c++ - How to debug segmentation fault?

Question

It works when, in the loop, I set every element to 0 or to entry_count-1. It works when I set it up so that entry_count is small, and I write it by hand instead of by loop (sorted_order[0] = 0; sorted_order[1] = 1; ... etc).

Please do not tell me what to do to fix my code. I will not be using smart pointers or vectors for very specific reasons. Instead focus on the question: What sort of conditions can cause this segfault? Thank you.

---- OLD -----

I am trying to debug code that isn't working on a unix machine. The gist of the code is:

int *sorted_array = (int*)memory;
// I know that this block is large enough
// It is allocated by malloc earlier

for (int i = 0; i < entry_count; ++i){
  sorted_array[i] = i;
}

There appears to be a segfault somewhere in the loop. Switching to debug mode, unfortunately, makes the segfault stop. Using cout debugging I found that it must be in the loop.

Next I wanted to know how far into the loop the segfault happend so I added:

std::cout << i << '
';

It showed the entire range it was suppose to be looping over and there was no segfault.

With a little more experimentation I eventually created a string stream before the loop and write an empty string into it for each iteration of the loop and there is no segfault.

I tried some other assorted operations trying to figure out what is going on. I tried setting a variable j = i; and stuff like that, but I haven't found anything that works.

Running valgrind the only information I got on the segfault was that it was a "General Protection Fault" and something about default response to 11. It also mentions that there's a Conditional jump or move depends on uninitialized value(s), but looking at the code I can't figure out how that's possible.

What can this be? I am out of ideas to explore.

Answer 1

This is clearly a symptoms of invalid memory uses within your program.This would be bit difficult to find by looking out your code snippet as it is most likely be the side effect of something else bad which has already happened.

However as you have mentioned in your question that you are able to attach your program using Valgrind. as it is reproducible. So you may want to attach your program(a.out).

$ valgrind --tool=memcheck --db-attach=yes ./a.out

This way Valgrind would attach your program in the debugger when your first memory error is detected so that you can do live debugging(GDB). This should be the best possible way to understand and resolve your problem.

Once you are able to figure it out your first error, fix it and rerun it and see what are other errors you are getting.This steps should be done till no error is getting reported by Valgrind.

However you should avoid using the raw pointers in modern C++ programs and start using std::vector std::unique_ptr as suggested by others as well.