Segmentation fault after declaring a large array.

oulevon · 11-04-2005, 11:57 PM

This program has a segmentation fault as it enters the for loop in the estimatepi function. Does anyone know why? I think it might have to do something with the array. Anyone have any ideas. Thanks for any help.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define MAXARRAY 50000000
double estimatepi(int n); 

int main (int argc, char * argv[]){

  double MyPi = 0.0;
  printf("I started with %f.\n", MyPi);  

  MyPi = estimatepi(10000000);
  

  
  return(0);
}

double estimatepi( int n ){
  
   int i;
   int count = 0;
  double numpair[MAXARRAY][2];
  
 
  for (i = 0; i < n; i++){ 
    numpair[i][0] = ((double) rand())/RAND_MAX;
    numpair[i][1] = ((double) rand())/RAND_MAX;
  }
  
     for (i = 0 ; i < n; i++){ 
       if( ((numpair[i][0] * numpair[i][0]) + (numpair[i][1] * numpair[i][1])) < 1.0) count++;
     }
     
     return((((double)count)/n) * 4.0);

}

perfect_circle · 11-05-2005, 12:23 AM

Are you aware you are declaring an array of size 781.250MB (in a pc )!!!

zhangmaike · 11-05-2005, 12:31 AM

I'm betting your array is too huge... but, looking at your code, you don't need an array anyway - just combine your loops and remove the array:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define MAXARRAY 50000000
double estimatepi(int n); 

int main (int argc, char * argv[]){

  double MyPi = 0.0;
  printf("I started with %f.\n", MyPi);  

  MyPi = estimatepi(10000000);
  

  
  return(0);
}

double estimatepi( int n ){
  
   int i;
   int count = 0;

// This isn't needed.
//  double numpair[MAXARRAY][2];
  
 
  for (i = 0; i < n; i++){ 
    /*numpair[i][0]*/
    double a = ((double) rand())/RAND_MAX;
    /*numpair[i][1]*/
    double b= ((double) rand())/RAND_MAX;

    // Code from old loop moved here
       if( ((a*a) + (b*b)) < 1.0) count++;

  }
       
     return((((double)count)/n) * 4.0);

}

paulsm4 · 11-05-2005, 12:33 AM

Works fine for me:

(gdb) p MyPi
$2 = 3.14113

Suggestions:

1. Compile with -g, step into the code with gdb, and troubleshoot with the debugger
2. Compile with -S, and look at how the stack is being allocated (it should be exactly what you're expecting)
3. Compile with different combinations of gcc "-fstack-check", " -fstack-limit-symbol" and/or "-fno-stack-limit"

zhangmaike · 11-05-2005, 12:37 AM

...you can still get the same result without arrays, and without using close to a gig of memory.

oulevon · 11-05-2005, 12:45 AM

Thanks for the help guys. It's not my program. I was trying to determine the seg fault for someone else. I assumed it was the array size, and I had run it through gdb already, but it still seg faults.

elluva · 11-08-2005, 02:41 AM

If an array is declared that is too large, I think you will get a null pointer. So you might insert some code to check that pointer.