Debugging realloc() failure with valgrind

I've been trying to solve the advent of code puzzles this year. Mainly to improve my python but I'm also using it as an opportunity to implement the same logic in C. The C implementations are actually helping me learn more than the python code.

I was working on the day 3 problem in C when I hit a problem with realloc() failing. The code ran fine otherwise and if I allocated a huge buffer to start with (basically never calling realloc()) then it gave correct answers. The realloc() call kept failing.

I spent a few minutes trying to figure out manually what was going wrong before giving up to try and find a better way. I had heard about valgrind but never used it. I decided to give it a go. You can find the wrong code on my github repo.

I compiled the code with a simple gcc puzzle.c and ran it with valgrind using valgrind ./a.out and there it was:

==23715== Invalid write of size 8
==23715==    at 0x10938A: main (in /home/aditya/Git/aoc_2018/day_3/a.out)
==23715==  Address 0x4a35830 is 0 bytes after a block of size 1,280 alloc'd
==23715==    at 0x4839D7B: realloc (vg_replace_malloc.c:826)
==23715==    by 0x1093CC: main (in /home/aditya/Git/aoc_2018/day_3/a.out)

Plain and simple. There is a write outside the allocated space. It was still hard to figure out why and where. That's when I headed to the freenode IRC channel for C. There someone mentioned using -g3 to compile my code and sure enough:

==24391== Invalid write of size 8
==24391==    at 0x10938A: main (puzzle.c:70)
==24391==  Address 0x4a35830 is 0 bytes after a block of size 1,280 alloc'd
==24391==    at 0x4839D7B: realloc (vg_replace_malloc.c:826)
==24391==    by 0x1093CC: main (puzzle.c:74)

The line#70 was the culprit. So I was writing to an uninitialized location from here:

fabrics[total_fabrics] = fabric;

My check to allocate more memory was missing one important thing. I had it set to only reallocate when total_items were more than 32 while my counting started at 0. I was writing to location 32 which is undefined. The fix was simple:

if(total_fabrics > (int)realloc_counter) => if(total_fabrics >= (int)realloc_counter)

That was it. The code was working again, this time reallocating the buffers as and when required. Happy coding!