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Manual page for STICKY(8)

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.

STICKY SYMBOLIC LINKS

Some file systems (such as autonfsmount(8)) will set the 'sticky bit' on symbolic links to indicate that the link points to a directory. This is done to indicate to other programs, such as ls(1) and Workspace, that they need not read the link to determine whether the link points to a directory. The advantage is that programs can improve their performance by avoiding the sometimes time-consuming read of the link.

For example, with autonfsmount(8), a remote NFS filesystem is not mounted until a sticky symbolic link is read. After the filesystem is mounted, the sticky bit is turned off and remains off until the filesystem gets unmounted. This scenario makes Workspace faster because it doesn't have to read the link unless the NFS filesystem is already mounted.

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.