Why is this linked-list initialization not working? - c

Question

I'm trying to initialize (or create) a linked-list with an empty node pointing to NULL, but it's returning an error and I don't know why. Can somebody help me?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct node {
int times;
char name[100];
char number[100];
struct node* next;
};
typedef struct node* node;
void mklist(node* n) {
(*n)->times = 0;
strcpy((*n)->name, "null");
strcpy((*n)->number, "null");
(*n)->next = (node)NULL;
}
int main(void) {
node n;
mklist(&n);
return 0;
}

Answer 1

So node is actually a pointer to a struct node, very confusing
typedef struct node* node;
In main() you declare a pointer and pass a pointer to a pointer to mklist()
node n;
mklist(&n);
In mklist(), n is actually a pointer to a pointer to the struct, so derefencing it, you get a pointer to a struct
void mklist(node* n){
(*n)->times=0;
but nowhere in your code have you allocated memory for an actual struct.
The most straightforward fix with the way your code is currently is to add a malloc()
void mklist(node* n) {
*n = malloc(sizeof(*(*n)));
// check for malloc() failure
(*n)->times = 0;
strcpy((*n)->name, "null");
strcpy((*n)->number, "null");
(*n)->next = (node)NULL;
}

Answer 2

You can do this easily in the global scope:
// nil = &nilObj, which means nil->next == nil.
//
// This way, there's no checking for both 'nil' and 'NULL'!
//
// As a consequence of this last point, you can make passing 'NULL'
// to most list functions invalid: it just means the user didn't
// use mklist() on every list they needed to manually work with.
static struct node nilObj = { 0, "null", "null", &nilObj };
node nil = &nilObj;
void mklist(node *n)
{
*n = nil;
}
As Stephen Docy mentioned, using typedef T *Tname; is generally a bad idea as it hides the fact that you're using a pointer, which can be confusing when you use Tname *n as (*n)->foo (I'd expect to use it as n->foo honestly). Some APIs do this, but they do it in a way that expresses that the variable is a pointer to an object rather than an object: instead of Node, something like NodeRef or NodePtr is used, signifying in the name that it's a pointer to a node, not a node. Apple's Core Foundation API uses names like this (e.g. CFStringRef). I highly suggest adopting a convention similar to this henceforth. The above code I posted might then look something like this:
static struct node nilObj = { 0, "null", "null", &nilObj };
nodeRef nil = &nilObj;
void mklist(nodeRef *n)
{
*n = nil;
}