freebsd-dev/share/man/man5/fdesc.5
1997-03-07 03:28:23 +00:00

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.\" Copyright (c) 1996
.\" Mike Pritchard <mpp@FreeBSD.org>. All rights reserved.
.\"
.\" Copyright (c) 1992, 1993, 1994
.\" The Regents of the University of California. All rights reserved.
.\" All rights reserved.
.\"
.\" This code is derived from software donated to Berkeley by
.\" Jan-Simon Pendry.
.\"
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.\"
.Dd December 14, 1996
.Dt fdesc 5
.Os BSD 4.4
.Sh NAME
.Nm fdesc
.Nd file-descriptor file system
.Sh SYNOPSIS
fdesc /dev fdesc rw 0 0
.Sh DESCRIPTION
The file-descriptor file system, or
.Nm fdesc ,
provides access to the per-process file descriptor
namespace in the global filesystem namespace.
The conventional mount point is
.Pa /dev
and the filesystem should be union mounted in order to augment,
rather than replace, the existing entries in
.Pa /dev .
.Pp
The contents of the mount point are
.Pa fd ,
.Pa stderr ,
.Pa stdin ,
.Pa stdout
and
.Pa tty .
.Pp
.Pa fd
is a directory whose contents
appear as a list of numbered files
which correspond to the open files of the process reading the
directory.
The files
.Pa /dev/fd/0
through
.Pa /dev/fd/#
refer to file descriptors which can be accessed through the file
system.
If the file descriptor is open and the mode the file is being opened
with is a subset of the mode of the existing descriptor, the call:
.Bd -literal -offset indent
fd = open("/dev/fd/0", mode);
.Ed
.Pp
and the call:
.Bd -literal -offset indent
fd = fcntl(0, F_DUPFD, 0);
.Ed
.Pp
are equivalent.
.Pp
The files
.Pa /dev/stdin ,
.Pa /dev/stdout
and
.Pa /dev/stderr
appear as symlinks to the relevant entry in the
.Pa /dev/fd
sub-directory.
Opening them is equivalent to the following calls:
.Bd -literal -offset indent
fd = fcntl(STDIN_FILENO, F_DUPFD, 0);
fd = fcntl(STDOUT_FILENO, F_DUPFD, 0);
fd = fcntl(STDERR_FILENO, F_DUPFD, 0);
.Ed
.Pp
Flags to the
.Xr open 2
call other than
.Dv O_RDONLY ,
.Dv O_WRONLY
and
.Dv O_RDWR
are ignored.
.Pp
The
.Pa /dev/tty
entry is an indirect reference to the current process's controlling terminal.
It appears as a named pipe (FIFO) but behaves in exactly the same way as
the real controlling terminal device.
.Sh FILES
.Bl -tag -width /dev/stderr -compact
.It Pa /dev/fd/#
.It Pa /dev/stdin
.It Pa /dev/stdout
.It Pa /dev/stderr
.It Pa /dev/tty
.El
.Sh SEE ALSO
.Xr tty 4 ,
.Xr mount_devfs 8 ,
.Xr mount_fdesc 8
.Sh HISTORY
The
.Nm
filesystem first appeared in
.Bx 4.4 . The
.Nm
manual page first appeared in
.Fx 2.2 .
.Sh AUTHOR
The
.Nm
manaul page was written by Mike Pritchard <mpp@FreeBSD.org>,
and was based on the
.Xr mount_fdesc 8
manual page written by Jan-Simon Pendry.