Seg Fault when initializing array

Question

I'm taking a class on C, and running into a segmentation fault. From what I understand, seg faults are supposed to occur when you're accessing memory that hasn't been allocated, or otherwise outside the bounds. 'Course all I'm trying to do is initialize an array (though rather large at that)

Am I simply misunderstanding how to parse a 2d array? Misplacing a bound is exactly what would cause a seg fault-- am I wrong in using a nested for-loop for this?

The professor provided the clock functions, so I'm hoping that's not the problem. I'm running this code in Cygwin, could that be the problem? Source code follows. Using c99 standard as well.

To be perfectly clear: I am looking for help understanding (and eventually fixing) the reason my code produces a seg fault.

#include <stdio.h>
#include <time.h>
int main(void){
   //first define the array and two doubles to count elapsed seconds.   
   double rowMajor, colMajor;
   rowMajor = colMajor = 0;
   int majorArray [1000][1000] = {};

   clock_t start, end;

   //set it up to perform the test 100 times.
   for(int k = 0; k<10; k++)
   {
   start=clock();
   //first we do row major
   for(int i = 0; i < 1000; i++)
   {
       for(int j = 0; j<1000; j++)
       {
           majorArray[i][j] = 314;
       }
   }
   end=clock();
   rowMajor+= (end-start)/(double)CLOCKS_PER_SEC;
   //at this point, we've only done rowMajor, so elapsed = rowMajor
   start=clock();
   //now we do column major
     for(int i = 0; i < 1000; i++)
   {
       for(int j = 0; j<1000; j++)
       {
           majorArray[j][i] = 314;
       }
   }
   end=clock();
   colMajor += (end-start)/(double)CLOCKS_PER_SEC;
   }
   //now that we've done the calculations 100 times, we can compare the values.
   printf("Row major took %f seconds\n", rowMajor);
   printf("Column major took %f seconds\n", colMajor);
   if(rowMajor<colMajor)
   {
     printf("Row major is faster\n");
   }
   else
   {
      printf("Column major is faster\n");
   }

   return 0;

}

Answer 1

You are trying to grab 1000 * 1000 * sizeof( int ) bytes on the stack. This is more then your OS allows for the stack growth. If on any Unix - check the ulimit -a for max stack size of the process.

As a rule of thumb - allocate big structures on the heap with malloc(3). Or use static arrays - outside of scope of any function.

In this case, you can replace the declaration of majorArray with:

int (*majorArray)[1000] = calloc(1000, sizeof majorArray);