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.\"     @(#)accept.2	8.2 (Berkeley) 12/11/93
.\"
.Dd October 27, 2019
.Dt ACCEPT 2
.Os
.Sh NAME
.Nm accept ,
.Nm accept4 ,
.Nm paccept
.Nd accept a connection on a socket
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/socket.h
.Ft int
.Fn accept "int s" "struct sockaddr * restrict addr" "socklen_t * restrict addrlen"
.Ft int
.Fn accept4 "int s" "struct sockaddr * restrict addr" "socklen_t * restrict addrlen" "int flags"
.Ft int
.Fn paccept "int s" "struct sockaddr * restrict addr" "socklen_t * restrict addrlen" "const sigset_t * restrict sigmask" "int flags"
.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
function
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 or polling 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 .
.Pp
The
.Fn accept4
function is equivalent to paccept with sigmask
.Dv NULL .
.Pp
The
.Fn paccept
function behaves exactly like
.Fn accept ,
but it also allows to set the following
.Fa flags
on the returned file descriptor:
.Bl -tag -width SOCK_NOSIGPIPEXX -offset indent
.It Dv SOCK_CLOEXEC
Set the close on exec property.
.It Dv SOCK_NONBLOCK
Sets non-blocking I/O.
.It Dv SOCK_NOSIGPIPE
Return
.Er EPIPE
instead of raising
.Dv SIGPIPE .
.El
.Pp
It can also temporarily replace the signal mask of the calling thread if
.Fa sigmask
is a
.Pf non- Dv NULL
pointer, then the
.Fn paccept
function shall replace the signal mask of the caller by the set of
signals pointed to by
.Fa sigmask
before waiting for a connection, and shall restore the signal mask
of the calling thread before returning.
.Sh RETURN VALUES
The
.Fn accept
and
.Fn paccept
calls return \-1 on error.
If they succeed, they return a non-negative
integer that is a descriptor for the accepted socket.
.Sh COMPATIBILITY
The
.Fn accept
implementation makes the new file descriptor inherit file flags
(like
.Dv O_NONBLOCK )
from the listening socket.
It's a traditional behaviour for BSD derivative systems.
On the other hand, there are implementations which don't do so.
Linux is an example of such implementations.
Portable programs should not rely on either of the behaviours.
The
.Pp
.Fn accept4
function is compatible with the Linux implementation.
.Sh ERRORS
The
.Fn accept
function will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The socket is marked non-blocking and no connections
are present to be accepted.
.It Bq Er EBADF
The descriptor is invalid.
.It Bq Er ECONNABORTED
A connection has been aborted.
.It Bq Er EFAULT
The
.Fa addr
parameter is not in a writable part of the
user address space.
.It Bq Er EINTR
The
.Fn accept
call has been interrupted by a signal.
.It Bq Er EINVAL
The socket has not been set up to accept connections (using
.Xr bind 2
and
.Xr listen 2 ) .
.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 EOPNOTSUPP
The referenced socket is not of type
.Dv SOCK_STREAM .
.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 .
The
.Fn accept4
function matches Linux semantics and appeared in
.Nx 8.0 .
The
.Fn paccept
function is inspired from Linux and appeared in
.Nx 6.0 .