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.\"     @(#)fd.4	8.1 (Berkeley) 6/9/93
.\" $FreeBSD: releng/12.0/share/man/man4/fd.4 263142 2014-03-14 03:07:51Z eadler $
.\"
.Dd June 9, 1993
.Dt FD 4
.Os
.Sh NAME
.Nm fd ,
.Nm stdin ,
.Nm stdout ,
.Nm stderr
.Nd file descriptor files
.Sh DESCRIPTION
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
Opening the files
.Pa /dev/stdin ,
.Pa /dev/stdout
and
.Pa /dev/stderr
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.
.Sh IMPLEMENTATION NOTES
By default,
.Pa /dev/fd
is provided by
.Xr devfs 5 ,
which provides nodes for the first three file descriptors.
Some sites may require nodes for additional file descriptors; these can be
made available by mounting
.Xr fdescfs 5
on
.Pa /dev/fd .
.Sh FILES
.Bl -tag -width /dev/stderr -compact
.It Pa /dev/fd/#
.It Pa /dev/stdin
.It Pa /dev/stdout
.It Pa /dev/stderr
.El
.Sh SEE ALSO
.Xr tty 4 ,
.Xr devfs 5 ,
.Xr fdescfs 5