Question

I have to call functions in the main executable from a shared library loaded with LD_PRELOAD.

The executable exports all symbols and contains debug information. Unfortunately I don’t have access to it’s source code.

Currently I’m getting undefined symbol errors when trying to load that shared library.
Is there a way to do this?

PS:
Target platform is FreeBSD/x86.

Answer 1

Can you create a function pointer by doing a typedef and use ‘dlsym()’ to get the address
of the symbol. You can then invoke the function through the function pointer like a normal
function call. Note: You do not need dlopen() since the main executable with symbols exported
is loaded into process address space.

Prototype:

void *dlsym(void *handle, const char *symbol);

Assume the exported function is:

int foo(char *arg);

Your function pointer:

typedef (int)(*fooPtr)(char *);

In you code:

/* You can send NULL for first argument */
fooPtr fp = dlsym(NULL, "foo");
assert(0 != fp);
int ret = fp("hello world");