sticky(7) - NetBSD Manual Pages

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STICKY(7)           NetBSD Miscellaneous Information Manual          STICKY(7)


NAME

     sticky -- Description of the `sticky' (S_ISVTX) bit functionality


DESCRIPTION

     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

   Sticky files
     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.

   Sticky directories
     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 direc-
     tory, 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.


HISTORY

     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.


BUGS

     Neither open(2) nor mkdir(2) will create a file with the sticky bit set.

NetBSD 6.1.2                     May 10, 2011                     NetBSD 6.1.2