Question

I’ve got the task of making a car rental program, which uses linked lists to control what cars are available for rent, are rented, or are in repair. The ‘car’ is a structure, and so is the rented list, available list, and repair list.

Heres my current issue. If the user wants to make a new car available, we must add it to our list of all possible cars, and we must add it to the list of available cars.

I have no problem adding it to the list of cars, but when i need to add it to the list of available cars, i get a segmentation fault.

I will now provide the code:

typedef struct vehicles
{
    char idNum[20];
    int miles;
    int rDate;
    struct vehicles *nextCar;

}car;

typedef struct list
{
    car * aCar;
    struct list *nextCar;
} carList;

The list of all cars is:

car * carHead, * carCur;

The list of all available cars is:

carList * availHead, * availCur;

Both are initialized to NULL.

I then create a new car, and put in the data the user has given me (mileage and ID number)

carCur = (car *)malloc(sizeof(car));
//set ID, Mileage
for(k=0;k<=19;k++)
{
     carCur->idNum[k] = idNum[k];
}
carCur->miles = miles;
carCur->nextCar = NULL;

This works perfectly fine. I call the function which actually adds it to the list, all is well.

Then i create a new carList structure to be added to the available cars list.

availCur = (carList *)malloc(sizeof(carList));
//set ID, Mileage
for(k=0;k<=19;k++)
{
    availCur->aCar->idNum[k] = idNum[k];
    printf("assigned\n");
}
availCur->aCar->miles = miles;
availCur->nextCar = NULL;

After some testing using printf statements,(which werent all included here for brevity), i found the seg fault occurs in this statement.

    availCur->aCar->idNum[k] = idNum[k];

I’m hoping someone can tell me why this assignment results in a segfault.
I’ve checked the idNum provided by the user is good, and it works for adding to the all-cars list, so I’m not sure what is wrong.

I appreciate the help!

Answer 1

You are right about the point of seg fault.
You are allocating memory correctly

availCur = (carList *)malloc(sizeof(carList));

at this point availCur->aCar is a dangling pointer. Next you are dereferencing that pointer here

availCur->aCar->idNum[k]
              ^^

and this causes the crash.

To fix this you need to allocate memory for a car object and make availCur->aCar point to it before you start stuffing it.