On Sun, Jul 28, 2024 at 10:20:01PM -0700, Paul Eggert 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. It's worth noting that while the comment always said that AC_FUNC_MMAP checked MAP_FIXED on a previously mapped address, the test was broken for 14 years after some cygwin support changes in 2009 [6]. This was fixed in December 2023 [7]. Unfortunately, this does mean there has been a behavior a change for AIX (at least unless "application has requested SPEC1170 complaint behavior" per the manpages). > 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]. I wonder if AIX builds are producing SPEC1170-requesting programs most of the time, but not when building the autoconf code fragments? Otherwise I'd expect things to be failing more. -- Brooks > > [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 > [6]: https://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=19629fa9fcb209d60012a75ba087a295d4b3c734 [7]: https://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=5d3f81d4b33218e00ac90bfa92d08eff06de9200
