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.\" @(#)accept.2 8.2 (Berkeley) 12/11/93
.\" $FreeBSD$
.\"
.Dd October 9, 2014
.Dt ACCEPT 2
.Os
.Sh NAME
.Nm accept ,
.Nm accept4
.Nd accept a connection on a socket
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.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"
.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
system call extracts the first connection request on the
queue of pending connections, creates a new socket,
and allocates a new file descriptor for the socket which
inherits the state of the
.Dv O_NONBLOCK
and
.Dv O_ASYNC
properties and the destination of
.Dv SIGIO
and
.Dv SIGURG
signals from the original socket
.Fa s .
.Pp
The
.Fn accept4
system call is similar,
but the
.Dv O_NONBLOCK
property of the new socket is instead determined by the
.Dv SOCK_NONBLOCK
flag in the
.Fa flags
argument,
the
.Dv O_ASYNC
property is cleared,
the signal destination is cleared
and the close-on-exec flag on the new file descriptor can be set via the
.Dv SOCK_CLOEXEC
flag in the
.Fa flags
argument.
.Pp
If no pending connections are
present on the queue, and the original socket
is not marked as non-blocking,
.Fn accept
blocks the caller until a connection is present.
If the original 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 argument that is filled-in with
the address of the connecting entity,
as known to the communications layer.
The exact format of the
.Fa addr
argument is determined by the domain in which the communication
is occurring.
A null pointer may be specified for
.Fa addr
if the address information is not desired;
in this case,
.Fa addrlen
is not used and should also be null.
Otherwise, the
.Fa addrlen
argument
is a value-result argument; 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
For some applications, performance may be enhanced by using an
.Xr accept_filter 9
to pre-process incoming connections.
.Pp
When using
.Fn accept ,
portable programs should not rely on the
.Dv O_NONBLOCK
and
.Dv O_ASYNC
properties and the signal destination being inherited,
but should set them explicitly using
.Xr fcntl 2 ;
.Fn accept4
sets these properties consistently,
but may not be fully portable across
.Ux
platforms.
.Sh RETURN VALUES
These calls return \-1 on error.
If they succeed, they return a non-negative
integer that is a descriptor for the accepted socket.
.Sh ERRORS
The
.Fn accept
and
.Fn accept4
system calls 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
argument is not in a writable part of the
user address space.
.It Bo Er EWOULDBLOCK Bc or Bq Er EAGAIN
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
.Pp
The
.Fn accept4
system call will also fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa flags
argument is invalid.
.El
.Sh SEE ALSO
.Xr bind 2 ,
.Xr connect 2 ,
.Xr getpeername 2 ,
.Xr getsockname 2 ,
.Xr listen 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr accept_filter 9
.Sh HISTORY
The
.Fn accept
system call appeared in
.Bx 4.2 .
.Pp
The
.Fn accept4
system call appeared in
.Fx 10.0 .
