Thread: C dynamically linked list, printing & pointers...

Hi, I'm new to the forum and have been trying to learn about dynamically linked lists. I'm having a problem with my code, adding nodes to a list seems to work but for some reason I can't manage to print (or to free memory for that matter). My thought is that there's something wrong with my pointers, but I can't put a finger on what and where I am going wrong.

My guess is that there's a simple fix with pointers...

Here's what I have gotten so far...

Code:

#include <stdio.h>
#include <stdlib.h>
#define SIZE_MAX  20
#define BUFFER_MAX 256
FILE *file;




/*Stuct*/
struct car {
char name[SIZE_MAX];
int year;
struct car *next;
};


typedef struct car Car;


int addToList(Car *head, char *, int);
void printList(Car *head);
void releaseMem(Car *head);


/*functions*/
int addToList(Car *head, char *name, int year ){


struct car *nd;
nd = malloc(sizeof(struct Car));
strcpy(nd->name, name);
nd->year = year;
//printf("%s %d\n",nd->name,nd->year);
nd->next = NULL;


if (head!=NULL){
    head->next = nd;
}
return 1;
}
void printList(Car *head){


        Car * current;
        current =    head;;


     while (current!=NULL){
           printf("%s %d\n",current->name,current->year);
           current = current->next;
           }


     }


void releaseMem(Car *head){
    Car * current = head;


        while (current != NULL) {
                //printf("remove %s %d",current->name  ,current->year);
                free(current);
                current = current->next;
        }
printf("Mem free'd.\n");


}


/*Main*/


int main (int argc, char *argv[]){


Car *head = NULL;
int year = 100 ;
char buffer [BUFFER_MAX];
char name[SIZE_MAX];




/*opening file*/
file = fopen(argv[1], "r");
if (file == NULL){
  printf("\n\tWARNING: No data found.\n");
  exit(1);
}
else{
    printf("Reading file %s. \n",argv[1]);
}
printf("Data read to list.\n");
while(fgets(buffer, BUFFER_MAX,file)!= NULL){
  sscanf(buffer,"%s%d",name,&year);
    //printf("%s %d\n",string, year);
    addToList(head,name,year);


}


printList(head);
releaseMem(head);
return 0;
}

1. addToList needs to return a new head pointer, when you add to an empty list.

2. Your releaseMem function accesses out of bound memory
free(current);
current = current->next; // this is out of bounds, you just freed the memory
You normally do something like
temp = current->next;
free(current);
current = temp;

Like so
Car *addToList(Car *head, char *name, int year );

You pass in the current head, and it returns the new (possibly different) head.