C中的链接列表：我在删除源时遇到问题，并且存在空错误

我编写的程序几乎没有问题。 其中之一是，当我输入信息时可以，但是当我想显示数据时，char字段有错误，源字段和目标字段显示为“空”。它应该类似于1234：从x到y。但是相反，它就像1234：从null到null。如何解决？我也有删除某些元素的问题。当我在列表中添加2条日志并尝试删除它们时，它告诉我没有这样的数字。如何解决这两个问题？

#include<stdio.h>
#include<stdlib.h>
#include <malloc.h>
#pragma warning(disable:4996)

struct flight {
    int value;
    char source;
    char destination;
    struct flight* next;
};

void insert();
void display();
void delete();
int count();

typedef struct flight DATA_NODE;

DATA_NODE* head_node, * first_node, * temp_node = 0, * prev_node, next_node;
int data;
char data2, data3;
struct flight f[];

int main() 
{
    int decision = 0;

    printf("Welcome to flight log editor.\n");

    while (decision < 5)
    {
        printf("\nOptions\n");
        printf("1: Add a flight \n");
        printf("2: Delete a flight\n");
        printf("3: Sort flights\n");
        printf("4: Display flights\n");
        printf("5: Exit\n");
        printf("Enter your option: ");
        scanf("%d", &decision);
        switch (decision) {
        case 1:
            insert();
            break;
        case 2:
            delete();
            break;
        case 3:
            display();
            break;
        case 4:
            display();
            break;
        case 5:
            return 0;
        }
    }
    return 0;
}

void insert()
{
    printf("\nEnter flight number: \n");
    scanf("%d", &data);
    printf("\nEnter flight source: \n");
    scanf("%s", &data2);
    printf("\nEnter flight destination: \n");
    scanf("%s", &data3);

    temp_node = (DATA_NODE*)malloc(sizeof(DATA_NODE));

    temp_node->value = data;
    temp_node->source = data2;
    temp_node->destination = data3;

    if (first_node == 0)
    {
        first_node = temp_node;
    }
    else
    {
        head_node->next = temp_node;
    }
    temp_node->next = 0;
    head_node = temp_node;
    fflush(stdin);
}

void delete() 
{
    int countvalue, pos, i = 0;
    countvalue = count();
    temp_node = first_node;
    printf("\nEnter flight number to delete: \n");
    scanf("%d", &pos);

    if (pos > 0 && pos <= countvalue) {
        if (pos == 1) {
            temp_node = temp_node->next;
            first_node = temp_node;
            printf("\nFlight log deleted.\n");
        }
        else {
            while (temp_node != 0) {
                if (i == (pos - 1)) {
                    prev_node->next = temp_node->next;
                    if (i == (countvalue - 1))
                    {
                        head_node = prev_node;
                    }
                    printf("\nFlight deleted successfully.\n");
                    break;
                }
                else {
                    i++;
                    prev_node = temp_node;
                    temp_node = temp_node->next;
                }
            }
        }
    }
    else
        printf("\nInvalid flight number.\n");
}

void display() 
{
    int count = 0;
    temp_node = first_node;
    printf("\nDisplaying flight list: \n");
    while (temp_node != 0) 
    {
        printf(" %d:", temp_node->value);
        printf(" from %s ", temp_node->source[temp_node]);
        printf(" to %s ", temp_node->destination[temp_node]);

        count++;
        temp_node = temp_node->next;
    }
}

int count() 
{
    int count = 0;
    temp_node = first_node;
    while (temp_node != 0) {
        count++;
        temp_node = temp_node->next;
    }
    return count;
}