Sticky bit trivia

If you have used any "recent" (the quotes are important) Unix-like system, you probably know what the sticky bit is used for: to restrict the deletion of files in a directory to, basically, the owner of such files. The sticky bit is used on /tmp (among other directories) so that anyone can create temporary files in it but only those that created the temporary files can delete them.

But that's not all that is to know.

The original purpose of the sticky bit was to mark frequently-used executable files so that the operating system kept their text segment on swap space. This speeded up subsequent executions of such programs because the system would not need to access the file system to load the binary: it could just use the image already kept in the swap space. This behavior became obsolete with the advent of memory-mapped executables.

Now it's clear why the sticky bit has the name it has, isn't it? But still, that's not all that is to know.

SunOS 4 introduced a new behavior for the sticky bit on regular, non-executable files: reads and writes to such files would bypass the buffer cache, thus basically telling the system to perform raw I/O on those files. This was particularly useful on swap files when NFS was involved.

With the above, I have just tried to summarize you the information that is in NetBSD's chmod(2) and sticky(7) manual pages; they contain much more detailed information. (And yep, that's right: contrary to what the Wikipedia article says, NetBSD does not support this behavior any more.) Hope you found this insightful if you did not know this little piece of history!