Since all you need to reconstruct the last try is the length of str1 you used, it is easy to reconstruct the winner even though you need to go too far to find the limit. You might even use bisection to find the right number! If str1 has 6 bytes, try 3, then 1 or 5, then 0, 2, 4, 6 to find the highest substring that works. Use the shorter length of str1 and str2, as the max overlap is that length and the min is always 0.

I helps in C to know what is going on, at least in a sensible model. Imagine a char* in a 32 bit system is a 4 byte unsigned integer offset from the bottom of memory. Values in the environment are in the heap below free memory. The heap grows up like stalagmites. Subroutine arguments and auto variables are on the stack at the top of memory growing down like stalactites. When you call, automatics may have whatever old ram content in them, so write before you read. When you exit a subroutine, the stack pointer rises above the automatics and passed variables, and the space is reused/overwritten on the next call. When you create a static or global variable, the compiler/linker allocates it on the heap. When you malloc, that is on the heap, too. Since you can free, someone has to keep track of the holes and try to reuse them. Fancy items like structs and such may be allocated on a mod 4 or 8 address, for speed. Some CPUs need aligned variables -- 4 byte integers have to have mod-4 addresses. So, space can be wasted when little and big items are mixed. If you mmap() a file region, address space is allocated, but rather than setting it up to swap to swap space, it is tied to the file. Some people do not like calling everything at the bottom 'the heap'. Dynamically linked code is mmap'd into several places. Itmight have initialized variables and constants with it, which are put in different areas, sometimes because code is on executable pages but data may be not executable, even not writable. If you run a command under truss, tusc or strace, you can see all this going on -- very educational.