I can't create doubly linked list properly in C - c

Question

My list head always points to tail. What's the problem?
My linked_list.h:
#ifndef LINKED_LIST
#define LINKED_LIST
struct node
{
char *data;
struct node *nextElement;
struct node *prevElement;
};
void createList(struct node **head, struct node **tail);
void fill_list (char *word, struct node **head, struct node **tail);
#endif
main.c:
#include <stdio.h>
#include <stdlib.h>
#include "linked_list.h"
#include <string.h>
int main()
{
FILE *dataFile;
char *word = (char *) calloc ( 255, sizeof(char) );
/* Create empty list */
struct node *head, *tail;
createList (&head, &tail);
/*------------------------*/
/* Data file open*/
dataFile = fopen("data.txt" ,"r");
if( dataFile == NULL )
{
perror("Error while opening the file.\n");
exit(EXIT_FAILURE);
}
/* Data reading */
while (( fscanf(dataFile, "%s", word) ) != EOF )
{
int i = 0;
int wordsCount = 0;
for (i = 0; i <= strlen(word); i++)
{
if ( (word[i] >= 'a') && (word[i] <= 'z') )
wordsCount = wordsCount + 1;
}
if ( wordsCount == strlen(word) )
{
fill_list ( word, &head, &tail );
}
}
fclose(dataFile);
return 0;
};
and linked_list.c:
#include <stdio.h>
#include <stdlib.h>
#include "linked_list.h"
void createList(struct node **head, struct node **tail)
{
*head = NULL;
*tail = NULL;
}
void fill_list ( char *word, struct node **head, struct node **tail )
{
struct node *elem, *temp;
if ( (*head) == NULL )
{
// printf("HEAD = NULL\n");
elem = (struct node *) malloc ( sizeof (struct node) );
elem -> data = word;
elem -> nextElement = NULL;
elem -> prevElement = NULL;
(*head) = elem;
*tail = elem;
// printf("%s\n", (*head) -> data );
}
else
{
// printf("HEAD != NULL\n");
elem = (struct node *) malloc ( sizeof (struct node) );
elem -> data = word;
elem -> nextElement = NULL;
elem -> prevElement = *tail;
*tail = elem;
// printf("%s\n", (*head) -> data );
}
}
My data file: qw erty b cc.
Firstly, head == NULL, so head -> data = 'qw' and it should be as head all the time, but it changes to erty, then b and cc after every loop step.
What I'm doing wrong?

Answer 1

The problem is that you use the same string for all input, and you use that for all nodes. This means all nodes will have its data member point to the same string. This string will of course only contain what you last read into it.
You may want to keep the string buffer in main as a normal array (instead of allocating it on the heap) and use e.g. strdup to duplicate the string for the nodes. Don't forget to free them later though.
A pointer is exactly what it sounds like, it's a variable that points to some other place in memory. You can have many pointers all pointing to the same memory.
In your case you pass the pointer word in the function main to all our calls to fill_list. This means that all nodes you create in fill_list will use the exactly same pointer, and they will all point to the exact same memory.
This means that all nodes in your list will have the data member seem to be the same value and it will always be the last string read into word in the main function.
If you use a function such as strdup it will duplicate the string. I.e. it will allocate completely new memory for the string and copy from the old area into the newly allocate area, and return a pointer to the newly allocated memory.

Answer 2

try this
void fill_list ( char *word, struct node **head, struct node **tail )
{
struct node *elem, *temp;
/* you need to create node first ! */
elem = (struct node *) malloc ( sizeof (struct node));
elem -> data = NULL ;
elem -> data = (char*) malloc( sizeof(char)*255));
if ( elem -> data == NULL )
{
perror("alloc data failed ");
}
if ( (*head) == NULL )
{
// printf("HEAD = NULL\n");
/* call memcpy() function need to #include <string.h> */
elem -> data = memcpy((void*)elem -> data,(void*)word,strlen(word));
elem -> nextElement = NULL;
elem -> prevElement = NULL;
(*head) = elem;
*tail = elem;
// printf("%s\n", (*head) -> data );
}
else
{
// printf("HEAD != NULL\n");
elem -> data = memcpy((void*)elem -> data,(void*)word,strlen(word));
elem -> nextElement = NULL;
elem -> prevElement = *tail;
*tail = elem;
// printf("%s\n", (*head) -> data );
}