/share/man/man4/fd.4

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 32.\"     @(#)fd.4	8.1 (Berkeley) 6/9/93
 33.\" $FreeBSD$
 34.\"
 35.Dd June 9, 1993
 36.Dt FD 4
 37.Os
 38.Sh NAME
 39.Nm fd ,
 40.Nm stdin ,
 41.Nm stdout ,
 42.Nm stderr
 43.Nd file descriptor files
 44.Sh DESCRIPTION
 45The files
 46.Pa /dev/fd/0
 47through
 48.Pa /dev/fd/#
 49refer to file descriptors which can be accessed through the file
 50system.
 51If the file descriptor is open and the mode the file is being opened
 52with is a subset of the mode of the existing descriptor, the call:
 53.Bd -literal -offset indent
 54fd = open("/dev/fd/0", mode);
 55.Ed
 56.Pp
 57and the call:
 58.Bd -literal -offset indent
 59fd = fcntl(0, F_DUPFD, 0);
 60.Ed
 61.Pp
 62are equivalent.
 63.Pp
 64Opening the files
 65.Pa /dev/stdin ,
 66.Pa /dev/stdout
 67and
 68.Pa /dev/stderr
 69is equivalent to the following calls:
 70.Bd -literal -offset indent
 71fd = fcntl(STDIN_FILENO,  F_DUPFD, 0);
 72fd = fcntl(STDOUT_FILENO, F_DUPFD, 0);
 73fd = fcntl(STDERR_FILENO, F_DUPFD, 0);
 74.Ed
 75.Pp
 76Flags to the
 77.Xr open 2
 78call other than
 79.Dv O_RDONLY ,
 80.Dv O_WRONLY
 81and
 82.Dv O_RDWR
 83are ignored.
 84.Sh IMPLEMENTATION NOTES
 85By default,
 86.Pa /dev/fd
 87is provided by
 88.Xr devfs 5 ,
 89which provides nodes for the first three file descriptors.
 90Some sites may require nodes for additional file descriptors; these can be
 91made available by mounting
 92.Xr fdescfs 5
 93on
 94.Pa /dev/fd .
 95.Sh FILES
 96.Bl -tag -width /dev/stderr -compact
 97.It Pa /dev/fd/#
 98.It Pa /dev/stdin
 99.It Pa /dev/stdout
100.It Pa /dev/stderr
101.El
102.Sh SEE ALSO
103.Xr tty 4 ,
104.Xr devfs 5 ,
105.Xr fdescfs 5