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  Calling A Function At ELF Shared Library Load Time
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This text was only tested under Linux 2.4.x, on x86 processor. It could be usefull to define a function in a library to be call at load time, like Windows do with 'DllMain()'.

On an ELF shared library, it is a bit more tricky, and I spent some hours on the problem. Here is the solution :

Let's say you have a library, and you want to call 'Elf_Init()' function BEFORE the main() (or _start) defined on the caller program.

Just define your function, like another one, but put a small piece of assembly in it :

int Elf_Init(void)
{
  	__asm__ (".section .init \n call Elf_Init \n .section .text\n");

if(!Init_some_stuff()) { exit(-1); } So_do_more_and_more();

return 1; }



So how does this work ?

According to ELF format (http://www.lnxscene.org/jylam/elf2.ps), there is a special section called '.init'. This section can contain code which will be launched immediadly after the shared library will be loaded by ld.so .

If you try to do this in assembly and think it'll be easy, be prepared to have some problems. In fact, the problem is that ld (the linker) will add some more code in this section, then put your own, and will fuck up the stack, then will lose the start entry address. That's why it's easier to put that in a C file, sometimes it's good to let the C compiler do the bad stuff :) Happy hacking.

-- Jean-Yves Lamoureux

The zip file viewer built into the Developer Toolbox made use of the zlib library, as well as the zlibdll source additions.

 

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