Question

I need to write a LinkedList in C,
I defined the construct in the file as struct element { int value; struct element * next; };
I also defined a head element.
Both of these are not local, they are variables that persist throughout runtime.
When I try to insert elements into the LinkedList using int-values from outside, I need to wrap an element around this int. I do this by creating a local variable struct element e = { value; 0 };. I make it the head if head is null, or else I append it using a for-loop.

The problem I figured is that the local variable e of type struct element is deleted upon terminating that function. So if I have my head point to e, it will continue pointing to an unallocated point of memory since the local variable e does not persist beyond the function call.

Thanks in advance!

Answer 1

That is because struct element e = { value; 0 }; allocates the element on the stack. Stacks are automatically deallocated (deleted) when the function scope terminates. Addressing stack memory of an active scope is perfectly valid, but that of a scope that has terminated is undefined behavior.

You need to allocate it on the heap via malloc(sizeof(struct element)); to make it persist through function scopes.

struct element *e = calloc(1, sizeof(struct element));
e->value = value;

return e;

Note: calloc allocates zeroed out memory, malloc allocates memory with undefined content.