Modifying a Linked List in C - c

Question

I want to change the value in a linked list. The list is defined as
struct car_elements {
char *car_rego;
double time_parked;
struct car_elements *next;
};
typedef struct car_elements car;
I have created a linked list that has 10 elements. I can change the time_parked easily but having trouble updating char *car_rego. To create the string
char *rego_array = malloc(7*sizeof(char));
CreateCarRego(rego_array);
The definition for CreateCarRego is
void *CreateCarRego(char *rego_array)
{
int temp = 0;
for (int w = 0; w < 6; w++)
{
if (w < 3)
{
temp = GenerateRandomNumber(26.0);
temp = temp + ASC_TO_LETTER;
rego_array[w] = (char)temp;
}
else
{
temp = GenerateRandomNumber(10.0);
temp = temp + ASC_TO_NUMBER;
rego_array[w] = (char)temp;
}
rego_array[7] = '\0';
}
}
Then to change the value
car *current_carpark = head; //head holds the linked list
for (int i = 1; i < carspace_number; i++) { //I know which element I want to change
current_carpark = current_carpark->next;
}
current_carpark->car_rego = (char*)calloc(strlen(rego_array), sizeof(char));
strncpy(current_carpark->car_rego, rego_array, strlen(rego_array));
This works but now I have a memory leak - I cannot free current_carpark. With the struct definition I have what is the best way to create a string (which is just 3 random letters and 3 random number) and update car_rego. I cannot just use
current_carpark->car_rego = rego_array;
As nothing is stored. I am new to C and yes this is an assignment however I have tried and tried to get this to work with no success

Answer 1

Code has out of bounds char * issues.
In CreateCarRego(), change
rego_array[7] = '\0';
to
rego_array[6] = '\0';
It is good to put the NUL terminator on, but it is done in the wrong place. rego_array is 7 bytes long and the 7th byte is accessed via rego_array[6].
Change
current_carpark->car_rego = (char*)calloc(strlen(rego_array), sizeof(char));
strncpy(current_carpark->car_rego, rego_array, strlen(rego_array));
to
current_carpark->car_rego = malloc(strlen(rego_array) + 1);
strcpy(current_carpark->car_rego, rego_array);
Not enough memory is allocated using only the length of the string, one needs the length of the string + 1. The following strncpy() limit is too tight, it should also be strlen(rego_array) + 1. As it is known that the destniation is long enough, strcpy() could be used.
or
Simplify the string duplication with
current_carpark->car_rego = strdup(rego_array);
A number of improvements could also be mentioned, but the above should get rid of memeory problems.