Getting infinite loop after entering 2 objects to the linked list in C

Question

I am trying to create a car renting project. I create a linked list in function createCarList(), where struct car is defined in f.h. When I enter one car, everything works fine. When I enter two cars, I always get infinite loops such as when printing the list with printAllCars() or if I try to check whether the car is already in the list.

Here is my code:

#include<stdio.h>
#include<stdlib.h>
#include<String.h>
#include<ctype.h>



/*car struct */

typedef struct car{
    unsigned int plate_number;
    unsigned int vin_number;
    char *Manufacturer_Name;
    char *model;
    char *colour;
    unsigned int year_Manufactured;
    unsigned int realseYear;
    unsigned int payment;
    unsigned int value;
    unsigned int engine_power;
    struct car *next;

}car;

car *cars;

/*function to scan and check the int input*/
int CheckIntinput(unsigned int *a)
{

    char temp[20];
    int i;
    int flag=1;
    while(1)
    {
    scanf("%s",temp);
    for(i=0;i<strlen(temp);i++)
    {
        if(!isdigit(temp[i]))
        {
            flag=0;
            printf("You did not enter a number , Please enter an argument's number\n");
            break;
        }
    }
    if(flag)
    {
        break;
    }
    flag=1;
  }
    
    *a=strtoul(temp,NULL,10);
    return 1;
}

/*function to check if the information of the car are correct*/
void checkifcarnputiscorrect(unsigned int *plate_number,unsigned int *vin_number,unsigned int *year_Manufactured,unsigned int *realseYear
,unsigned int *payment,unsigned int *value,unsigned int *engine_power){
    int flag=1;

     while(1)
     {
        printf("enter plate number :");
        CheckIntinput(plate_number);
        if(*plate_number>9999999 || *plate_number<1000000)
         {
            printf("plate number should be 7 digits try again \n");
            flag=0;
         }
         /*
         if(!checkifcaralreadyinlist(*plate_number))
            {
                printf("the car is already in the list \n");
                flag=0;
            }*/
         if(flag){break;}
         flag=1;
     }

     while(1)
     {
         printf("enter vin number :");
         CheckIntinput(vin_number);
     if(*vin_number>99999 || *vin_number<10000)
     {
         printf("vin_number should be 5 digits try again\n");
         flag=0;
     }
     if(flag){break;}
     flag=1;
     }

    while(1)
    {
        printf("enter year manfucatured :");
        CheckIntinput(year_Manufactured);
     if(*year_Manufactured>9999||*year_Manufactured<1000)
     {
         printf("yeaer_manfucatured should be 4 digits try again\n");
         flag=0;
     }
     if(flag){break;}
     flag=1;
    }

    while(1)
    {
        printf("enter realese year :");
        CheckIntinput(realseYear);
     if(*realseYear>9999||*realseYear<1000){
         printf("realse year  should be 4 digits try again \n");
        flag=0;
     }
     if(flag){break;}
     flag=1;
    }

    while(1){
        printf("enter payment :");
       CheckIntinput(payment);
     if(*payment>9999999||*payment<0){
         printf("payment should be between 0-9999999 digits\n");
         flag=0;
     }
     if(flag){break;}
     flag=1;
    }

    while(1){
        printf("enter value :");
        CheckIntinput(value);
     if(*value>9999999||*value<0){
         printf("value should be between 0-9999999 digits\n");
         flag=0;
     }
     if(flag){break;}
     flag=1;
    }

    while(1){
        printf("enter engine_power :");
       CheckIntinput(engine_power);
     if(*engine_power>9999||*engine_power<1000){
         printf("engine power be 4 digits\n");
         flag=0;
     }
     if(flag){break;}
     flag=1;
}
}

