Why is my program segfaulting?

Question

I'm having some difficulty figuring out why my program is segfaulting. Any help? Some context: This is a coding assignment where I run this method through a program that executes with various inputs and determines if the function is correct or not. The purpose of this assignment is to read assembly and construct a function based on the given assembly.

I don't think it's the comp4 method, because I've compared it to the given assembly, and it's identical, which suggests it's in the problem4 method, but I have no idea what's in problem 4 that could even cause a seg fault. Could anyone offer any help?

long int comp4(long int * a, long int b) {
  long int output = *a;
  long int newa = output + b;
  *a = newa;
  return output;
}

long int problem4(long int a, long int b){
int i;
long int* ap;
*ap = a;
long int sum;
long int holder;
for (i=0;i<b;i++){
  sum = sum + i*8;
  holder = comp4(ap,2);
  sum = sum + holder;
}
return sum;
}

Run it under your debugger. Which line generates the segfault? — Martin James, Apr 01 '16 at 19:27
Here: `*ap = a` you dereference an uninitialised pointer. Did you mean `ap = &a`? That makes `ap` point to `a`. — M Oehm, Apr 01 '16 at 19:29

user007 · Accepted Answer · 2016-04-01T19:59:35.030

0

ap is an uninitialized pointer. When you declare long int *ap it allocates a memory on the stack that can hold the address of a long int * but it does not allocate the memory for a variable to which this pointer can point.

So, you need to initialize your pointer value, before you can dereference it using the * operator.

A more appropriate approach would be ::

long int *ap = &a;

or

long int *ap = malloc(sizeof(long int));
*ap = a;

Dereferencing a non-initialized pointer is the reason your program segfaults!

edited Apr 01 '16 at 19:59

answered Apr 01 '16 at 19:42

user007

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What language is your second code sample supposed to be written in? It's not C, at least not without sprinkling on some preprocessor magic. – John Bollinger Apr 01 '16 at 19:48
@JohnBollinger Updated! :) – user007 Apr 01 '16 at 20:00
Thank you! I realize that these segfault questions can get repetitive but I appreciate you answering anyway. – wtk219 Apr 01 '16 at 20:21

Why is my program segfaulting?

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