Structs in Linked List - c

Question

I'm still fairly new to C programming and I'm having some trouble.
I've written some code to read a file that has information on people's names, ages, weight, and heights. Each new line in the file represents a new person. This information is then stored in a struct, and this struct is then added to a linked list. Essentially I am trying to use this linked list as a queue, with each node being a struct.
This is what the file I am reading will look like: (the format is age, weight, height, name).
20,60,170,Joe
23,70,175,Mike
My issue is that for some reason, every node in the linked list is the exact same, which is the last line of the file text. So for example every time I create a struct and add it to the linked list, each node will have the name 'Mike', age '23', and so forth. I've played around with the code and I notice that there is no issue with the file reading, and the values assigned to the struct members are correct (I printed out the values to check). However when I view the top of the Linked List, then dequeue (remove top), then view the top again, the values are always the same. So I am not sure where my problem is.
Note: I wrote a generic linked list code, and there is no issue with the linked list itself as I have tested it extensively.
This is my code:
#include <stdio.h>
#include "macros.h"
#include <string.h>
typedef struct
{
int AGE;
int WEIGHT;
int HEIGHT;
char NAME[51];
}Person;
int main()
{
void* vdPtr;
Person* topVal;
Person person;
LinkedList* list = createLinkedList();
char line[100];
int i=0;
int numPeople=0;
FILE* fp;
int age, weight, height;
char name[51];
fp = fopen("text.txt","r");
while(fgets(line,100,fp) != NULL)/*reading file to see how many lines, and how many people*/
{
numPeople++;
}
rewind(fp);
while (i<numPeople)/*storing file data in the appropriate member for Person struct*/
{
fscanf(fp,"%d,%d,%d,%s",&age,&weight,&height,name);
person.AGE = age;
person.WEIGHT = weight;
person.HEIGHT = height;
strcpy(person.NAME,name);
insertLast(list,&person);
/*printing to see whether the values added to struct members are correct*/
printf("%d %d %d %s\n",person.AGE,person.WEIGHT,person.HEIGHT,person.NAME);
i++;
}
/*testing*/
printf("\nCount %d",getLinkedListCount(list));/*seeing how many nodes in the linkedList*/
/*using void pointer to get the top value, then typecasting it to Person pointer*/
vdPtr = viewTop(list);
topVal = (Person*)(vdPtr);
/*viewing the top person*/
printf("\ntop is %s %d",topVal->NAME,topVal->AGE);
/*deqeue*/
removeTop(list);
printf("\nCount %d",getLinkedListCount(list));
vdPtr = viewTop(list);
topVal = (Person*)(vdPtr);
printf("\ntop is %s %d",topVal->NAME,topVal->AGE);
}
Can anyone tell me where I went wrong and what to do to fix it?

Answer 1

You are continually adding the same pointer to the list. On every new line you overwrite the same variable. So in the end you have one big list of pointers all pointing to the same thing. You have to make a new person struct for every person like this:
#include <stdio.h>
#include "macros.h"
#include <string.h>
typedef struct
{
int AGE;
int WEIGHT;
int HEIGHT;
char NAME[51];
}Person;
int main()
{
void* vdPtr;
Person* topVal;
LinkedList* list = createLinkedList();
char line[100];
int i=0;
int numPeople=0;
FILE* fp;
int age, weight, height;
char name[51];
fp = fopen("text.txt","r");
while(fgets(line,100,fp) != NULL)/*reading file to see how many lines, and how many people*/
{
numPeople++;
}
rewind(fp);
while (i<numPeople)/*storing file data in the appropriate member for Person struct*/
{
Person *person = (Person *)malloc(sizeof(Person)); //make new person object
fscanf(fp,"%d,%d,%d,%s",&age,&weight,&height,name);
person->AGE = age;
person->WEIGHT = weight;
person->HEIGHT = height;
strcpy(person->NAME, name);
insertLast(list, person);
/*printing to see whether the values added to struct members are correct*/
printf("%d %d %d %s\n",person.AGE,person.WEIGHT,person.HEIGHT,person.NAME);
i++;
}
/*testing*/
printf("\nCount %d",getLinkedListCount(list));/*seeing how many nodes in the linkedList*/
/*using void pointer to get the top value, then typecasting it to Person pointer*/
vdPtr = viewTop(list);
topVal = (Person*)(vdPtr);
/*viewing the top person*/
printf("\ntop is %s %d",topVal->NAME,topVal->AGE);
/*deqeue*/
removeTop(list);
printf("\nCount %d",getLinkedListCount(list));
vdPtr = viewTop(list);
topVal = (Person*)(vdPtr);
printf("\ntop is %s %d",topVal->NAME,topVal->AGE);
}
I recommend reading up on how pointers, and memory management work. Also keep in mind that this code does not free the memory it allocates, which is something you still need to do.

Answer 2

Without seeing code for insertLast() it is difficult too answer but it may be that you would create a new struct for each person and then pass it to insertLast :
Person* newPerson;
while (i<numPeople)/*storing file data in the appropriate member for Person struct*/
{
newPerson = (Person *)malloc(sizeof(Person))
fscanf(fp,"%d,%d,%d,%s",&age,&weight,&height,name);
newPerson->AGE = age;
newPerson->WEIGHT = weight;
newPerson->HEIGHT = height;
strcpy(newPerson.NAME,name);
insertLast(list,newPerson);
...
The list seems to save only the address of the data, that why you need to create new data for each person.

Answer 3

A struct that is to be used in a linked list needs to be self referential. i.e. one of the members (typically the last one) is a pointer to the struct itself. This is what allows segments of data to be linked together . (other members can be thought of as the payload, or data.) For example, your struct:
typedef struct
{
int AGE;
int WEIGHT;
int HEIGHT;
char NAME[51];
}Person;
Can be refactored for example as:
struct Person
{
int AGE;
int WEIGHT;
int HEIGHT;
char NAME[51];
struct Person *node;
};
Look at this simple starter tutorial for creating and using linked lists. (Click the C tab for example code.)