C QueueLinkedList string variable not working - c

Question

#include<stdlib.h>
struct customer
{
int phoneNum;
char *name;
struct customer *nxt;
};
struct customer *initialNode(int i, char *n);
int isEmpty(struct customer *head, struct customer *tail);
void enqueue(int phNum1, char *cName1, struct customer *tail);
void dequeue(struct customer *head, struct customer *tail);
void display(struct customer *head, struct customer *tail);
int main()
{
int phNum;
char *cName;
int choice;
struct customer *head;
struct customer *tail;
head = initialNode(0, NULL);
tail = initialNode(0, NULL);
head->nxt = tail;
tail->nxt = head;
while (choice !=4)
{
printf("\nEnter your option:");
printf("\n1. Enqueue new customer. \n2. Dequeue customer. \n3. Display customer in queue. \n4. Exit. \nOption: ");
scanf("%d", &choice);
switch(choice)
{
case 1:
{
printf("\nEnter customer phone number: ");
scanf("%d", &phNum);
printf("\nEnter customer name: ");
scanf("%s", cName);
enqueue(phNum, cName, tail);
break;
}
case 2:
{
dequeue(head, tail);
break;
}
case 3:
{
display(head, tail);
break;
}
default:
{
if (choice < 1 || choice > 4)
printf("\nInvalid option!\n");
break;
}
}
}
}
struct customer *initialNode(int i, char *n)
{
struct customer *newCustomer = malloc(sizeof(struct customer));
newCustomer->phoneNum = i;
newCustomer->name = n;
newCustomer->nxt = NULL;
return newCustomer;
}
int isEmpty ( struct customer *head, struct customer *tail)
{
if (head->nxt == tail)
return 1;
else
return 0; //cannot is full because it is not array, have no size limitation
}
void enqueue(int phNum1, char *cName1, struct customer *tail)
{
struct customer *nC = malloc(sizeof(struct customer));
nC = initialNode(phNum1, cName1);
nC->nxt = tail;
tail->nxt->nxt = nC;
tail->nxt = nC;
printf("\nSuccessfully enqueue a customer!\n");
}
void dequeue(struct customer *head, struct customer *tail)
{
struct customer *removedCustomer = malloc(sizeof(struct customer));
if (isEmpty(head, tail) == 1)
{
printf("\nThe queue is empty!\n");
}
else
{
removedCustomer = head->nxt;
printf("\nFirst customer is removed.\n");
head->nxt = head->nxt->nxt;
}
}
void display(struct customer *head, struct customer *tail)
{
struct customer *tempH = malloc(sizeof(struct customer));
tempH = head;
tempH = tempH->nxt;
if (isEmpty(head, tail) == 1)
{
printf("\nThe queue is empty!\n");
}
else
{
printf("\n===Current customer list===");
while (tempH != tail)
{
printf("\n\tCustomer phone number : %d", tempH->phoneNum);
printf("\n\tCustomer name: %s\n", tempH->name);
tempH = tempH->nxt;
}
}
}
The code is suppose to make queue of a list of customer that user input their phone number and name, code is working well but when i input a new customer, the customer before's name all somehow become the latest customer's name..
How do i solve this? Do i declare a 2D string array to collect different set of customer's name?

Answer 1

I see the below issues with the code.
The head and tail should be initialized to NULL. This signifies an empty list.
In the initialnode function you are directly assigning the scanned pointer as newCustomer->name = n. This is wrong. As you have declared the struct as having a character pointer, you need to also allocate space for each string separately.
The enqueue and dequeue functions can change the value of the head and tail pointers in the main function. You can change the function declarations to pass a double pointer for this.
You also need to free the struct on dequeue. This is strictly not necessary in a small program but is a good practice anyway to avoid memory leaks in larger programs.
In the display function you are having an unnessary malloc which needs to be removed. This function does not need to allocate memory.
The variable choice in main is not initailized.
Please see the code below
#include<stdlib.h>
#include <stdio.h>
#include <string.h>
struct customer
{
int phoneNum;
char *name;
struct customer *nxt;
};
struct customer *initialNode(int i, char *n);
int isEmpty(struct customer *head, struct customer *tail);
void enqueue(int phNum1, char *cName1, struct customer **head, struct customer **tail);
void dequeue(struct customer **head, struct customer **tail);
void display(struct customer *head, struct customer *tail);
int main(void)
{
int phNum;
char cName[51]; // max name length supported
int choice = 0;
struct customer *head;
struct customer *tail;
head = NULL;
tail = NULL;
while (choice !=4)
{
printf("\nEnter your option:");
printf("\n1. Enqueue new customer. \n2. Dequeue customer. \n3. Display customer in queue. \n4. Exit. \nOption: ");
scanf("%d", &choice);
switch(choice)
{
case 1:
{
printf("\nEnter customer phone number: ");
scanf("%d", &phNum);
printf("\nEnter customer name: ");
scanf("%s", cName);
enqueue(phNum, cName, &head, &tail);
break;
}
case 2:
{
dequeue(&head, &tail);
break;
}
case 3:
{
display(head, tail);
break;
}
default:
{
if (choice < 1 || choice > 4)
printf("\nInvalid option!\n");
break;
}
}
}
return 0;
}
struct customer *initialNode(int i, char *n)
{
int len;
struct customer *newCustomer = malloc(sizeof(struct customer));
newCustomer->phoneNum = i;
len = strlen(n);
newCustomer->name = malloc(len +1);
strcpy(newCustomer->name, n);
newCustomer->nxt = NULL;
return newCustomer;
}
int isEmpty ( struct customer *head, struct customer *tail)
{
if ((head == NULL) && (tail == NULL))
return 1;
else
return 0; //cannot is full because it is not array, have no size limitation
}
void enqueue(int phNum1, char *cName1, struct customer **head, struct customer **tail)
{
struct customer *nC = malloc(sizeof(struct customer));
nC = initialNode(phNum1, cName1);
if (isEmpty(*head,*tail))
{
*head = nC;
*tail = nC;
}
else if (*head == *tail) // single element
{
(*head)->nxt = nC;
*tail = nC;
}
else
{
(*tail)->nxt = nC;
*tail = nC;
}
printf("\nSuccessfully enqueue a customer!\n");
}
void dequeue(struct customer **head, struct customer **tail)
{
struct customer *removedCustomer = malloc(sizeof(struct customer));
if (isEmpty(*head, *tail) == 1)
{
printf("\nThe queue is empty!\n");
}
else if (*head == *tail) // single element
{
removedCustomer = *head;
free(removedCustomer->name);
free(removedCustomer);
*head = NULL;
*tail = NULL;
}
else
{
removedCustomer = *head;
*head = (*head)->nxt;
free(removedCustomer->name);
free(removedCustomer);
printf("\nFirst customer is removed.\n");
}
}
void display(struct customer *head, struct customer *tail)
{
struct customer *tempH;
tempH = head;
if (isEmpty(head, tail) == 1)
{
printf("\nThe queue is empty!\n");
}
else
{
printf("\n===Current customer list===");
while (tempH != NULL)
{
printf("\n\tCustomer phone number : %d", tempH->phoneNum);
printf("\n\tCustomer name: %s\n", tempH->name);
tempH = tempH->nxt;
}
}
}