How to resolve segmentation error in reading/writing to binary file in C

Question

This is the struct definition that I am trying to write copies of to and read from binary file

   typedef struct carType Car;
struct carType {
   int vehicleID;
   char make[20];
   char model[20];
   int year;
   int mileage;
   double cost;
   Car *next;
};

This is my code for writing to a binary file(state of the Car)

   void writeBinFile(Car *headPointer)
{
     char fileName[20];
     //prompt user for name of textfile to print to 
    scanf("%s", fileName);
    FILE *ftp;
    Car *start =  headPointer->next;
    ftp = fopen(fileName, "wb");
    char separator = '0';
    while(start != NULL)
    {
       //write out 1 cell of data, cell contains 4 bytes
        fwrite(&start->year,sizeof(int), 1, ftp);
        fwrite(start->make,sizeof(char), strlen(start->make), ftp);
        fwrite(&separator, sizeof(char), 1, ftp); 
        fwrite(start->model, sizeof(char), strlen(start->make), ftp);
        fwrite(&separator, sizeof(char), 1, ftp);
        fwrite(&start->cost, sizeof(float), 1, ftp);
        fwrite(&start->mileage, sizeof(float),1,ftp);
        fwrite(&start->vehicleID, sizeof(int), 1, ftp);
        start = start->next;
    }
    fclose(ftp);
}

This is my code for reading from a binary file(to state of the car)

    void readFromBinFile(Car *headPointer)
{
      char fileName[20];
     //prompt user for name of textfile to print to 
    scanf("%s", fileName);
    FILE *ftp;
    Car *previous =  headPointer;
    ftp = fopen(fileName, "rb");
    Car *current;
    //go until the end of file is reached
    while(!feof(ftp))
    {
            current = (Car *)malloc(sizeof(Car));
            previous->next = current;
             //program receives 1 cell, that cell contains 4 bytes
             fread(&current->year, sizeof(int),1,ftp);
             printf("%d\n",current->year);
             char make[25];
             int count = 0;
             char oneAtATime= 'a';
             while(oneAtATime != '0') 
             { 
                    fread(&oneAtATime, sizeof(char),1,ftp);
                   if(oneAtATime!='0')
                   {
                      make[count] = oneAtATime;
                        count++;
                   }
            } 
            make[count] = 0;
           strcpy(current->make, make);
             char model[25];
              count = 0;
              oneAtATime= 'a';
             while(oneAtATime != '0') 
             { 
                    fread(&oneAtATime, sizeof(char),1,ftp);
                   if(oneAtATime!='0')
                   {
                      model[count] = oneAtATime;
                        count++;
                   }
            } 
            model[count] = 0;
           strcpy(current->model, model);
           fread(&current->cost, sizeof(float),1, ftp);
           fread(&current->mileage, sizeof(int),1,ftp);
           fread(&current->vehicleID, sizeof(int),1,ftp);
         previous = previous->next;
    } 
    fclose(ftp);
} 

Last time I got a segmentation error from not allocating memory to the new car Why am I getting a segmentation failure?. I made sure to do that this time. I checked this one Segmentation fault when reading a binary file into a structure and Segmentation fault while reading binary file in C but my fields were values , not pointers. Does anyone see a glaring issue? I can't test anything bc whenever i try to run this, i get that error. The problem seems to be the reading but i am not sure if some code in the writing is causing the reading to fail

Answer 1

'0' is not the null terminator. 0 or '\0' are (note the lack of quotes on the first and the escape char on 2nd). '0' is value 48, not zero.

These are valid options.

char separator = 0;

or

char separator = '\0';

You have an error:

fwrite(start->model, sizeof(char), strlen(start->make), ftp);   // 'make' size used to write 'model'

Secondly, you can simplify your code rather than writing the separator null as a separate step, just write out the full string, including null terminator.

fwrite(start->make, 1, strlen(start->make) + 1, ftp);

However, how do you intend to read the strings back in? What function call are you going to use to read a string in binary that maybe variable length, with a null terminator? Better would be to just write the padded buffer using sizeof instead of strlen.

fwrite(start->make, 1, sizeof(start->make), ftp);

However, even this is brittle, because sizeof() will silently return a different value if your struct members are changed from a fixed character array to a character string (pointer). You are safer using constants.

const int SMALL_STRING_LEN = 20;
const int MAKE_LEN = SMALL_STRING_LEN;
const int MODEL_LEN = SMALL_STRING_LEN;

char make[MAKE_LEN];
char make[MODEL_LEN];

fwrite(start->model, 1, MODEL_LEN, ftp);
fwrite(start->make, 1, MAKE_LEN, ftp);