Determine size of dynamically allocated memory in C

Question

Is there a way in C to find out the size of dynamically allocated memory?

For example, after

char* p = malloc (100);

Is there

Answer 1

This code will probably work on most Windows installations:

template 
int get_allocated_bytes(T* ptr)
{
 return *((int*)ptr-4);
}

template 
int get_allocated_elements(T* ptr)
{
 return get_allocated_bytes(ptr)/sizeof(T);
}