Re: Doubts about mmap(2)

[Alex Colomar <alx.manpages@gmail.com>]

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On 5/18/23 10:53, Alex Colomar wrote:
> Hi!
> 
> I believe mmap(2) is confusing (see the emboldened parts):
> 
> DESCRIPTION
>         mmap()  creates  a  new mapping in the virtual address space of
>         the calling process.  **The starting address for the new  mapping
>         is specified in addr.**  The length argument specifies the length
>         of the mapping (which must be greater than 0).
> 
>         If addr is NULL, then the kernel chooses the (page‐aligned) ad‐
>         dress at which to create the mapping; this is the most portable
>         method  of  creating  a new mapping.  **If addr is not NULL, then
>         the kernel takes it as a hint about where to place the mapping;
>         on Linux, the kernel will pick a nearby page boundary** (but  al‐
>         ways    above    or   equal   to   the   value   specified   by
>         /proc/sys/vm/mmap_min_addr) and attempt to create  the  mapping
>         there.   If  another  mapping  already exists there, the kernel
>         picks a new address that may or may not  depend  on  the  hint.
>         The address of the new mapping is returned as the result of the
>         call.
> 
> 
> The first emboldened sentence contradicts the second.  It's not the
> starting address, but just a hint to find a nearby (>=) starting

I was a bit wrong there.  It's >= mmap_min_addr, not >= hint.  But the
suggestion below holds.

> address.  How about saying this?:
> 
>      A hint for the starting address for the new mapping is specified in
>      addr.
> 
> Cheers,
> Alex
> 

-- 
<http://www.alejandro-colomar.es/>
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