Question

I’m trying to identify a bug in I have in my code where I get seg. fault while trying to assign value to a pointer from a vector (it is describe better in the link). When I run the code using valgrind I don’t get the seg.fault.

What does valgrind do differently. I think that I need to consider the memory management differences between valgrind session and regular c++ session but I don’t really know

Answer 1

From Valgrind FAQ:

4.4. My program crashes normally, but doesn’t under Valgrind, or vice
versa. What’s happening?

When a program runs under Valgrind, its environment is slightly
different to when it runs natively. For example, the memory layout is
different, and the way that threads are scheduled is different.

Most of the time this doesn’t make any difference, but it can,
particularly if your program is buggy. For example, if your program
crashes because it erroneously accesses memory that is unaddressable,
it’s possible that this memory will not be unaddressable when run
under Valgrind. Alternatively, if your program has data races, these
may not manifest under Valgrind.

There isn’t anything you can do to change this, it’s just the nature
of the way Valgrind works that it cannot exactly replicate a native
execution environment. In the case where your program crashes due to a
memory error when run natively but not when run under Valgrind, in
most cases Memcheck should identify the bad memory operation.

So you can nothing to do with it. Actually you need not worry that you program not crashes under Valgrind. You should read error messages from it and fix them. Start with Invalid read/Invalid write errors. They are almost always indicate the bug in code. In this particular case you can also run your code in infinite loop from simple bash script utill it produces error message. Most likely you are working with invalid iterators and it is Undefined Behaviour in C++.