incredible program crash call stack

Question

I have one incredible segment crash call stack in C, could anyone help me out?

int function(struct A *a) 
{
    ... some other declare
    struct B *b = a->b;
    int count, len;
    ... some other declare
    struct C *c = b->c;  /* not crash here */
    ....some other decalre

    if (b->e) { /*crash log indicate b is NULL and crash here,first line instrucment code*/
    .....
    }
    ....
}

My question is why the crash did not happened at the struct C *c = b->c.

GCC is the compiler.

Tony

Have you seen anything that will guarantee a "crash" in documentation? — Eugene Sh., Feb 10 '17 at 23:05
Accessing invalid pointers is Undefined Behaviour. The compiler is not obliged to guarantee a crash as soon as UB is encountered. By definition UB means the behaviour is unpredicatable. — kaylum, Feb 10 '17 at 23:05
What's an incredible segment? I'd like to know more about these! — Daniel Kamil Kozar, Feb 10 '17 at 23:23
The code following if (b->e) will use the c. If the b is one null pointer, I think it should crash at the c = b->c , not at the following code, I met such crash before with the same compiler parameter. — orasy, Feb 10 '17 at 23:29

score 0 · Accepted Answer · answered Feb 10 '17 at 23:14

The compiler does not guarantee ordered execution of instructions unless there is a data flow dependency. It has numerous optimizations, one of which is data flow optimization.

Also, if you don't use the variable c at all, the compiler will optimize it out of the code (dead code optimization) and you may not see a crash at all (if the rest of the program works fine).

One easy way to understand which optimization is happening is to access the c variable somewhere. Another way is to disable optimizations (-O0 in GCC).

incredible program crash call stack

1 Answers1