Airline Reservation System using linked list - c

Question

Im doing airline reservation system in c programming, using linked list. It reserves a seat, views the reservation, cancels the reservation. The program runs properly for the first time without adding any data.
But when i add data and want to delete it, it gives this error "Exception thrown at 0x0FFAFBB3 (ucrtbased.dll) in ConsoleApplication1.exe: 0xC0000005: Access violation writing location 0x0004B000."
For deleting data, im comparing the phone number data. And when it compares, it deletes the record. But instead it is giving error. Anyone identify the error area and provide suggestion??
Below is the code for making reservation and deleting a reservation.
void new_reservation()
{
curr = start;
if (start == NULL)
{
//Empty list
start = curr = (struct passenger *)malloc(sizeof(struct passenger));
dataentry();
curr->next = NULL;
printf("\n\t Reservation successful");
return;
}
//Reach end of list
while (curr->next)
curr = curr->next;
curr->next = (struct passenger *)malloc(sizeof(struct passenger));
curr = curr->next;
dataentry();
curr->next = NULL;
printf("\n\t Reservation successful");
printf("\n\t Saved to Reservation list");
}
void del()
{
struct passenger *temp; // assigning a temporary pointer to struct airplane
char str[20];
printf("Enter phone number:");
gets(str);
fflush(stdin);
curr = start;
while (curr)
{
if (strcmp(start->Phone, str) == 0)
{
temp = start;
start = start->next;
free(temp);
return;
}
if (strcmp(curr->next->Phone, str) == 0)
{
temp = curr->next;
curr->next = curr->next->next;
free(temp);
break;
}
else if (strcmp(curr->next->Phone, str) != 0)
{
printf("\n\n No reservations found!!");
break;
}
}
printf("\n\n Deletion successful!!");
}

Answer 1

if (strcmp(start->Phone, str) == 0){...}
if (strcmp(curr->next->Phone, str) == 0){...}
else if (strcmp(curr->next->Phone, str) != 0){...}
This method is too complicated, it is bound to run in to errors unless you carefully check the pointers and that pointer's next member. You can simplify this and make only one comparison. Just make sure to keep track of the node's position, if the node being deleted is the start or it is another node. Example:
void del()
{
char str[20];
struct passenger *walk = start;
if (!walk)
return;
printf("Enter phone number:");
scanf("%s", str);
struct passenger *previous = 0;
while (walk)
{
if (strcmp(walk->Phone, str) == 0)
{
struct passenger *next = walk->next;
free(walk);
if (previous == 0)
{
//deleting the first item
start = next;
}
else
{
//deleting an item in middle or end
previous->next = next;
}
return;
}
previous = walk;
walk = walk->next;
}
}
new_reservation is probably not causing any errors, but it too can be simplified. You only need to allocate once, then insert it either at the end, or assign it as start. Also you can use scanf to read a string.
void new_reservation()
{
//create new item
curr = malloc(sizeof(struct passenger));
curr->next = NULL;
//I don't know this part of your code...
dataentry();
if (start == NULL)
{
//Empty list
start = curr;
}
else
{
//find the end of the list
struct passenger *walk = start;
while (walk->next)
walk = walk->next;
walk->next = curr;
}
}