Is the address of a var in c passed by value?

Question

this how the variable is passed from the main func:

Node merged = NULL; 
ErrorCode result = mergeSortedLists(left, right, &merged);

and this is the signatue of the func.

ErrorCode mergeSortedLists(Node list1, Node list2, Node *merged_out); 

If an error occurs in func. mergedsortedlists we should return res=Error and the merged list should be NULL

the tutor said we should assign merged_out=NULL; but isn't the address passed by value to the func.? shouldn't we assign the var it points to, to NULL i.e. *merged_out=NULL;

Thanks in advance!

Answer 1

Node merged = NULL; merged is pointer to user defined struct. (its done by typedef operator) for example:

struct S_Node{
    int m_var;
    struct S_Node* p_m_next;
};
typedef struct S_Node* Node;  /*now Node is new type. its pointer to S_Node*/

so merged_out is pointer to pointer. so in case of some error in function, we set it to NULL in this code snippet:

ErrorCode mergeSortedLists(Node list1, Node list2, Node *merged_out)
{
    if(!list1 || !list2) /*lets assume that this is the error case*/
    {
        *merged_out=NULL;
        return Error;
    }
}