Thread: Program runs fine but crashes when i press enter to exit the exe. window

Hi everyone,

So I'm working on a stacks project and i every time i run the following function, the program runs fine until the end but it only crashes once i press enter to exit out of the command screen. I'm thinking it may be some sort of memory access error.. but i can't figure out exactly where it goes wrong. S->data is a pointer to an array of stackItems and itemPtr is just a pointer to a stackItem.

Please help! and thank you!!

Code:

int stack_peek( stackItem *itemPtr, Stack S )
/*This function takes as arguments a Stack S and a pointer to a stackItem 
itemPtr. If S is not empty, the top item is copied into the space pointed to by
 itemPtr. It returns 1 of S is not empty, 0 if it is empty, and -1 of any of 
 its arguments is NULL.*/
{
     if (S){                              // check if S exists
            if (S->count==0) {            // check if S is empty
                             return 0;    
                             }
            else {                           // if Stack is not empty
                 *itemPtr = S->data[S->top]; // copy value of top item to itemPtr
                 return 1;  
            }
     }
     else {                 // if S is NULL, return -1
          return -1;
          }
}

Probably you're feeding it an invalid pointer in itemPtr. (Possibly something weird is happening with S->top, but usually writes cause this sort of thing and reads don't.)

Hey so i fixed the problem but i'm not really sure why it works now.
you were right, it was something with itemPtr. in main when i had called the function stack_peek. I declared an itemPtr as: stackItem *itemPtr; but i didnt give an address or any value to it so i guess it wasnt dereferenced to anything and maybe that's why it crashed?

because now i just made a stackItem variable and deferenced itemPtr to that variable first and then when i called the function stack_peek, nothing crashed.

but i'm not really sure what the reasoning is behind it?..

Because you declared a pointer variable, but didn't give it a value, that meant that it was pointing to something important (this is Murphy's Law). Then when you do *itemPtr = whatever, the value is stored where itemPtr points to. Since itemPtr pointed to something important, that something important got overwritten, which meant when you needed to do the something important (namely "close the window") it wasn't there anymore.

ah.. i see, that makes a lot more sense now. thank you!!