linked list - unable to modify global variable in c -

i'm learning c, , in program i'm trying implement simple linked list. each node of list contains integer, , pointer next node. pointer head points first node in list, list empty, initialized head = null.

i want 2 operations on list - populate it, , print it.

to populate list, i'm calling function insert_node 2 arguments: head, , integer want insert.

the problem need function insert_node change value of head (so points updated list, instead of null). i'm not sure how that, made head global variable, , i'm trying change value. reason, though value of head changed inside function insert_node, when call function again, head still has value of null.

questions:

1. why global variable value not changed globally?

2. i'm aware using global variables not practice, how can update pointer list? thinking having insert_node function return pointer list, way?

#include<stdio.h> #include<stdlib.h>   struct node {    int data;   struct node *link; };  void insert_node(struct node *head, int n);  void print_list(struct node *head);  struct node *head = null;  main() {    int i;    for(i=1; i<5; i++)         insert_node(head, i*i);    print_list(head);  }  void print_list(struct node *head) {    if(head == null)         return;    else {        printf("%i ", head->data);         print_list(head->link);   }    return; }   void insert_node(struct node *head, int n) {    struct node n = {n, null};   struct node *next, *prev;   int prev_data = 0;    //case one: list empty - point head n, , set n.link null    if(head == null)         head = &n;    //case two: n less first element in list:    else if(n < head->data) {        n.link = head;       head = &n;   }     else {        next = head;    //case three: n.data equal existing element, nothing:        while(next != null) {            if(n == next->data) {                printf("this element exists.\n\n");               return;            }           prev = next;          //save current element           next = next->link;    //look @ next element       }    //case four: n.data greater last element:        if(n > prev->data) {            prev->link = &n;           return;       }    //case five: n.data in between list elements:        next = head;        while(next != null) {            prev_data = next->data;   //save current element           prev = next;              //save pointer current element           next = next->link;        //look @ next element            if((n > prev_data) && (n < next->data)) {                prev->link = &n;               n.link = next;               return;           }       }   }    return;  } 

1. because pass global head by value function insert_node().
   function insert_node() makes local variable (which, way, has name head might confuse you, because it's local , not global). modifies local head , changes not visible in global variable head. it's called shadowing (variable same name within local scope distinct other variable same name).
2. pass address of head function , make function parameter pointer pointer structure.

declaration

void insert_node(struct node **ptr_to_head, int n); 

usage

insert_node(&head, 5); 

now can modify head dereferencing ptr_to_head in insert_node function:

(*ptr_to_head)=&new_node;      ^            ^      |            |    head       =  value returned malloc  

and yes, can return head insert_node function, don't forget make assignment head in main function.