Getting segmentation fault exception when trying to write data inside a pointer

Question

I am getting an exception: Segmentation fault on the line 54, where I put in a brand name. C language. I am using VScode with C/C++ Compile Run extension.

    do
    {
        printf("Enter brand: ");
        scanf("%s", *tab[i].brand);

        if (!isalpha(*tab[i].brand))
        {
            printf("Brand must consist of letters!\n");
        }
    } while (!isalpha(*tab[i].brand));

When putting in an brand name it points the error to stdio.h file on line 291: " __retval = __mingw_vfscanf( stdin, __format, __local_argv );". I am trying to put data inside an pointer that is inside an struct. Although I am not sure I do it right, chatgpt gives me similar answers that do not resolve the issue. Can some point out what am I doing wrong, pointers are hard for me to understand.

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

struct Car
{
    double price;
    char *brand;
};


void createAndPrint(int n, struct Car *tab)
{
    for (int i = 0; i < n; i++)
    {
        tab[i].brand = malloc(sizeof(char) * 100);
        
        printf("\nENTER DETAILS FOR CAR NO %d\n", i + 1);
        
        do
        {
            printf("Enter price: ");
            scanf("%lf", &tab[i].price);
            
            if (tab[i].price < 0)
            {
                printf("Price cannot be < 0!\n");
            }
        } while (tab[i].price < 0);

         do
        {
            printf("Enter brand: ");
            scanf("%s", *tab[i].brand);

            if (!isalpha(*tab[i].brand))
            {
                printf("Brand must consist of letters!\n");
            }
        } while (!isalpha(*tab[i].brand)); 
        

        free(tab[i].brand);
    }
        for (int i = 0; i < n; i++)
        {
            printf("\nCAR INFORMATION NO %d\n", i + 1);
            printf("Price: %.2lf\n", tab[i].price);
            printf("Brand: %s\n", *tab[i].brand);
        }
}

int main()
{
    int n;
    printf("Enter the number of cars: ");
    scanf("%d", &n);

    struct Car *tab = malloc(n * sizeof(struct Car));

    createAndPrint(n, tab);

    free(tab);
}

Answer 1

This part of your code

 do
{
    printf("Enter brand: ");
    scanf("%s", *tab[i].brand);

    if (!isalpha(*tab[i].brand))
    {
        printf("Brand must consist of letters!\n");
    }
} while (!isalpha(*tab[i].brand)); 

    

free(tab[i].brand);

does not make sense.

If you want to enter a single character then instead of this call of scanf

scanf("%s", *tab[i].brand);

you need to write

scanf(" %c", *tab[i].brand);

If you want to enter a string then you need to write

scanf("%99s", tab[i].brand);

and in the last case this message

printf("Brand must consist of letters!\n");

also does not make sense because only the first character of the entered string is checked.

This call of free

free(tab[i].brand);

at once frees the allocated array.

As a result this for loop

for (int i = 0; i < n; i++)
{
    printf("\nCAR INFORMATION NO %d\n", i + 1);
    printf("Price: %.2lf\n", tab[i].price);
    printf("Brand: %s\n", *tab[i].brand);
}

invokes undefined behavior because it tries to access the already freed character array brand.