Can some throw bright light on Linux Memory Allocation

[Sergey Skorokhodov]

Hi,

> Hello,
> 
> I have installed Ubuntu on a machine with following
> config.
> 
> RAM :  2 GB
> Machine:  Intel Core 2 Duo
> OS:  Ubuntu 64 bit.
> Disk:  250 GB.
> 
> 
> I have attached a simple program to test memory allocation on linux
> and something strange is happening.
> 
> 1.    If I invoke large_memory function then maximum
>        size I am allowed is 245GB.
>  
> 2.    If I use less than 245GB  ( say 244 GB) then the
>        second stage is also passed.  Where from OS is
>        allocating 244GB ?
>  
> 3.   Strange enough, if I use chuck memory, I can
>       goto 3817GB before I get segmentation fault.
>       ( and buf == NULL is never reached ).
> 
> 4.   Much more strange, GoogleHeapProfiler, and
>       ValGrindHeap profile also add to my uneasiness
>       and report that 3817GB allocation is successful.
> 
> What is happening ? Please help.
> 
I guess that the result of

size_t nb	=	1024*1024*1024;
nb		*=	255;	// 0x3FC0000000

is much larger than the maximal value of size_t on 32-bit systems. So it
is truncated silently to 4 GB. Try 

size_t nb = 1024*1024*1024*255;

and you get a warning. C/C++ consider that the programmer knows what
he/she is doing in the former case and keep silent but you can turn some
more rigorous checking on if you like.

Actually, a program is allocated with address space some less that 4 GB
on 32-bit system as some address space is used for memory mapped io and
other hardware needs.

-- 
Best regards,
Serge