pointer to a pointer in a linked list

Question

I'm trying to set a linked list head through pointer to a pointer. I can see inside the function that the address of the head pointer is changing but as i return to the main progran it becomes NULL again. can someone tell me what I'm doing wrong ??

           #include <stdio.h>
           #include <stdlib.h>

           typedef void(*fun_t)(int);
           typedef struct timer_t {
           int   time;
           fun_t func;
           struct timer_t *next;
           }TIMER_T;

          void add_timer(int sec, fun_t func, TIMER_T *head);

             void run_timers(TIMER_T **head);

           void timer_func(int);

           int main(void)
            {
            TIMER_T *head = NULL;
            int time = 1;

            fun_t func = timer_func;

            while (time < 1000) {
              printf("\nCalling add_timer(time=%d, func=0x%x, head=0x%x)\n", time,     

              func, &head);
               add_timer(time, func, head);
               time *= 2;
              }  
              run_timers(&head);

              return 0;
             }

            void add_timer(int sec, fun_t func, TIMER_T *head)
            {
           TIMER_T ** ppScan=&head;
               TIMER_T *new_timer = NULL;
           new_timer = (TIMER_T*)malloc(sizeof(TIMER_T));
               new_timer->time = sec;
               new_timer->func = func;
               new_timer->next = NULL;

               while((*ppScan != NULL) && (((**ppScan).time)<sec))
               ppScan = &(*ppScan)->next;

               new_timer->next = *ppScan;
               *ppScan = new_timer;
               } 

Answer 1

You got it the wrong way round. The function needs to take a double pointer, and the caller needs to take the address-of:

{   // caller
    TIMER_T *head = NULL;
    do_something(&head);
}

void do_something(TIMER_T ** p)  // callee
{
    *p = malloc(sizeof(TIMER_T*));
    // etc.
}

There have been many , many similar answers like this before.

Answer 2

Since C function arguments are passed by their value and not by their address and you don't pass the address of any variable in your call:

add_timer(time, func, head);

so none of them will be changed outside of add_time function scope.

What you probably need to do is pass the address of head :

add_timer(time, func, &head);

and:

void add_timer(int sec, fun_t func, TIMER_T **head)
{
    TIMER_T ** ppScan = head;
    // ...
}