emscripten and mmap()

[Jeroen Demeyer <jdemeyer@cage.ugent.be>]

Moving discussion from bug #1912 to pari-dev

> This patch causes the mmap code to be more fragile and to break in a
> non-obvious way on systems where it used to work.

With "this patch" I assume that you mean my collection of commits 
involving mmap() with PROT_NONE and that "systems where it used to work" 
refers to emscripten?

As I tried to say several times already, emscripten needs to *emulate* 
mmap(). It has no access to the operating system, so it cannot do an 
actual mmap() system call. Apparently, the emulation of mmap() in 
emscripten lacks features: for example, MAP_FIXED and the PROT_... flags 
are simply not supported. For that reason, I think it's a bad idea to 
use mmap() with emscripten.

On systems which properly implement mmap (as described by POSIX), my 
patch should work.

It would be easy to add a configure check to check whether 
mmap(MAP_FIXED, ...) actually works. It seems that config/has_mmap.c is 
only *compiled* though, not *run*. Testing for MAP_FIXED support can 
only be done by *running* code.


Cheers,
Jeroen.