libc/sys/accept.2

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.\"     @(#)accept.2	8.2 (Berkeley) 12/11/93
.\" $FreeBSD$
.\"
.Dd December 11, 1993
.Dt ACCEPT 2
.Os
.Sh NAME
.Nm accept
.Nd accept a connection on a socket
.Sh LIBRARY
.Lb libc
.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 as
.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
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 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.
.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 IMPLEMENTATION NOTES
In the non-threaded library
.Fn accept
is implemented as the
.Va accept
syscall.
.Pp
In the threaded library, the
.Va accept
syscall is assembled to
.Fn _thread_sys_accept
and
.Fn accept
is implemented as a function which locks
.Fa s
for read and write, then calls
.Fn _thread_sys_accept .
If the call to
.Fn _thread_sys_accept
would block, a context switch is performed.
Before returning,
.Fn accept
unlocks
.Fa s .
.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 EINTR
The
.Fn accept
operation was interrupted.
.It Bq Er EMFILE
The per-process descriptor table is full.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er ENOTSOCK
The descriptor references a file, not a socket.
.It Bq Er EINVAL
.Xr listen 2
has not been called on the socket descriptor.
.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 ECONNABORTED
A connection arrived, but it was closed while waiting
on the listen queue.
.El
.Sh SEE ALSO
.Xr bind 2 ,
.Xr connect 2 ,
.Xr getpeername 2 ,
.Xr listen 2 ,
.Xr select 2 ,
.Xr socket 2
.Sh HISTORY
The
.Fn accept
function appeared in
.Bx 4.2 .