Re: [TUHS] mmap origin (was Systematic approach to command-line interfaces) - TUHS

1 Oct 2021

Dan wrote:

...
  3BSD and I think 4.1BSD had vread() and vwrite(),
which looked like 
 regular read() and write() but accessed pages only on demand. I was a 
 grad student at Berkeley at the time and remember their genesis. Bill 
 and I were eating lunch from Top Dog on the Etcheverry Hall plaza, and 
 were talking about memory-mapped I/O. I remember suggesting the actual 
 names, perhaps as a parallel to vfork(). I had used both TENEX and 
 Multics, which both had page mapping. Multics' memory-mapped segments 
 were quite fundamental, of course. I think we were looking for something 
 vaguely upward compatible from the existing system calls. We did not 
 leap to an mmap() right away just because it would have been a more 
 radical shift than continuing the stream orientation of UNIX. I did not 
 implement any of this: it was just a brainstorming session. 
Thank you for reminding me of these. 

On a substrate with a unified buffer cache and copy-on-write, vread/vwrite would have been
very close to regular read/write and maybe could have been subsumed into them, using flags
to open() as the differentiator. The user discernible effect would have been the alignment
requirement on the buffer argument.

John Reiser wrote that he "fretted” over adding a 6 argument system call. Perhaps he
was considering something like the above as the alternative, I never asked.

I looked at the archives and vread/vwrite were introduced with 3BSD, present in 4BSD but
marked deprecated, and absent from 4.1BSD. This short lifetime suggests that using vread
and vwrite wasn’t entirely satisfactory in 1980/81 practice. Maybe the issue was that
there was no good way to deallocate the buffer after use.