segmentation 11 in C - c

Question

I'm writing a homework program in C. The program should take records from an input file and write those record to an output file. It seems like there is something wrong with the print_to_file function. I keep getting segmentation fault 11. Please help. My code is as below.
#include <stdio.h>
#include <stdlib.h>
typedef struct car { // create a struct type Car
char *license_plate;
int parking_spot;
int num_tickets;
int time_left;
} Car;
#define LICENSEPLATELENGTH 10
Car* import_cars(char *filename, int numCars);
void print_to_file(char* filename, Car* garage, int numCars);
int main(int argc, char * argv[]) {
if(argc != 4)
printf("Incorrect input.\n");
else {
int number = atoi(argv[1]);
Car* parked_car = (Car*)malloc(sizeof(Car) * number);
parked_car = import_cars(argv[2], number);
print_to_file(argv[3], parked_car, number);
free(parked_car);
}
return 0;
}
Car* import_cars(char* filename, int numCars)
{
Car* inCar = (Car*)malloc(sizeof(Car) * numCars);
inCar->license_plate = (char*)malloc(sizeof(char) * 8);
//Question: How do I do if I the plate length is varied. How to malloc space to it?
FILE* inFilePtr;
if((inFilePtr = fopen(filename, "r")) == NULL)
printf("Error! Unable to open file %s. Check again.\n", *filename);
else
{
int i = 0;
fscanf(inFilePtr, "%s", inCar[i].license_plate);
fscanf(inFilePtr, "%d%d%d", inCar[i].parking_spot, inCar[i].num_tickets, inCar[i].time_left);
printf("%s %d %d %d \n", inCar[i].license_plate, inCar[i].parking_spot, inCar[i].num_tickets, inCar[i].time_left);
for(i = 1; i < numCars; i++)
{
fscanf(inFilePtr, "%s", inCar[i].license_plate);
fscanf(inFilePtr, "%d%d%d", inCar[i].parking_spot, inCar[i].num_tickets, inCar[i].time_left);
printf("%s %d %d %d \n", inCar[i].license_plate, inCar[i].parking_spot, inCar[i].num_tickets, inCar[i].time_left);
}
}
fclose(inFilePtr);
return(inCar);
//free(inCar.license_plate); `
//Question: Do I need to free space here would it remove the value
//stored in the variable which passed to main?
}
void print_to_file(char* filename, Car* garage, int numCars) {
FILE* outFilePtr;
if((outFilePtr = fopen(filename, "w+")) == NULL){
printf("Error! Cannot Open File %s!", *filename);
printf("here\n");
} else {
int i = 0;
for(i = 0; i < numCars; i++) {
printf("%s\n%d %d %d\n", garage[i].license_plate, garage[i].parking_spot, garage[i].num_tickets, garage[i].time_left);
fprintf(outFilePtr, "%s\n%d %d %d\n", garage[i].license_plate, garage[i].parking_spot, garage[i].num_tickets, garage[i].time_left);
}
}
fclose(outFilePtr);
}
This is my input command.
./a.out 6 garage.txt output.txt
Here is what print in my terminal.
fi590dz 20 2 25
57fjgmc 8 0 55
7dkgjgu 25 1 15
f9e829d 1 2 60
4jgfd81 12 2 10
Segmentation fault: 11
By the way, I'm pretty new in programming and really bad with debugging. Could you give me some tips of how to debug or any debugging tools? I use a mac so gdb doesn't work.

Answer 1

Not a complete answer, because it’s a homework problem and you want to figure it out yourself, but here are some hints.
First, you really want to learn how to run your program in a debugger and get it to tell you which line crashed the program, and on which data.
Second, make sure you initialize the pointers for every element of the array before you try to read or write them.
Third, you’ll save yourself a lot of trouble if you initialize all your dynamic and local variables to zeroes, not garbage. It will make a lot of bugs reproducible, make a lot of bugs crash immediately instead of corrupting memory, and also make it obvious when you debug that you’re using uninitialized data.
Therefore, I suggest you get in the habit of allocating your dynamic arrays with calloc(), not malloc().

Answer 2

The problem lies within your parked_car = import_cars(argv[2], number); and Car* import_cars(char* filename, int numCars);functions.
Indeed in Car* import_cars(char* filename, int numCars); you are doing this:
Car inCar;
inCar.license_plate = (char*)malloc(sizeof(char) * 8);
So you are creating a local variable that is not accessible outside of the function (many different things can happen to the memory after the end of the function).
So when you do: parked_car = import_cars(argv[2], number); you are assigning to parked_car a freed variable.
A solution is to simply use the parked_caras an argument of your import_cars() function. All modifications made within the function will still be valid after it returns. So you should have:
void import_cars(char* filename, int numCars, Car* car);

Answer 3

For everyone who met the issue here, I found the problem in my program. The problem is that I didn't allocate space for each of the license_plate pointer in the structure. So my way to solve it is add a line as below in the for loop of the import_cars function.
inCar[i].license_plate = (char*)malloc(sizeof(char) * LICENSEPLATELENGTH);