Question

I’m very new to C, but have no idea why this program breaks. The program compiles and runs if I remove the lines that have to do with i, but if I assign i, I can no longer assign anything to *ptr without the program breaking.

int main(void)
{
    int i;
    int *ptr;

    i = 2;
    *ptr = 5;
    printf("%d",*ptr);
}

Answer 1

You leave the pointer with uninitialized value. So when you dereference it (*ptr), you access arbitrary place in memory, resulting in a segmentation fault.

Point ptr at something by assigning to ptr itself (not *ptr) an address of a variable (like &i) or some freshly allocated memory (like malloc(sizeof(int))).