Question

I writed my LD_PRELOAD module and i want to add some initialization code before my overrided functions work. Maybe LD_PRELOAD have any loaded event or something like this?

Thanks!

Answer 1

I’m not sure about a “loaded” event, however if you’re using gcc, you may find the constructor attribute useful. Take for example:

testlib.c:
#include

void testing(void) __attribute__((constructor));

void testing(void)
{
  printf("It worked!\n");
}

hworld.c:

#include <stdio.h>

int main(void)
{
  printf("Hello world!\n");
  return 0;
}

$ gcc -o hworld hworld.c
$ gcc -shared -fPIC -o testlib.so testlib.c
$ export LD_PRELOAD=./testlib.so
$ ./hworld 
It worked!
Hello world!

The constructor attribute means that the function should be executed before main(). Alternatively, if you’re using C++, you could create a static global instance of a class whose constructor does the initialization, which would achieve the same effect as using constructor.