/*function to scan and check if the input is only chars*/
void Checkstringifonlychars(char *str){
    int i;
    int flag=1;
   while(1)
   {
       scanf("%s",str);
       for(i=0;i<strlen(str);i++)
       {
           if(!isalpha(str[i]))
           {
               flag=0;
               printf("wrong input try input only chars.");
               break;
           }
       }
       if(flag)
       {
           break;
       }
       flag=1;
   }
   
   
}



int CheckLonginput(unsigned long *a)
{

    char temp[20];
    int i;
    int flag=1;
    while(1)
    {
    scanf("%s",temp);
    for(i=0;i<strlen(temp);i++)
    {
        if(!isdigit(temp[i]))
        {
            flag=0;
            printf("You did not enter a number , Please enter an argument's number\n");
            break;
        }
    }
    if(flag)
    {
        break;
    }
    flag=1;
  }
    
    *a=strtoul(temp,NULL,10);
    return 1;
}

/*creating empty car list */


int createCarList()
{
    car *cars=(car*)malloc(sizeof(car));
    if(!cars)
    {
        return 0;
    }
    cars->next=NULL;
    return 1;
}

int addNewCar()
{
    unsigned int plate_number,vin_number,realseYear,year_Manufactured,payment,value,engine_power;
    char *Manufacturer_Name,*model,*colour,c;
    car * temp=NULL;
    car * p=NULL;
    temp=NULL;
    temp=(car*)malloc(sizeof(car));
    while(1){
        checkifcarnputiscorrect(&plate_number,&vin_number,&year_Manufactured,&realseYear,&payment,&value,&engine_power);

        Manufacturer_Name=(char*)malloc(5 * sizeof(char));
        printf("enter Manufacturer Name:");
        Checkstringifonlychars(Manufacturer_Name);
    
        model=(char*)malloc(5 * sizeof(char));
        printf("enter model:");
        Checkstringifonlychars(Manufacturer_Name);
    
        colour=(char*)malloc(5 * sizeof(char));
        printf("enter colour of the car:");
        Checkstringifonlychars(Manufacturer_Name);
    
        temp->plate_number=plate_number;
        temp->vin_number=vin_number;
        strcpy(temp->Manufacturer_Name,Manufacturer_Name);
        strcpy(temp->model,model);
        strcpy(temp->colour,colour);
        temp->year_Manufactured=year_Manufactured;
        temp->realseYear=realseYear;
        temp->payment=payment;
        temp->value=value;
        temp->engine_power=engine_power;
        temp->next=NULL;

        if(cars==NULL)
        {
            cars=temp;
        }
        else
        {
            p=cars;
            while(p->next!=NULL)
            {
                p=p->next;
            }
            p->next=temp;
        }  

        printf("\n done adding? if yes press y ");
        scanf(" %c",&c);
        if(c=='y')
        {
            break;
        }

   
    }
    if(cars==NULL)
    {
        return 0;
    }
    return 1;
}



void printAllCars(void){

    car *tmp=cars;

    if(!cars){ 
        printf("No Cars in list to Shown\n");
        return;
    }
    
    printf("\nCar List :\n\n");
    printf("LicenseNum | chassisNum | makerName | modelName | color | productionYear | onRoadYear |  price  |  paidMoney  | engine");
    printf("\n----------------------------------------------------------------------------------------------------------------------\n");
    while(tmp!=NULL)
    {
        printf("%-11u ",tmp->plate_number);
        printf("%-13u ",tmp->vin_number);
        printf("%-11s ",tmp->Manufacturer_Name);
        printf("%-11s ",tmp->model);
        printf("%-7s ",tmp->colour);
        printf("%-16u ",tmp->year_Manufactured);
        printf("%-11u ",tmp->realseYear);
        printf("%-9u ",tmp->value);
        printf("%-14u ",tmp->payment);
        printf("%u\n",tmp->engine_power);
        tmp=tmp->next;
    }

    printf("\n");
}

int main()
{
    createCarList();
    addNewCar();
    printAllCars();
    return 0;
}

input:

enter plate number :1234567
enter vin number :12345
enter year manfucatured :1994
enter realese year :1995
enter payment :30000
enter value :25000
enter engine_power :3000
enter Manufacturer Name:bmw
enter model:bmw
enter colour of the car:red

output:

plate number :1234567
vin number :12345
year manfucatured :1994
realese year :1995
enter payment :30000
enter value :25000
engine_power :3000
Manufacturer Name:bmw
model:bmw
colour of the car:red

when i added a secound car or more.

