Trouble with insertion of C string into linked list - c

Question

I am writing a program to read a file and then store the data into a linked List.
linkedList.h
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<stdbool.h>
struct linked_list
{
char *stock_name;
double stock_price;
struct linked_list *next;
};
typedef struct linked_list NODE;
NODE* insert(NODE *head, double stock_price, char *stock_name);
void printList(NODE *head);
linkedList.c
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<stdbool.h>
#include"linkedList.h"
void printList(NODE *head)
{
NODE *this = head;
while(this != NULL)
{
printf("stock name:%s , stock price:%lf\n", this->stock_name, this->stock_price);
this = this->next;
}
}
NODE* insert(NODE *head, double stock_price, char *stock_name)
{
NODE *newNode = malloc(sizeof(NODE));
if(head == NULL)
{
newNode->stock_price = stock_price;
newNode->stock_name = stock_name;
head = newNode;
}
else
{
newNode->stock_price = stock_price;
newNode->stock_name = stock_name;
newNode->next = head;
head = newNode;
}
return head;
}
main.c
#include <stdio.h>
#include <stdbool.h>
#include <string.h>
#include "linkedList.h"
NODE *head;
bool headNode = true;
void insertIntoLinkedList(char *stock_name, double stock_price);
int main ( int argc, char *argv[] )
{
head = malloc(sizeof(NODE));
double stock_price;
char stock_name[100];
int stock_name_counter = 0;
**..then I read the file..**
stock_name[stock_name_counter] = '\0'; //to end my C string
stock_name_counter = 0; //this is used for reading char
insertIntoLinkedList(stock_name, stock_price); //I double checked here,the name and price is correct
**......**
printList(head); //**Not the output I want**
fclose( file );
void insertIntoLinkedList(char *m_stock_name, double m_stock_price)
{
if(headNode == true)
{
head = insert(NULL, m_stock_price, m_stock_name);
headNode = false; //this is used to insert data to my linked list for the first time
}
else
{
head = insert(head, m_stock_price, m_stock_name);
}
}
Here is the problem: if the file contains:
YAHOO 120
GOOGLE 10
APPLE 199
my printList() gave me this:
APPLE 120
APPLE 10
APPLE 199
I have been trying to debug for hours and still cannot figure why the name is not stored in my linked list properly(but the price is store properly..)...any help will be appreciate :)

Answer 1

The address of stock_name in main is always constant through out the program and you store that address to newNode->stock_name and so you will always get the last stored string in stock_name.
Modification
NODE* insert(NODE *head, double stock_price, char *stock_name)
{
NODE *newNode = malloc(sizeof(NODE));
newNode->stock_name = malloc(strlen(stock_name)+1);
if(head == NULL)
{
newNode->stock_price = stock_price;
strcpy(newNode->stock_name, stock_name);
head = newNode;
}
else
{
newNode->stock_price = stock_price;
strcpy(newNode->stock_name, stock_name);
newNode->next = head;
head = newNode;
}
return head;
}
Don't forget to free the allocated memory.

Answer 2

You need to make a copy of the string stored in stock_name when you call insert. As is, all the nodes point to the same buffer, and every time you read a new line, you overwrite the buffer. In the end, this means that all of the nodes have the text APPLE because that's the last content that's read into the shared buffer. If you either copy the buffer's contents (instead of just the pointer to the buffer) or allocate a new buffer every time you read from the file, this will be fixed.

Answer 3

Your basic problem is that C does not have a "String" type. A string is simple an array of characters, and an array "decays" to a pointer when used as an argument to a function. Based on the way you're using it, your insert() function should do a second malloc() to allocate storage for the string and use strdup() or similar to store it.
Your other problem is that you've got a logic flaw in the insert() function. You aren't initializing newNode->next if head is NULL. You should be doing newNode->next = head either way. If it's NULL, good. Your list tail will then not be pointing at an indeterminate place.