Unexpected behavior? [duplicate] - c

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Creating large arrays in C
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I am trying to optimize the best I can a DAXPY procedure. However,
there is a mistake in the following code that I cannot spot:
#include <stdio.h>
#include <time.h>
void daxpy(int n, double a, double *x, double *y) {
int i;
double y0, y1, y2, y3, x0, x1, x2, x3;
// loop unrolling
for (i = 0; i < n; i += 4) {
// multiple accumulating registers
x0 = x[i];
x1 = x[i + 1];
x2 = x[i + 2];
x3 = x[i + 3];
y0 = y[i];
y1 = y[i + 1];
y2 = y[i + 2];
y3 = y[i + 3];
y0 += a * x0;
y1 += a * x1;
y2 += a * x2;
y3 += a * x3;
y[i] = y0;
y[i + 1] = y1;
y[i + 2] = y2;
y[i + 3] = y3;
}
}
int main() {
int n = 10000000;
double a = 2.0;
double x[n];
double y[n];
int i;
for (i = 0; i < n; ++i) {
x[i] = (double)i;
y[i] = (double)i;
}
clock_t start = clock();
daxpy(n, a, x, y);
clock_t end = clock();
double time_elapsed = (double)(end - start) / CLOCKS_PER_SEC;
printf("Time elapsed: %f seconds\n", time_elapsed);
return 0;
}
When executing the binary I am getting the following error message:
Segmentation fault: 11
Using LLDB I am getting:
Process 89219 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x7ff7bb2b2cc0)
frame #0: 0x0000000100003e4f daxpy.x`main at daxpy.c:38:14
35 double y[n];
36 int i;
37 for (i = 0; i < n; ++i) {
-> 38 x[i] = (double)i;
39 y[i] = (double)i;
40 }
41 clock_t start = clock();
Target 0: (daxpy.x) stopped.
But I can't spot what is the mistake on lines 38 and 39. Any help?

Answer 1

Assuming sizeof(double) == 8 you try to allocate 160MB on the stack. The stack is not usually as big.
You need to allocate it from the heap
size_t n = 10000000;
double a = 2.0;
double *x = malloc(sizeof(*x) * n);
double *y = malloc(sizeof(*y) * n);
for (i = 0; i < n; where n is the array size.
What happens if you
y1 = y[i + 1];
y2 = y[i + 2];
y3 = y[i + 3];
You may access the array outside the bounds.