Re: Memory allocations in linux for processes

[Niroj Pokhrel <nirojpokhrel@xxxxxxxxx>]

On Tue, Mar 19, 2013 at 8:27 PM, Mulyadi Santosa <mulyadi.santosa@xxxxxxxxx> wrote:

Hi Niroj....

Please see inlined answer below...

On Mon, Mar 18, 2013 at 5:28 PM, Niroj Pokhrel <nirojpokhrel@xxxxxxxxx> wrote:
> Hi all,
> As I have read, whenever we execute a program, the memory is allocated in
> different sections viz stack, text and data segment.

Yup, that is correct

> But, since we have used
> loader and linker the three allocations will happen for them too.

Correct again. Linker and loader will be the part of process address space.

> But when I ran a program and did pmap pid, I saw several other fields which
> I have no idea of. What is the [anon] and what is it doing here ?

Anon is short of anonymous. It is a region of memory created by
function e.g malloc() or mmap()

>Why the
> memory is being implemented for it and there are so many of them ?

Drawing from above explanation, we can easily conclude somewhere in
the program, it does malloc() or mmap().

> And also the stack ( 3rd last element), is it the process stack??

Yes, it's process' stack

>Can
> somebody please explain.
> Thanking you all in advance.
>
> 5448:   ./a.out
> 0000000000400000      4K r-x--  /home/n.pokhrel/Personal/mywork/a.out
> 0000000000600000      4K r----  /home/n.pokhrel/Personal/mywork/a.out
> 0000000000601000      4K rw---  /home/n.pokhrel/Personal/mywork/a.out
> 00007f57f5e18000   1512K r-x--  /lib/libc-2.11.1.so
> 00007f57f5f92000   2044K -----  /lib/libc-2.11.1.so
> 00007f57f6191000     16K r----  /lib/libc-2.11.1.so
> 00007f57f6195000      4K rw---  /lib/libc-2.11.1.so
> 00007f57f6196000     20K rw---    [ anon ]
> 00007f57f619b000    128K r-x--  /lib/ld-2.11.1.so
> 00007f57f6396000     12K rw---    [ anon ]
> 00007f57f63b8000      8K rw---    [ anon ]
> 00007f57f63ba000      4K r----  /lib/ld-2.11.1.so
> 00007f57f63bb000      4K rw---  /lib/ld-2.11.1.so
> 00007f57f63bc000      4K rw---    [ anon ]
> 00007fffa2f37000     84K rw---    [ stack ]

> 00007fffa2fa0000      4K r-x--    [ anon ]
> ffffffffff600000      4K r-x--    [ anon ]

ok, one of the region above, I think it's VDSO (virtual dynamic shared
object). It's a region that handle system call. When a process does
system call, it jumps to VDSO, and it is the VDSO that does the
actuall syscall

--
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

Hi Mulyadi .
Thank you very much.... But I still have a minor confusion .........
All I ran was this short program

#include<stdio.h>
int main()
{
    while(1)
    {
    }
    return 0;
}

I
 don't understand where does mmap or malloc come in to play in this
 code. It would be great if you can give me some insight.
Thanking you in advance.

Yours,
Niroj Pokhrel

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