Getting Segmentation fault (Core dumped) error while executing the C++ program

Question

Below is the code I am trying to run, I keep getting the error Segmentation fault. Added Compare. In the below program I need to consider an array of 5000000 or less(this is n) numbers and implement quicksort on those numbers.

int compare (const void * a, const void * b)
{
  const double *da = (const double *) a;
  const double *db = (const double *) b;

  return (*da > *db) - (*da < *db);
  }
int main(){

int n = rand()% 5000000;
double arr[n];
for (int i=0; i<n; i++)
{
    arr[i] = (double)rand();
}
qsort(arr,n, sizeof(double), compare);
 for (int  i=0;i<n;i++){
     cout<<arr[i]<<"\n";
 }
     return 0;
 }

Can you show us the code for `compare` too? – miniBill Jan 21 '16 at 17:18 — miniBill, Jan 21 '16 at 17:18

score 2 · Answer 1 · answered Jan 21 '16 at 16:04

2

1) rand usually cannot generate numbers that large. It is commongly capped around 32k

2) double arr[n]; is not legal if n is not a compile-time constant. You are probably using unportable compiler extention.

3) Stack space (where arr resides) is usually very limited. I doubt it can accomodate so much data which is the reason to crash.

answered Jan 21 '16 at 16:04

Revolver_Ocelot

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Is there any way to fix it? I have to consider an array and the length of the array can go upto 5000000 – KSK Jan 21 '16 at 16:06
@KSK (1) [use c++11 `random` library to generate your number](http://en.cppreference.com/w/cpp/numeric/random). (2) and (3): allocate array dynamically in heap space. – Revolver_Ocelot Jan 21 '16 at 16:08
1) really depends on system, windows uses INT16_MAX standard gnu header uses INT32_MAX – Kai Iskratsch Jan 21 '16 at 16:08
if you don't have c++11, you can also use multiple random calls to get more bits – Kai Iskratsch Jan 21 '16 at 16:10

score 1 · Accepted Answer · edited Jan 21 '16 at 16:06

1

The problem is that with the double arr[n] syntax you are allocating the array on the stack.

Now, the stack is a limited resource, and n can grow up to almost 5 megabytes, which is way beyond the normal size of the stack

You should allocate the array on the heap with

double * arr = new double[n];

and then free it at the end of the function with

delete[] arr;

edited Jan 21 '16 at 16:06

Ed Heal

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answered Jan 21 '16 at 16:02

miniBill

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4

delete []arr I guess is better – sestus Jan 21 '16 at 16:04
How do I modify to fix the error? I have to consider an array and the length of the array can go upto 5000000 – KSK Jan 21 '16 at 16:05
@KSK use the heap, not the stack – johnbakers Jan 21 '16 at 16:07
1

Or perhaps use `std::vector` – Ed Heal Jan 21 '16 at 16:07
@johnbakers I am at beginner level of C++ programming, can you tell how can I change from stack to heap. – KSK Jan 21 '16 at 16:08
@KSK the others on this page have already shown you how to do that – johnbakers Jan 21 '16 at 16:26

score 0 · Answer 3 · answered Jan 21 '16 at 16:45

Sorry this is a very partial answer but I can't add comments (low reputation...) Still, there should be a RAND_MAX value defined in stdlib (0x7FFF in my case) So to my understanding rand() returns a value between 0 and 32767. If you want a value between 0 and 5000000 you can try

unsigned int n = ((double)rand()/RAND_MAX) * 5000000;
double * arr = new double[n];
...
delete[] arr;

Also, check that your compare structure is a strict comparator.

Hope it helps

Getting Segmentation fault (Core dumped) error while executing the C++ program

3 Answers3