Re: do_mmap

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

On Fri, 2002-05-31 at 18:30, Linus Torvalds wrote:
> >> is it possible to have 0 as a valid address? - if not, this should
> >> be the return on errors.
> >
> >SuS explicitly says that 0 is not a valid mmap return address.
> 
> But if so, SuS is _very_ _very_ wrong.
> 
> The fact is, if you use something like vm86 mode, you absolutely _need_
> to be able to explicitly mmap at address 0. 
> 
> So it is correct (and in fact there is no other sane way to do it) to
> say
> 
> 	addr = mmap(NULL, 1024*1024,
> 		PROT_READ | PROT_WRITE ,
> 		MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED,
> 		-1, 0);
> 
> and if SuS says that mmap must not return NULL for this case, then SuS
> is so full of sh*t that it's not worth worrying about.

SuS doesn't have this requirement in the case of MAP_FIXED.

For normal maps it says

"When the implementation selects a value for pa, it never places a mapping 
at address 0, nor does it replace any extant mapping."

For MAP_FIXED it says the return value shal be that of pa (first
argument) exactly