Re: do_mmap

[Richard B. Johnson]

On Fri, 31 May 2002, Thomas 'Dent' Mirlacher wrote:

> --snip/snip
> 
> > >> 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.
> > 
> > In short, under Linux 0 _is_, and will always be (at least on x86) a
> > perfectly valid return address from mmap() and friends. It's only going
> > to be returned when you explicitly ask for it with MAP_FIXED, but it
> > absolutely is a valid return.
> 
> ok. so 0 or (NULL) is not an option, and also unnneccessary once someone
> know how the error retun is used. - wouldn't it be much more cleaner
> to convert this _ugly_ unsigned long vals into void * wherever these vals
> are carrying an address? (well at least for do_mmap*) and use ERR_PTR
> for returning, and IS_ERR for checking for an error?
> 
> btw, is err should (according to alans explaination be):
> 
> 	return (unsigned long)ptr > (unsigned long)-1024UL;
> 
> 	tm
> 

At the user-mode API, we get to (void *) -1, defined in sys/mman.h
(actually (__ptr_t) -1); so whatever you do, the 'C' runtime library
has to 'know' about your return values if this propagates to
sys-calls.

Cheers,
Dick Johnson

Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).

                 Windows-2000/Professional isn't.