c - Where do malloc() and free() store allocated sizes and addresses?

Question

Where do malloc() and free() store the allocated addresses and their sizes (Linux GCC)? I've read that some implementations store them somewhere before the actual allocated memory, but I could not confirm that in my tests.

The background, maybe someone has another tip for this:

I'm experimenting a little bit with analyzing the heap memory of a process in order to determine the current value of a string in the other process. Accessing the process heap memory and strolling through it is no problem. However, because the value of the string changes and the process allocates a new part of the memory each time, the string's address changes. Because the string has a fixed format it's still easy to find, but after a few changes the old versions of the string are still in the heap memory (probably freed, but still not reused / overwritten) and thus I'm not able to tell which string is the current one.

So, in order to still find the current one I want to check if a string I find in the memory is still used by comparing its address against the addresses malloc() and free() know about.

ciao, Elmar

Answer 1

There are lots of ways in which malloc/free can store the size of the memory area. For example, it might be stored just before the area returned by malloc. Or it might be stored in a lookup table elsewhere. Or it might be stored implicitly: some areas might be reserved for specific sizes of allocations.

To find out how the C library in Linux (glibc) does this, get the source code from http://ftp.gnu.org/gnu/glibc/ and look at the malloc/malloc.c file. There is some documentation at the top, and it refers to A Memory Allocator by Doug Lea.