An action on executable's library load - c++

Question

Let's say, I have an executable a.out. It has shared libraries libcrypto.so, libmylib.so. I'd like to make some action at every of these libs load. Is there some function I can add my code to? So I will define some function inside my executable and it will be called twice for this example.
I'm aware of __attribute__ ((constructor)), but it should be defined for every library specifically, which is not possible.
Thanks.

Answer 1

I'd like to make some action at every of these libs load. Is there some function I can add my code to?
Your code, no. But you could LD_PRELOAD a dlopen interposer.
Example:
// foo.c
#include <stdio.h>
__attribute__((constructor))
void ctor()
{
printf("In %s:%d\n", __FILE__, __LINE__);
}
// main.c
#include <dlfcn.h>
#include <stdio.h>
int main()
{
printf(">>> main\n");
dlopen("./foo.so", RTLD_LAZY);
dlopen("./bar.so", RTLD_LAZY);
printf("<<< main\n");
return 0;
}
gcc -shared -fPIC -o foo.so foo.c
gcc -shared -fPIC -o bar.so foo.c
gcc main.c -ldl
./a.out
>>> main
In foo.c:6
In foo.c:6
<<< main
Now let's add a dlopen interposer:
// dlopen_preload.c
#define _GNU_SOURCE
#include <stdio.h>
#include <dlfcn.h>
typedef void* (*FN)(const char *, int);
void *dlopen(const char *fname, int flags)
{
FN real_dlopen = (FN)dlsym(RTLD_NEXT, "dlopen");
void *ret = real_dlopen(fname, flags);
printf("my dlopen(%s, 0x%x) -> %p\n", fname, flags, ret);
return ret;
}
gcc -shared -o dlopen_preload.so dlopen_preload.c
LD_PRELOAD=./dlopen_preload.so ./a.out
>>> main
In foo.c:6
my dlopen(./foo.so, 0x1) -> 0x565039670690
In foo.c:6
my dlopen(./bar.so, 0x1) -> 0x565039670c90
<<< main
Voilà.