Finding the start and end of memory areas

Finding the start and end of memory areas

[Shaun Jackman]

First off, this posting is somewhat off-topic. My apologies in
advance. If anyone knows of a better forum for this topic, I'd
appreciate being pointed in the right direction. Now, on to my
question...

I'd like to find the start and end of the following memory areas
during run-time...

text
data
bss - &__bss_start to &__bss_end
heap - &__bss_end to sbrk(0)
(gap)
stack - stack pointer to ?

Presumably, they appear in that order in memory, so then end of one
should be the start of the next. Note that I'm doing this at run-time
by the program itself (introspection, as it were) and not with tools
like readelf and objdump. I've filled in the few values I know. If you
can provide any others I'd appreciate it.

If it affects the results, the target architectures are
i686-pc-linux-gnu and arm-elf.

Many thanks!
Shaun

Re: Finding the start and end of memory areas

[Nick Clifton]

Hi Shaun,

> First off, this posting is somewhat off-topic. My apologies in
> advance. If anyone knows of a better forum for this topic, I'd
> appreciate being pointed in the right direction. 

Actually this is the right forum for this kind of question.

> I'd like to find the start and end of the following memory areas
> during run-time...
> 
> text
> data
> bss - &__bss_start to &__bss_end
> heap - &__bss_end to sbrk(0)
> (gap)
> stack - stack pointer to ?

The simplest way is to have the cooperation of the linker script used to 
create your executable.  If the script places symbols at the start and 
end of each of these sections then you can easily access them at run-time.

If you cannot modify the linker script then you are going to have to 
locate the executable on disk (presumably via argv[0]) and then load and 
process the ELF headers and section tables themselves.

Cheers
   Nick

Re: Finding the start and end of memory areas

[Shaun Jackman]

Thanks for pointing me towards the linker script. From 'ld --verbose'
I found ld was using /usr/lib/ldscripts/elf_i386.xc. From this I found
the following information...

text ? to &_etext
data &_etext to &_edata
bss - &__bss_start to &_end
heap - &_end to sbrk(0)
(gap)
stack - stack_ptr to ?

From a snippet of code in newlib (sys/arm/syscalls.c), I found that
stack_ptr can be defined as such:
register void *stack_ptr asm ("sp");

On a Linux system, the beginning of the text and the top of the stack
are constants, 0x08048000 and 0xbffff7a0 respectively. I haven't yet
found a way to find these values at run-time in a cross-platform
manner.

I found the above values give somewhat different results than 'size'.
For exampled, on my sample program...
_etext = 0x8057baf
_edata = 0x805ce18
_end = 0x80b1cd8
_etext - 0x08048000 = 64431 // size of text
_edata - _etext = 21097 // size of data
_end - _edata = 347840 // size of bss
_end - 0x08048000 = 433368 // total
$ size a.out
   text    data     bss     dec     hex filename
  74477    7704  347832  430013   68fbd a.out

The results are similar, but not identical.

Cheers,
Shaun


> The simplest way is to have the cooperation of the linker script used to
> create your executable.  If the script places symbols at the start and
> end of each of these sections then you can easily access them at run-time.
> 
> If you cannot modify the linker script then you are going to have to
> locate the executable on disk (presumably via argv[0]) and then load and
> process the ELF headers and section tables themselves.
> 
> Cheers
>    Nick