read / write structures to file - c - c

Question

I'm working on creating a student database in C. The final thing I need to be able to do read and write the database I create to a file. So I've already got an array full of pointers to student structures, and I need to write it to a file. Once I have it written, I need to be able to read it back into my array as well.
I'm really not sure how to do it though. This is my struct:
typedef struct student Student;
struct student
{
char name[300];
int age;
char course1[300];
char course2[300];
char remarks[300];
};
Student *Data[30];
And these are the functions I've written to work with file:
void writeDisk()
{
FILE *file = fopen("disk.dat", "ab");
fwrite(&Data, sizeof(Student), count, file);
fclose(file);
}
void loadDisk()
{
FILE *file = fopen("disk.dat", "rb");
if (file != NULL)
{
fread(&Data, sizeof(Student), count, file);
fclose(file);
}
}
void emptyDisk()
{
FILE *file = fopen("disk.dat", "rw");
fopen("disk.dat", "wb");
}
I can see that the size of my disk.dat file changes when I write to it, and it goes to zero when I call empty, but loading does not work at all. If the student array in my program is zero, it just stays at zero when I call load and try to display it, but if there is something in the array, I get a segmentation fault when I call load and then try to display it.
I may be doing this entirely wrong. I'm really not sure what I'm doing. I was thinking that I could just write the whole array to a file, but I'm not sure that's true. I was just wondering if someone could look at this and let me know where I'm going wrong.
EDIT
I've edited my code to look like this:
void writeDisk()
{
int i = 0;
FILE *file = fopen("disk.dat", "ab");
for(i; i <count; i++)
{
fwrite(Data[i], sizeof(Student), 1, file);
}
fclose(file);
}
void loadDisk()
{
char buffer[300];
int i = 0;
clearData();
FILE *file = fopen("disk.dat", "rb");
while(Data[i] != NULL)
{
Data[i] = malloc(sizeof(Student));
fread(Data[i], sizeof(Student), 1, file);
i++;
}
fclose(file);
}
This still doesn't work though. The write seems to work better,but I don't see it writing the age of the student over to the file. The clearData() function in the load file just clears anything that's in the Data array to begin with, so that we can have a clean slate to read the file into.

Answer 1

I believe instead of
Student *Data[30];
You want
Student Data[30];
Because the first one is an array of pointers, while the second one is an array of the struct you want.
When you write
fread(&Data, sizeof(Student), count, file);
It reads the data from the file right into the location of Data. It looks like you want to read and write the actual structs, so you have to put them directly into the array, as opposed to using pointers.

Answer 2

I think this is the culprit:
Student *Data[30];
That's an array of pointers to Student structures. There is no storage allocated for actual Students.
Remove the *, throughout the rest of the code you seem to properly use Data as if it was a plain array, so it should need no modification.
EDIT on an unrelated note, you can declare a structure and its alias on the same statement, like this:
typedef struct {
...
} Student;

Answer 3

If Data is indeed an array of pointers to your structures, then what you are saving is just the pointers and not your actual data. In fact, you should never save actual pointers as the next time you run, malloc may return different pointers for storing your data.
What you want to do, for saving, is to iterate over your array and write the actual structure data to the file, something like:
for (i = 0; i < numberOfStudents; i++) {
fwrite(Data[i], sizeof (Student), 1, file);
}
For restoring, you'll need to loop over the students, malloc storage and read in the structure, and then set the Data pointer, something like:
for (i = 0; i < numberOfStudents; i++) {
Student *p = malloc(sizeof (Student));
fread(p, sizeof (Student), 1, file);
Data[i] = p;
}
Also, in general, you should check the return values from fopen, fread, fwrite, and fclose to detect errors.