Matrix multiplication in C++ gives Runtime Error -

i'm using following c++ code matrix multiplication , runs fine size = 500. when size = 600 or above code fails. (runtime error)

i ran on ideone.com ouputs "runtime error time: 0 memory: 3292 signal:11"

and in local machine giving me error

#include <cstdlib> #include<iostream> #include <stdio.h> #include <sys/time.h>  using namespace std; class timer { private:  timeval starttime;  public:  void start(){     gettimeofday(&starttime, null); }  double stop(){     timeval endtime;     long seconds, useconds;     double duration;      gettimeofday(&endtime, null);      seconds  = endtime.tv_sec  - starttime.tv_sec;     useconds = endtime.tv_usec - starttime.tv_usec;      duration = seconds + useconds/1000000.0;      return duration; }  static void printtime(double duration){     printf("%5.6f seconds\n", duration); } }; using namespace std; const int size = 600; // size*size matrix  void multiplymatricessequential(double a[][size],double b[][size],double ans[][size]); int i,j,k; double s; /*  *   */ int main(int argc, char** argv) { double a[size][size], b[size][size], ans[size][size]; // assign numbers matrix , b (i = 0; < size; i++) {     (j = 0; j < size; j++) {         a[i][j]=(double)rand()/rand_max;         b[i][j]=(double)rand()/rand_max;     } } multiplymatricessequential(a,b,ans); return 0; }  void multiplymatricessequential(double a[][size],double b[][size],double ans[][size]) {    timer timer = timer();     timer.start();  (i = 0; < size; i++) {     (j = 0; j < size; j++) {         (k = 0; k < size; k++)             s += a[i][k] * b[k][j];         ans[i][j] = s;                  s = 0.0;     }  }  double duration = timer.stop(); cout << "sequential method time elapsed size " << size << " : "; timer.printtime(duration);   } 

so doing wrong here ?

note : still same when timer not used.

        #include <cstdlib>         #include<iostream>         #include <stdio.h>         #include <sys/time.h>          using namespace std;         const int size = 500; // size*size matrix          void multiplymatricessequential(double a[][size],double b[][size],double ans[][size]);         int i,j,k;         double s;         /*          *           */         int main(int argc, char** argv) {           double a[size][size], b[size][size], ans[size][size];             // assign numbers matrix , b             (i = 0; < size; i++) {                 (j = 0; j < size; j++) {                     a[i][j]=(double)rand()/rand_max;                     b[i][j]=(double)rand()/rand_max;                 }             }             multiplymatricessequential(a,b,ans);             return 0;         }          void multiplymatricessequential(double a[][size],double b[][size],double ans[][size])         {                (i = 0; < size; i++) {                 (j = 0; j < size; j++) {                     (k = 0; k < size; k++)                         s += a[i][k] * b[k][j];                     ans[i][j] = s;                              s = 0.0;                 }              }           } 

first of - if you're going read data unknown size - recommend using dynamic memory allocation storing instead of pre-allocating fixed-size arrays. if not case or don't want listen advice better allocate bigger arrays in global namespace instead on stack. means code that:

    using namespace std;             const int size = 500; // size*size matrix              void multiplymatricessequential(double a[][size],double b[][size],double ans[][size]);             int i,j,k;             double s;             /*              *               */     double g_a[size][size], g_b[size][size], g_ans[size][size];              int main(int argc, char** argv) {                  // assign numbers matrix , b                 (i = 0; < size; i++) {                     (j = 0; j < size; j++) {                         g_a[i][j]=(double)rand()/rand_max;                         g_b[i][j]=(double)rand()/rand_max;                     }                 }                 multiplymatricessequential(g_a,g_b,g_ans);                 return 0;             }  //.......... 

note added "g_" prefix "matrices" names.

also know stack or "local storage" used storing temporary variables , function arguments allocated when function in belong called , "removed" when returns. stack size fixed , if there no space in create them program crashes. global variables on other hand doesn't have memory limit space need automatically allocated on compile-time. life-time equals application run-time "matrices" because had created them "main" function. choice simple - want waste program time allocating & "removing" data , risking stack overflow or big data without hassle - using global variables. or if using real data - becoming friendlier dynamic memory allocation.