Cannot create linked list in C using information in structure array

Question

I'm basically trying to create a linked list / queue out of the data in a struct array, but it refuses to work.

Here's the code, tried allocating memory to front but to no avail, the output is just the while loop infinitely looping.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct node
{
    char ID[12];
    long arrtime;
    double priority;
    struct node *next;
} request;

void main()
{
    request *front;
    request *rear;
    request *new = (request *)malloc(sizeof(request));
    request *arr = (request *)malloc(100 * sizeof(request));
    for (int k = 0; k < 4; k++)
    {
        printf("ID:: ");
        scanf("%s", arr[k].ID);
        printf("Arrival time:: ");
        scanf("%ld", &arr[k].arrtime);
        printf("Priority::");
        scanf("%lf", &arr[k].priority);
    }
    for (int k = 0; k < 4; k++)
    {  printf("\n %s %ld %lf\n", arr[k].ID, arr[k].arrtime, arr[k].priority);


    }
    strcpy(new->ID, arr[0].ID);
    new->arrtime = arr[0].arrtime;
    new->priority = arr[0].priority;

    front = new;
    rear = front;
    for (int i = 1; i < 4; i++)
    {
        strcpy(new->ID, arr[i].ID);
        new->arrtime = arr[i].arrtime;
        new->priority = arr[i].priority;
      
        rear->next = new;
        rear = new;
    }
    // print test!!
    request *walker = front;
    printf("\n............QUEUE CONTENT.........\n");
    while (walker != NULL)
    {
        printf("\n%s %ld %.1lf\n", walker->ID, walker->arrtime, walker->priority);
        walker = walker->next;
    }

    return;
}

Remy Lebeau · Accepted Answer · 2022-03-09 00:32:24Z

You are not initializing new->next = NULL; for each item in the linked list (malloc() does not zero out the allocated memory), and you are not allocating a separate request object for each item in the linked list.

Try this instead:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct node
{
    char ID[12];
    long arrtime;
    double priority;
    struct node *next;
} request;

int main()
{
    request *front = NULL, *rear = NULL;
    request *arr = (request *)malloc(4 * sizeof(request));

    for (int k = 0; k < 4; ++k)
    {
        printf("ID:: ");
        scanf("%s", arr[k].ID);
        printf("Arrival time:: ");
        scanf("%ld", &arr[k].arrtime);
        printf("Priority::");
        scanf("%lf", &arr[k].priority);
    }

    for (int k = 0; k < 4; ++k)
    {
        printf("\n %s %ld %lf\n", arr[k].ID, arr[k].arrtime, arr[k].priority);
    }

    request *new = (request*)malloc(sizeof(request));
    strcpy(new->ID, arr[0].ID);
    new->arrtime = arr[0].arrtime;
    new->priority = arr[0].priority;
    new->next = NULL;

    front = rear = new;

    for (int i = 1; i < 4; ++i)
    {
        new = (request*)malloc(sizeof(request));

        strcpy(new->ID, arr[i].ID);
        new->arrtime = arr[i].arrtime;
        new->priority = arr[i].priority;
        new->next = NULL;
      
        rear->next = new;
        rear = new;
    }

    // print test!!
    request *walker = front;
    printf("\n............QUEUE CONTENT.........\n");
    while (walker != NULL)
    {
        printf("\n%s %ld %.1lf\n", walker->ID, walker->arrtime, walker->priority);
        walker = walker->next;
    }

    walker = front;
    while (walker != NULL)
    {
        request *next = walker->next;
        free(walker);
        walker = next;
    }

    free(arr);

    return 0;
}

Alternatively, you don't need to allocate the 1st node outside of the loop, eg:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct node
{
    char ID[12];
    long arrtime;
    double priority;
    struct node *next;
} request;

int main()
{
    request *front = NULL, *rear = NULL;
    request *arr = (request *)malloc(4 * sizeof(request));

    for (int k = 0; k < 4; ++k)
    {
        printf("ID:: ");
        scanf("%s", arr[k].ID);
        printf("Arrival time:: ");
        scanf("%ld", &arr[k].arrtime);
        printf("Priority::");
        scanf("%lf", &arr[k].priority);
    }

    for (int k = 0; k < 4; ++k)
    {
        printf("\n %s %ld %lf\n", arr[k].ID, arr[k].arrtime, arr[k].priority);
    }

    request **ptr = &front;
    for (int i = 0; i < 4; ++i)
    {
        request *new = (request*)malloc(sizeof(request));

        strcpy(new->ID, arr[i].ID);
        new->arrtime = arr[i].arrtime;
        new->priority = arr[i].priority;
        new->next = NULL;
      
        *ptr = rear = new;
        ptr = &(rear->next);
    }

    // print test!!
    request *walker = front;
    printf("\n............QUEUE CONTENT.........\n");
    while (walker != NULL)
    {
        printf("\n%s %ld %.1lf\n", walker->ID, walker->arrtime, walker->priority);
        walker = walker->next;
    }

    walker = front;
    while (walker != NULL)
    {
        request *next = walker->next;
        free(walker);
        walker = next;
    }

    free(arr);

    return 0;
}

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