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 (

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.

Answer 2

That is hard, I used valgrind tools to debug seg-faults and it usually pointed to violations.

Likely your problem is freed memory that you are writing to i.e. sorted_array gets out of scope or gets freed. Adding more code hides this problem as data allocation shifts around.

Answer 3

After a few days of experimentation, I figured out what was really going on.

For some reason the machine segfaults on unaligned access. That is, the integers I was writing were not being written to memory boundaries that were multiples of four bytes. Before the loop I computed the offset and shifted the array up that much:

int offset = (4 - (uintptr_t)(memory) % 4) % 4;
memory += offset;

After doing this everything behaved as expected again.

Answer 4

Valgrind and GDB are very useful.

The most previous one that I used was GDB- I like it because it showed me the exact line number that the Segmentation Fault was on.

Here are some resources that can guide you on using GDB:

GDB Tutorial 1

GDB Tutorial 2

If you still cannot figure out how to use GDB with these tutorials, there are tons on Google! Just search debugging Segmentation Faults with GDB!

Good luck :)