linked list and pointers : ??????

Hi !

This is a program I wrote which compiles fine but gives me and error on running it .. I need some help on finding what I am doing wrong in this ????



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

Code:

/*  ********************************************************** */
/*  This program creates a linked  list */
/*  ********************************************************** */

typedef struct node* link;

struct node
{
       int item; 
       link next;
};
void makelist(link x, int y);
void printlist(link x);

int main()
{
    link x;
    makelist(x, 5);
    printlist(x);
    return 0;
}


void makelist(link x, int num)
{
     int i;
     if(0 == num)
     {
         x = NULL;
         return;
     }
     link a = malloc(sizeof *a);
     x = a;
     a->item = 1;
     if(1 == num)
     {
          a->next = NULL;
          return;
     }
     for(i = 2; i <= num; i++)
     {
         a->next = malloc(sizeof *a);
         a = a->next;
         a->item = i;
         if(i == num)
          {
               a->next = NULL;
               break;
          }
     }
}

void printlist(link x)
{
    link b = x;
    while(b != NULL)
    {
         printf("%d, ", b->item);
         b = b->next;   
    }
}

Probably a seg fault error right? Wow I must be a psychic. Have you tried running a debugger? Try and narrow down what function or even what line the error is occurring on.

Code:

link a = malloc(sizeof *a);
     x = a;

This won't work, if you wanted to change the "link x" in the main function you need to pass a pointer-to-pointer for your insert function. If you don't want to do that you can make the return type of the insertion function return the head node instead of being a void.

yeah I changed the function to " link makelist(link x, int num)" and it worked but I don't understand why do I need to do that ?

can you please tell why do I need to do that ??
can you give me a function declaration to the second option of a pointer to pointer ??

It's to do with local variables, whenever you call a function - the function will make "copies" of the parameters you send in on the stack. So the link x you modify will not be the same as the link x in the main function, which is why you need to send a pointer back to link x in the main to point to the right thing after the insertion function modified it.

A pointer-to-pointer prototype is like so:

Code:

void makelist(link* x, int num);

but link x is a pointer to a structure itself ???

Which is a pointer to "pointer to a structure". Hence pointer to a pointer.

good stuff, thanks a lot !!!