Why is this C program crashing? - c

Question

I have a simple test program in C to scramble an array of values on the heap. Sidenote: I know the random logic here has a flaw that will not allow the "displaced" value to exceed RAND_MAX, but that is not the point of this post.
The point is that when I run the code with N = 10000, every once in a while it will crash with very little information (screenshots posted below). I'm using MinGW compiler. I can't seem to reproduce the crash for lower or higher N values (1000 or 100000 for example).
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
const int N = 10000;
int main() {
int i, rand1, rand2, temp, *values;
/* allocate values on heap and initialize */
values = malloc(N * sizeof(int));
for (i = 0; i < N; i++) {
values[i] = i + 1;
}
/* scramble */
srand(time(NULL));
for (i = 0; i < N/10; i++) {
rand1 = (int)(N*((double)rand()/(double)RAND_MAX));
rand2 = (int)(N*((double)rand()/(double)RAND_MAX));
temp = values[rand1];
values[rand1] = values[rand2];
values[rand2] = temp;
}
int displaced = 0;
for (i = 0; i < N; i++) {
if (values[i] != (i+1)) {
displaced++;
}
}
printf("%d numbers out of order\n", displaced);
free(values);
return 0;
}

Answer 1

it may be because rand() generates a random number from 0 to RAND_MAX inclusive so (int)(N*((double)rand()/(double)RAND_MAX)) can be N, which exceeds the array boundary. however, i don't see why that would vary with array size (it does explain why it only crashes sometimes, though).
try /(1+(double)RAND_MAX) (note that addition is to the double, to avoid overflow, depending on the value of RAND_MAX) (although i'm not convinced that will always work, depending on the types involved. it would be safer to test for N and try again).
also, learn to use a tool from Is there a good Valgrind substitute for Windows? - they make this kind of thing easy to fix (they tell you exactly what went wrong when you run your program).