how to find out which variable malloc() is being assigned to?

Question

I'm trying to track the usage of malloc'ed area through variables that point to the are in a profiler. For example, for the following assignment inside function func().

uint64_t *   dictionary = (uint64_t *) malloc(sizeof(uint64_t)*128);

I need to figure out the variable name (which is 'dictionary' in the above example) that points to the malloc'ed memory region. I instrumented malloc() to record the start address and size of the allocation. However, still no knowledge of variable 'dictionary', what I'm thinking is to examine the stack frame of function func(), finding out the local pointer variable pointing to a data type that matches that of malloc'ed type. The approach would need to instrument malloc() to go back one frame to func() to find out the possible local variables, and then fuzzy match by type. Wondering whether there are any other neat ways to implement this.

Are you building a tool like Valgrind or are you trying to debug your own programs? In the latter case, take a look at http://stackoverflow.com/questions/262439/create-a-wrapper-function-for-malloc-and-free-in-c — Klas Lindbäck, Nov 05 '13 at 08:49
Hi Klas, I'm trying to build a profiling tool, so yes a tool like Valgrind. — user1147800, Nov 05 '13 at 09:16

score 2 · Answer 1 · answered Nov 05 '13 at 08:47

In general, I would expect this to be impossible. :)

You can't, of course, assume that the variable name is available, the best bet in general would be (I guess) a stack offset in the calling function's frame. If debugging symbols are available you might perhaps be able to map that to a name, though.
I guess it's possible that there is no name; that the return address is put in a register and perhaps manipulated there, before (if ever) being written to memory. If this means your code needs to start analyzing the calling code to track what it does with the return value, that sounds difficult.

What do you want to do with the variable reference once you've isolated it? I assume you're instrumenting malloc() for debugging purposes, so probably you're going to store it somewhere.

how to find out which variable malloc() is being assigned to?

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