c - 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
", rowMajor);
   printf("Column major took %f seconds
", colMajor);
   if(rowMajor<colMajor)
   {
     printf("Row major is faster
");
   }
   else
   {
      printf("Column major is faster
");
   }

   return 0;

}

Answer 1

Your program works correctly on my computer (x86-64/Linux) so I suspect you're running into a system-specific limit on the size of the call stack. I don't know how much stack you get on Cygwin, but your array is 4,000,000 bytes (with 32-bit int) - that could easily be too big.

Try moving the declaration of majorArray out of main (put it right after the #includes) -- then it will be a global variable, which comes from a different allocation pool that can be much bigger.

By the way, this comparison is backwards:

if(rowMajor>colMajor)
{
  printf("Row major is faster
");
}
else
{
   printf("Column major is faster
");
}

Also, to do a test like this you really ought to repeat the process for many different array sizes and shapes.