Writing a ldd like utility.

Writing a ldd like utility.

[vamsi krishna]

Hello All,

I need to write some for a program which needs the virtual address
where the shared libraries (I need to enumerate each of them ) are
mapped in the virtual adress space.

o I can read the link_maps of the dynamic linker get the details of
where the shared libraries are mapped, but I'am little skeptical about
its portability (do all the linuxes and unixes have the same way to
access the link_maps ?).

o I see the there are some .dynamic sections in the elf binary, so the
information on where (virtual address) these dynamic shared libraries
are mapped is put in this section?

o Was wondering how ldd does that ? (does it fork the executable and
get the link_maps from it?)

Thank you for your comments.

Regards,
Vamsi.

Re: Writing a ldd like utility.

[Ian Lance Taylor]

"vamsi krishna" <vamsi.krishnak@gmail.com> writes:

> o I can read the link_maps of the dynamic linker get the details of
> where the shared libraries are mapped, but I'am little skeptical about
> its portability (do all the linuxes and unixes have the same way to
> access the link_maps ?).

GNU/Linux always does, but other Unix systems do not.

> o I see the there are some .dynamic sections in the elf binary, so the
> information on where (virtual address) these dynamic shared libraries
> are mapped is put in this section?

No.  The virtual addresses are only known at run time, and therefore
can not be in the binaries.

> o Was wondering how ldd does that ? (does it fork the executable and
> get the link_maps from it?)

On GNU/Linux, ldd is a shell script.  You can see exactly what it
does.  Basically it invokes the dynamic linker with some environment
variables which cause the dynamic linker to print out information
about which shared libraries are loaded where.  Note that on a system
with address space randomization turned on
(/proc/sys/kernel/randomize_va_space != 0) the dynamic libraries will
not be loaded in the same location every time.

Ian