How to make library for linked-list functions - c

Question

I want to create static or dynamic library for linked list functions, but when I try to compile function files with gcc to get object file it gives me error ‘struct node’ declared inside parameter list
int GetNth(struct node* head, int index) & dereferencing pointer to incomplete type ‘struct node’ if(i == index) return head->data;. Not sure what's going on, may be because I haven't declared the structure in the file
#include <stdlib.h>
int count(struct node* head, int n) //counts the occurrence of n in the list
{
int c = 0;
while(head != NULL){
if(head->data == n) c+=1;
head = head->next;
}
return c;
}
But if I'll declare it inside this file, I think it'll violate 1 definition rule. What to do?
structure is declared in the main function as
struct node{
int data;
struct node *next;
}

Answer 1

Prior to using objects (you use it, when have access to its members) you must completely define them. So, to use struct node, declare it before its first appearance in the source code. It's the rule of C programming language.
Put the declarations into header file and include the file into source files if you want to create useful libraries, e.g.:
...
struct node
{
...
};
int count(struct node *head, int n);
...

Answer 2

Continuing from the comment, in your code, the compiler does not know what node is because you have not provided its declaration. You can provide a declaration including a typedef for convenience. The following code declares a typedef to struct _node {...} as node; and then declares two nodes (statically for simplicity reasons), iterates over the list, outputting the data values and then ends, e.g.
#include <stdio.h>
typedef struct _node {
int data;
struct _node *next;
} node;
int main (void) {
node n1 = {.data = 1},
n2 = {.data = 2, .next = NULL};
n1.next = &n2;
/* iterate over list outputting data values */
for (node *n = &n1; n != NULL; n = n->next)
printf ("%d\n", n->data);
return 0;
}
Example Use/Output
$ ./bin/llstatic
1
2
As mentioned, you can create a header file for your list and include your declaration of struct _node and the typedef there, or if you simply use node within this source-file, you can declare it here. Let me know if you have further questions.

Answer 3

The compiler tells you it does not recognize struct node. You need to Declare it first. Something like struct node; (decleration) or struct node{...}; (definition) and the error should go away. If you have already declared it in another file header.h, just add #include "header.h" in the start of your code
Since you want struct node to contain a field called data, you need to add it when you define the struct:
struct node{
int data;
}