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
