To shorten, my issue is simply understanding why would this code:
int main() {
typedef int* ASElement;
int zero = 0;
int one = 1;
int two = 2;
int three = 3;
ASElement *elements = (int**)malloc(4 * sizeof(ASElement));
*elements = (int*)malloc(4*sizeof(int));
*(elements) = &zero;
*(*(elements+1)) = one; //segementation fault here
printf("%d", *(*(elements+1)));
return 0;
}
not work?
Your program, as written, uses the contents at address elements+1 uninitialized when it does *(elements+1). You have never written at that address before, and it is inside a block allocated by malloc, so the value it contains is indeterminate and you are not allowed to use this value.
The crash you observed can be explained by the fact that when you executed the program, the contents of that memory location did not form a valid pointer. Perhaps the contents were zeroes, perhaps a number that did not happen to be a valid address for your program. You were lucky: the program might not have crashed.
Maybe you intended to write:
*(elements+1) = &one;
*elements = (int*)malloc(4*sizeof(int)); - here, I have allocated the first pointer, at the array of int pointers, to point at a block of size 4 allocated by malloc. *(*(elements+1)) = one - here, should'nt it simple assign the value of the first item at the array *(elements+0) to the number one?
elements+1 is an address, the address of the second element of the array allocated by the first call to malloc. You have never written to this address, but you read from it. And then, you dereference what you have read. This cannot go well. I am not sure what you are trying to do but it cannot involve reading uninitialized array elements and accessing to the address they represent.*(elements + 1) is an uninitialized value. *(*(elements+1))=…; uses an uninitialized value as address to write to.*((*elements)+1) = one; was what I ment to do.zero, producing an invalid address that happens not to crash your program when you write to it: taas.trust-in-soft.com/tsnippet/t/c12c3e8a
*(*(elements+1)) = one;what doeselements+1point to?*(elements) = &zero;<- here you overwrite the allocation of the previous line, since*elementsand*(elements)are the same. What did you intend to do? perhaps*(*(elements)) = zero?*(elements) = &zerosimply make the first item of the *(elements) array the value of zero?