This is the sticky bit, usually 01000.
For a directory it gives permission to delete a file in that directory
only if you own that file. Ordinarily, a user can either delete all the
files in a directory or cannot delete any of them (based on whether the
user has write permission for the directory). The same restriction
applies—you must have both write permission for the directory and own
the file you want to delete. The one exception is that the owner of the
directory can delete any file in the directory, no matter who owns it
(provided the owner has given himself write permission for the
directory). This is commonly used for the /tmp directory, where
anyone may create files but not delete files created by other users.
Originally the sticky bit on an executable file modified the swapping
policies of the system. Normally, when a program terminated, its pages
in core were immediately freed and reused. If the sticky bit was set on
the executable file, the system kept the pages in core for a while as if
the program were still running. This was advantageous for a program
likely to be run many times in succession. This usage is obsolete in
modern systems. When a program terminates, its pages always remain in
core as long as there is no shortage of memory in the system. When the
program is next run, its pages will still be in core if no shortage
arose since the last run.
On some modern systems where the sticky bit has no useful meaning for an
executable file, you cannot set the bit at all for a non-directory.
If you try,
chmod fails with
see Setting Permissions.
Some systems (particularly SunOS) have yet another use for the sticky
bit. If the sticky bit is set on a file that is not executable,
it means the opposite: never cache the pages of this file at all. The
main use of this is for the files on an NFS server machine which are
used as the swap area of diskless client machines. The idea is that the
pages of the file will be cached in the client's memory, so it is a
waste of the server's memory to cache them a second time. With this
usage the sticky bit also implies that the filesystem may fail to record
the file's modification time onto disk reliably (the idea being that
no-one cares for a swap file).
This bit is only available on BSD systems (and those derived from
them). Therefore one has to use the
_BSD_SOURCE feature select
macro to get the definition (see Feature Test Macros).