The /tmp directory shall be made available for programs that require temporary files.
Although data stored in /tmp may be deleted in a site-specific manner, it is recommended that files and directories located in /tmp be deleted whenever the system is booted.
Programs shall not assume that any files or directories in /tmp are preserved between invocations of the program.
IEEE standard P1003.2 (POSIX, part 2) makes requirements that are similar to the above section.
FHS added the recommendation that /tmp be cleaned at boot time on the basis of historical precedent and common practice, but did not make it a requirement because system administration is not within the scope of this standard.