I am trying to provide memory wrappers on CentOS and using clang compiler/linker. I wrote wrappers for the allocation functions (malloc et al) and rerouted the calls using -Wl,-wrap,malloc.
This all works fine and I can see it in action.
void* mem = malloc(10); // routes to __wrap_malloc
free(mem);// routes to __wrap_free
However, the problem I am seeing is that any memory allocated within libc is not being routed to my wrapper but the application is making the free call that gets intercepted (and crash as a result). For example,
char* newStr = strdup("foo"); // The internal malloc in libcdoes not come to wrapper
free(newStr); // The free call makes it to the wrapper
My program is in C++. I created a mallocimpl.cpp and did something like
extern "C"{
void* __wrap_malloc(size_t size)
{
// Route memory via custom memory allocator
}
//Similarly, __wrap_calloc, __wrap_realloc, __wrap_memalign and __wrap_free
Any ideas what I am doing wrong? Do I need any special compiler/linker flags?
Thanks in advance.
The recommended way to replace the glibc malloc implementation is ELF symbol interposition:
This way, you do not have to recompile everything, including glibc, and your malloc replacement will still be called once glibc removes the malloc hooks.
The __wrap approach does not work without recompiling (or at least rewriting) everything because all other libraries (including glibc) will use non-wrapped symbols.