enter plate number :1234567
enter vin number :12345
enter year manfucatured :1994
enter realese year :1995
enter payment :30000
enter value :25000
enter engine_power :3000
enter Manufacturer Name:bmw
enter model:bmw
enter colour of the car:red

output: prints first car then infinite time secound car.

I checked the while loops for all the functions, but I couldn't find the error. What could be the problem?

Answer 1

I found 5 bugs in your code. After fixing these bugs, your program seems to work. I haven't tested all kinds of input, though.

---

First bug:

In the function createCarList, the line

car *cars=(car*)malloc(sizeof(car));

does not write to your global variable cars. Instead, it creates a new local variable with that name, and writes to that one instead. If you want to write to the global variable, you should write cars=(car*)malloc(sizeof(car)); instead. However, I don't see much point in that, as this will do nothing else than make cars point to a mostly uninitialized node. It would probably be best to remove the function createCarList completely.

---

Second bug:

In the function addNewCar, in the lines

model=(char*)malloc(5 * sizeof(char));
printf("enter model:");
Checkstringifonlychars(Manufacturer_Name);

you need to replace Manufacturer_Name with model. You make the same mistake later in your code with the variable colour:

colour=(char*)malloc(5 * sizeof(char));
printf("enter colour of the car:");
Checkstringifonlychars(Manufacturer_Name);

---

Third bug:

In the function addNewCar, in the line

strcpy(temp->Manufacturer_Name, Manufacturer_Name);

you write to the address of the uninitialized pointer temp->Manufacturer_Name, which causes undefined behavior. You must first allocate memory for the new string (for example with malloc) and make temp->Manufacturer_Name point to that memory. However, since you already did that for Manufacturer_Name, it would probably be sufficient not to use strcpy, but to instead simply copy the pointer, like this:

temp->Manufacturer_Name = Manufacturer_Name;

The following lines have the same problem as described above:

strcpy(temp->model,model);
strcpy(temp->colour,colour);

---

Fourth bug:

With the line

Manufacturer_Name = (char*)malloc(5 * sizeof(char));

you allocate space for 5 characters, including the terminating null character. Since you use the %s scanf format specifier, if the user enters "Honda", this will cause a buffer overflow, because storing that string requires 6 bytes including the terminating null character. Such a buffer overflow causes undefined behavior. Therefore, instead of reading input with the line

scanf("%s",str);

it would be better to use

scanf("%4s",str);

which limits the input to 4 characters, so that a buffer overflow cannot occur.

---

Fifth Bug:

In the function addNewCar, the line

temp = (car*)malloc(sizeof(car));

is only executed once, because it is outside the while loop. You must move it inside the while loop, so that it is executed once for every car added. Otherwise, your program will reuse the same memory address for every linked list node, so that every node will point to itself. This will later cause an infinite loop when printing out the nodes in the function printAllCars.

---

Additional concerns:

• The functions CheckIntinput, checkifcarnputiscorrect, Checkstringifonlychars and CheckLonginput have misleading names. The word "check" in this context implies that the functions merely perform input validation, but not that the functions actually read input.
• It is unsafe to use scanf without checking the return value. Also, for line-based input, I recommend that you always use fgets instead of scanf. See this page for further information: A beginners' guide away from scanf()
• You should always check the return value of malloc in order to verify that it is not NULL.
• Your casts of the result of malloc are not necessary. You may want to read the following question: Do I cast the result of malloc?