2,v 1.12 2001/03/11 05:02:29 aaron Exp $

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.\"     @(#)accept.2    8.2 (Berkeley) 12/11/93

.\"

.Dd February 15, 1999

.Dt ACCEPT 2

.Os

.Sh NAME

.Nm accept

.Nd accept a connection on a socket

.Sh SYNOPSIS

.Fd #include 

.Fd #include 

.Ft int

.Fn accept "int s" "struct sockaddr *addr" "socklen_t *addrlen"

.Sh DESCRIPTION

The argument

.Fa s

is a socket that has been created with

.Xr socket 2 ,

bound to an address with

.Xr bind 2 ,

and is listening for connections after a

.Xr listen 2 .

The

.Fn accept

argument extracts the first connection request on the queue of pending

connections, creates a new socket with the same properties of

.Fa 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,

.Fn 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,

.Fn accept

returns an error as described below.

The accepted socket may not be used to accept more connections.

The original socket

.Fa s

remains open.

.Pp

The argument

.Fa 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

.Fa addr

parameter is determined by the domain in which the communication

is occurring.

The

.Fa addrlen

is a value-result parameter; it should initially contain the

amount of space pointed to by

.Fa 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

.Dv SOCK_STREAM .

.Pp

It is possible to

.Xr select 2

or

.Xr poll 2

a socket for the purposes of doing an

.Fn accept

by selecting it for read.

.Pp

For certain protocols which require an explicit confirmation, such as

.Tn ISO

or

.Tn DATAKIT ,

.Fn 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.

.Pp

One can obtain user connection request data without confirming

the connection by issuing a

.Xr recvmsg 2

call with an

.Fa msg_iovlen

of 0 and a non-zero

.Fa msg_controllen ,

or by issuing a

.Xr getsockopt 2

request.

Similarly, one can provide user connection rejection information

by issuing a

.Xr sendmsg 2

call with providing only the control information, or by calling

.Xr setsockopt 2 .

.Sh 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.

.Sh ERRORS

The

.Fn accept

will fail if:

.Bl -tag -width Er

.It Bq Er EBADF

The descriptor is invalid.

.It Bq Er ENOTSOCK

The descriptor references a file, not a socket.

.It Bq Er EOPNOTSUPP

The referenced socket is not of type

.Dv SOCK_STREAM .

.It Bq Er EINVAL

The referenced socket is not listening for connections (that is,

.Xr listen 2

has not yet been called).

.It Bq Er EFAULT

The

.Fa addr

parameter is not in a writable part of the user address space.

.It Bq Er EWOULDBLOCK

The socket is marked non-blocking and no connections

are present to be accepted.

.It Bq Er EMFILE

The per-process descriptor table is full.

.It Bq Er ENFILE

The system file table is full.

.It Bq Er ECONNABORTED

A connection has been aborted.

.El

.Sh SEE ALSO

.Xr bind 2 ,

.Xr connect 2 ,

.Xr listen 2 ,

.Xr poll 2 ,

.Xr select 2 ,

.Xr poll 2 ,

.Xr socket 2

.Sh HISTORY

The

.Fn accept

function appeared in

.Bx 4.2 .