NAME
sticky - persistent text and append-only directories

DESCRIPTION
The sticky bit (file mode bit 01000, see chmod(2)) is used to
indicate special treatment for certain executable files and
directories.

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. Shareable text segments are normally placed in a
least-frequently-used cache after use, and thus the ‘sticky bit’
has little effect on commonly-used text images.

Sharable executable files are made by the -n and -z options of
ld(1).

Only the super-user can set the sticky bit on a sharable executable
file.

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.

Any user may create a sticky directory. See chmod(1) for details
about modifying file modes.

BUGS
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.

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