Unix Section 9 Manual Page: Kernel Routines (.9)
The 9 file extension refers to Section 9 of the Unix manual pages, specifically covering Kernel Routines. These manual pages provide detailed information on kernel APIs, internal structures, and other essential aspects of kernel development. Developed by Dennis Ritchie and Ken Thompson at AT&T Bell Laboratories in 1971, Section 9 is crucial for kernel developers but is not universally available due to its specialized nature.