Question

So i have my definition of the list as a global variable:

typedef struct center {

  char center_name[100];

  char hostname[100];

  int port;

  struct center *next_center;

} center;

I need to add elements to the list. But these elements that i need to add are on a file, so:

 int main(int argc, char** argv) {
     center *head = NULL; 
     parse(argv, head);
  }

parse is a function, which read the file and add those read elements to a new center (all of this works, it double checked)

void parser (char** argv, center *head) {
   //read the elements i need to add
   //creates a newCenter and adds the elements read to the new center
   //this far it works
  addToCenter(newCenter, head); 
}

where:

addToCenter(center *newCenter, center *head){
   //adds newCenter to the list
   if (head == null)
      head = newCenter;
   else {
      //find last element
      lastelement.next_center = newCenter; 
   }

}

Everything works, except that the list on Main always return as null. In other words, the reference is not being modified. I dont understand why, because i am passing a pointer to the list.

another solution i though is to create the head variable of the list as a global variable, but its better to avoid those situations.

Thanks in advance.

Answer 1

Your list head is being passed by value. You need to pass the head pointer by address in case it is modified (which it will be).

Example:

addToCenter(center *newCenter, center *head) // <=== note: passed by value
{
   //adds newCenter to the list
   if (head == null)
      head = newCenter; // <=== note: modified local stack parameter only.
   else {
      //find last element
      lastelement.next_center = newCenter; 
   }
}

should be:

addToCenter(center *newCenter, center **head) // <=== note: now passed by address
{
   //adds newCenter to the list
   if (*head == null)
      *head = newCenter;  // <=== note: now modifies the source pointer.
   else {
      //find last element
      lastelement.next_center = newCenter; 
   }
}

Likewise with parse:

void parser (char** argv, center **head) // <=== again, the head-pointer's *address*
{
   //read the elements i need to add
   //creates a newCenter and adds the elements read to the new center
   //this far it works
  addToCenter(newCenter, head);  // <=== just forward it on.
}

And finally back in main:

int main(int argc, char** argv) 
{
     center *head = NULL; 
     parse(argv, &head);  // <=== note: passing address of the head-pointer. (thus a dbl-pointer).
}