Why do I get segmentation fault in C? [duplicate]

Question

Why this question is not duplicate? The problem can't be at scanf, as it is given by the hackerrank and I can't modify the code! Also, test cases are automated, given by hackerrank. We can trust them with input reading. There is some bug in my logic of finding two elements in array, couldn't find it though :|

Note: I can't modify main method, as mentioned earlier. Re-iterating it. Why can't I modify? It is because the method is given by the portal. I can't change it. It reads the input and calls my method through some automated scripts, I've no control over it. So, please don't tell me to modify main method.

I was trying some practice problems in hackerrank in C language and some of the test cases started throwing Segmentation fault for following problem. I've tried understanding it for more than 2 hours now and couldn't think of any test case which will throw segmentation fault. I'm stuck :|

Problem statement:

Given an array and a number. Find if there are 2 elements in array, whose sum is equal to given number. If number are present return 1, otherwise 0.

pretty easy, huh? I thought the same and coded it as follows,

int compFunc (const void * a, const void * b)
{
   return ( *(int*)a - *(int*)b );
}


int isSumPossible(int a[], int L, int N ){
/*L: Length of the array */
    if(a==NULL) return 0;
    if(L<=1) return 0;
    int left=0, right=L-1;

    qsort(a,L,sizeof(int),compFunc);

    while(left<right)
        {
        if(a[left]+a[right]==N) return 1;
        else if(a[left]+a[right]<N) left++;
        else right--;    
    }
    return 0;
}
int main() //given by hackerrank, I can't modify main method
{
 int N; scanf("%d", &N); //fixed &N - it was correctly given in code, I missed it while typing it here.
 int a[100004], i=0;
 //read input into array a, based on N
 int x;scanf("%d", &x); //fixed &x - it was correctly given in code, I missed it while typing it here.
 printf("%d", isSumPossible(a,N,x));
}

Please assume all the header files required are included. Now, when I ran the code, most of the test cases passed and for some test cases they showed Segmentation fault. Unfortunately test cases aren't visible to me :|. I've gone through my code more than 10 times & I don't understand in which scenario I would get Segmentaion fault. Can anyone help me in understand which scenario am I missing in my code and why do I see segmentation fault?

Used a debugger to step through it? That's usually the best way to find these kinds of problems. — Almo
Also when asking other people to debug your code, please use spaces, like this: if(a[left] + a[right] == N) — Almo
scanf needs address arguments to store the parsed values. i.e. scanf("%d",&N);, scanf("%d",&x);. — Jean-Baptiste Yunès
Using a 100004 size array on the stack is a terrible idea. You should really modify the main method, even if you say you can't. — Bregalad
May I suggest you provide prompts? When I run the program I'll just get a cursor. I might guess I am supposed to enter something, but what? Six months down the line this will help you too, as you are unlikely to remember. The N and x in the program source won't be much help, as those names have no meaning. printf("Enter the (whatever)\n"); — Weather Vane

Dom Dom · Accepted Answer · 2015-05-25T18:17:13

The problem is in the scanfs in the main part of the code.

If you look at documentation the function description even says:

Reads data from stdin and stores them according to the parameter format into the locations pointed by the additional arguments.

This means you must give it a memory address to write data to. By adding & to both of your integer variables in your main the problem is solved.

int main() 
{
 int N; scanf("%d", &N);
 int a[100004], i=0;
 //read input into array a, based on N
 int x;scanf("%d", &x);
 printf("%d", isSumPossible(a,N,x));
}

Why do I get segmentation fault in C? [duplicate]

1 Answers