thread 1: EXC_BAD_ACCESS(code = 1, address = 0x7fff00000001) - c

Question

I'm trying to pass an array to a function that sums up all the elements in the array, but I get a bad access error at the line sum+=a[i]; how can i fix this? Here is the code:
#import <Foundation/Foundation.h>
int sum(int*, int);
int main() {
#autoreleasepool {
int size = 0;
int a[size];
int x;
NSLog(#"Enter a size for the array ");
scanf("%i", &size);
NSLog(#"Enter %i numbers to populate the array ", size);
for (int i = 0; i < size; i++) {
scanf("%i", &a[i]);
}
x = sum(a, size);
NSLog(#"The sum of the array is %i ", x);
}
return 0;
}
int sum(int *a, int n) {
int sum = 0;
for (int i = 0; i < n; i++) {
sum += a[i];
}
return sum;
}

Answer 1

It is because your array is 0 in size. Writing / Reading from a[i] may / may not crash as it's behavior is undefined.
Instead of
int size = 0;
int a[size];
int x;
NSLog(#"Enter a size for the array ");
scanf("%i", &size);
you should do this instead:
int size = 0;
int *a;
int x;
NSLog(#"Enter a size for the array ");
scanf("%i", &size);
a = malloc(sizeof(int) * size);
By dynamically allocate the array a, your program should no longer crash.
And after we use malloc, we have to free it when we don't need it anymore. Put this before return 0;
free(a);
Hope this helps.

Answer 2

You've defined an array of size 0. Since an array is a block of memory, and in this case a block of "no" memory, you cannot store anything into it.
You can use malloc/free as indicated in #Owen's answer. C99 also added the ability to declare arrays on the stack (so-called VLA, Variable Length Arrays). This saves you using malloc/free but leaves you at risk of using up all your stack space. For values which you know for a fact will be constrained, a VLA might make sense:
int size;
NSLog(#"Enter a size for the array ");
scanf("%i", &size);
int arr[size];
....
Note that in C89/C90/ANSI you would not be able to do that, since the size of an array must be a compile-time constant.