libc/sys/accept.2

.\"	$OpenBSD: accept.2,v 1.20 2007/05/31 19:19:32 jmc Exp $
.\"	$NetBSD: accept.2,v 1.7 1996/01/31 20:14:42 mycroft Exp $
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.\"     @(#)accept.2	8.2 (Berkeley) 12/11/93
.\"
.Dd $Mdocdate: May 31 2007 $
.Dt ACCEPT 2
.Os
.Sh NAME
.Nm accept
.Nd accept a connection on a socket
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/socket.h>
.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
call 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 structure
.Li sockaddr_storage
exists for greater portability.
It is large enough to hold any of the types that may be returned in the
.Fa addr
parameter.
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,
.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 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 EXAMPLES
The following code uses struct
.Li sockaddr_storage
to allocate enough space for the returned address:
.Bd -literal -offset indent
#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");
.Ed
.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 EINTR
A signal was caught before a connection arrived.
.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
or
.Fa addrlen
parameter is not in a valid part of the process 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 socket 2
.Sh HISTORY
The
.Fn accept
function appeared in
.Bx 4.2 .