Re: ancient OS history, ARM is sort of channeling the IBM 360

John Levine <johnl@taugh.com> writes:

As I think I said before, these days you map the file into your
address space and read or write it like a long string, so its all
locate all the time.

Reading with mmap() appears to be quite common, either directly or
through libaries like glibc which use mmap() for buffered input where
possible.  For writing, using mmap() poses some difficulties:

* Writing to an mmap()ed region has weaker atomicity semantics than
  write().

* mmap() works on pages (not in the interface, fortunately, but
  internally), but for writing you want byte granularity.  The
  difference is not a big problem for reading, but it is a bigger one
  for writing.

* You have to extend the file length with a separate system call, and
  then mmap() the new area, so you might just as well use write().

I have not looked at what glibc does for buffered writing, but I have
been advisor on a master's thesis that tried to extend the reach of
mmap()
<https://www.complang.tuwien.ac.at/Diplomarbeiten/syrowatka00.ps.gz>;
at the start I was very enthusiastic about the possibilities, at the
end I concluded that mmap() is good for reading (when it can be
applied), but usually not appropriate for writing.

- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>