sticky
—
Description of the `sticky' (S_ISVTX) bit functionality
A special file mode, called the sticky bit (mode
S_ISVTX
), is used to indicate special treatment for
directories. See chmod(2) or the
file /usr/include/sys/stat.h
For regular files, the use of mode S_ISVTX
is reserved
and can be set only by the super-user. NetBSD does not
currently treat regular files that have the sticky bit set specially, but this
behavior might change in the future.
A directory whose “sticky bit” is set becomes 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.
Any user may create a sticky directory. See
chmod(1) for details about
modifying file modes.
The sticky bit first appeared in V7, and this manual page appeared in section 8.
Its initial use was to mark sharable executables that were frequently used so
that they would stay in swap after the process exited. Sharable executables
were compiled in a special way so their text and read-only data could be
shared amongst processes. vi(1) and
sh(1) were such executables. This is
where the term “sticky” comes from - the program would stick
around in swap, and it would not have to be fetched again from the file
system. Of course as long as there was a copy in the swap area, the file was
marked busy so it could not be overwritten. On V7 this meant that the file
could not be removed either, because busy executables could not be removed,
but this restriction was lifted in BSD releases.
To replace such executables was a cumbersome process. One had
first to remove the sticky bit, then execute the binary so that the copy
from swap was flushed, overwrite the executable, and finally reset the
sticky bit.
Later, on SunOS 4, the sticky bit got an additional meaning for
files that had the bit set and were not executable: read and write
operations from and to those files would go directly to the disk and bypass
the buffer cache. This was typically used on swap files for NFS clients on
an NFS server, so that swap I/O generated by the clients on the servers
would not evict useful data from the server's buffer cache.