usr/src/share/man/man8/sticky.8

.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
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.\"     @(#)sticky.8    6.2 (Berkeley) %G%
.\"
.TH STICKY 8 ""
.UC 4
.SH NAME
sticky \- persistent text and append-only directories
.SH DESCRIPTION
The
.I "sticky bit"
(file mode bit 01000, see
.IR chmod (2))
is used to indicate special treatment
for certain executable files and directories.
.SH "STICKY TEXT EXECUTABLE FILES"
While the `sticky bit'
is set on a sharable executable file,
the text of that file will not be removed from the system swap area.
Thus the file does not have to be fetched from the file system
upon each execution.
As long as a copy remains in the swap area, the
original text cannot be overwritten in the file system,
nor can the file be deleted.
(Directory entries can be removed so long as one link remains.)
.PP
Sharable executable files are made by the
.B \-n
and
.B \-z
options of
.IR ld (1).
.PP
To replace a sticky text executable file that has been used,
perform the following steps:
(1) Clear the sticky bit with
.IR chmod (1).
(2) Execute the old program to flush the swapped copy.
This can be done safely even if others are using it.
(3) Overwrite the sticky file.
If the file is being executed by any process,
writing will be prevented; it suffices to simply remove the file
and then rewrite it, being careful to reset the owner and mode with
.I chmod
and
.IR chown (2).
(4) Set the sticky bit again.
.PP
Only the super-user can set the sticky bit
on a sharable executable file.
.SH "STICKY DIRECTORIES"
A directory whose `sticky bit' is set
becomes an append-only directory, or, more accurately,
a directory in which the deletion of files is restricted.
A file in a sticky directory may only be removed or renamed
by a user if the user has write permission for the directory and
the user is the owner of the file, the owner of the directory,
or the super-user.
This feature is usefully applied to directories such as /tmp
which must be publicly writable but
should deny users the license to arbitrarily
delete or rename each others' files.
.PP
Any user may create a sticky directory.
See
.IR chmod (1)
for details about modifying file modes.
.SH BUGS
Sticky text executable files are
largely unnecessary on the VAX.
The sticky attribute is useful only for large programs that
will page heavily to start, since text pages are normally cached in core
as long as possible after all instances of a text image exit.
.PP
Since the text areas of sticky text executables are stashed in the swap area,
abuse of the feature can cause a system to run out of swap.
.PP
Neither
.IR open (2)
nor
.IR mkdir (2)
will create a file with the sticky bit set.