Hey Paul, Thank you very much for your exceptionally informative summary! Always very nice hearing these stories straight from the horse’s mouth ;-)
All the best, Yury > On 29. Jul 2024, at 07:20, Paul Eggert <egg...@cs.ucla.edu> wrote: > > On 2024-07-28 15:40, Bob Friesenhahn wrote: > >> Whoever wrote it had an expectation that it would be used for this purpose. >> It would be good to investigate the history of this macro to see if its >> purpose was always the same or if its scope was later narrowed. > > AC_FUNC_MMAP has always checked for MAP_FIXED, ever since it was introduced > to Autoconf in 1994[1] by Mike Haertel and Jim Avera. Mike wrote in 2010[2] > that he built GNU grep to use mmap for speed, so I think the Autoconf macro > was for GNU grep, which did use MAP_FIXED. Although that sort of thing made > sense on some platforms in the early 1990s, I noticed even at the time that > it actually made things slower in significant cases, and in 2010 Paolo > Bonzini noticed something similar and removed mmap usage from GNU grep[3]. > I've never been tempted to bring it back. > > Because POSIX+XSI requires support for MAP_FIXED[4], it's reasonable for > AC_FUNC_MMAP to check for MAP_FIXED, and AIX mmap should support MAP_FIXED if > AIX says it conforms to XSI. > > I suppose that an app that needs mmap but not MAP_FIXED (which is allowed by > POSIX sans XSI) could write its own special-purpose Autoconf macro for that. > That being said, if you're planning to use mmap on AIX good luck; you may > need it[5]. > > [1]: > https://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=8202df5faf642cde4e5b1fa02cd9cc4411a7a477 > [2]: > https://lists.freebsd.org/pipermail/freebsd-current/2010-August/019310.html > [3]: > https://git.savannah.gnu.org/cgit/grep.git/commit/?id=f17211efe374200d532a9f27173e15b018e5538e > [4]: > https://pubs.opengroup.org/onlinepubs/9799919799/functions/mmap.html#tag_17_345_03 > [5]: https://news.ycombinator.com/item?id=19805675
