Loading a heap analyser with LD_PRELOAD, and global scope objects in C++

Question

We have written a heap analyser (for reference known as lib_debug) which overrides free and malloc (and others), primarily to monitor for errors such as memory leaks in our applications, which are deployed on a PPC system running Linux. lib_debug is compiled as a shared library, which is loaded with LD_PRELOAD as and when needed.

This debugger has worked great up until now; whenever malloc is called, a memory monitor singleton is created on the stack, which reports on any un-freed memory when this singleton goes out of scope on exit.

However, one of our applications now allocates a std::vector at the global scope. It appears that this global vector is being created before our heap analyser is, and most importantly destroyed afterwards. Hence, once in main if we insert anything into this vector, lib_debug thinks we have a memory leak on shutdown.

To illustrate, this is what I believe the sequence of events is:

1. LD_PRELOAD loads the symbols from lib_debug;
2. std::vector is created on stack at global scope;
3. lib_debug initialisation occurs - malloc has been called somewhere, which causes the actual heap analyser singleton to be created on the stack;
4. Enter main;
5. std::vector::push_back() occurs, allocating memory on the heap;
6. main exits;
7. The lib_debug heap analyser singleton goes out of scope, which sees that the vector hasn't freed its memory - reports error;
8. The vector goes out of global scope, releasing its memory;
9. Program terminates.

Therefore, is there any way we can guarantee that our LD_PRELOADed singleton is always the last thing to be deleted, and therefore catch every single free that occurs in our application?

Answer 1

This works if you are using GCC as your compiler. You can define a function in your library to act as a constructor, so that code gets executed when your library is loaded.

static int initialize_lib_debug () __attribute__((constructor));

static int initialize_lib_debug () {
    /* ... */
}