'fread'ing a dynamically allocated struct - c

Question

So I am currently working on Learn C the Hard Way and stuck on the extra credit in exercise 17.
What's needed is to convert the fixed sized database from http://c.learncodethehardway.org/book/ex17.html into a dynamically sized one where you take the number of rows and max string length allowed from the user.
I managed to get the program to create the database and write it to a file, however I cannot figure out how to read the program from the file again.
The program is compiling, but it's segfaulting whenever I try to do anything other than creating a database. I've checked with valgrind and the immediate reason appears to be reading from nonallocated memory. I believe the problem is in the Database_load function, located below.
Note that this is my first time posting a question, apologies if I am posting too much.
Loading it up:
struct Connection *Database_open(const char *filename, char mode)
{
struct Connection *conn = malloc(sizeof(struct Connection));
if(!conn) die("Memory error", conn);
conn->db = malloc(sizeof(struct Database));
if(!conn->db) die("Memory error", conn);
// If we're creating, write a new file otherwise load it up.
if(mode == 'c') {
conn->file = fopen(filename, "w");
} else {
conn->file = fopen(filename, "r+");
if(conn->file) {
Database_load(conn);
}
}
if(!conn->file) die("Failed to open the file", conn);
return conn;
}
void Database_load(struct Connection *conn)
{
int rc = fread(conn->db, sizeof(struct Database), 1, conn->file);
if(rc != 1) die("Failed to load database.", conn);
const int MAX_ROWS = conn->db->MAX_ROWS;
const int MAX_DATA = conn->db->MAX_DATA;
int i = 0;
// I want to allocate memory for the rows and the strings here...
// Clearly the freads are failing, how would I actually allocate properly?
for(i = 0; i < MAX_ROWS; i++) {
rc = fread(conn->db->rows, sizeof(struct Address),
1 , conn->file);
if(rc != 1) die("Failed to load rows.", conn);
rc = fread(&conn->db->rows[i], sizeof(char),
MAX_DATA, conn->file);
if(rc != MAX_DATA) die("Failed to load characters.", conn);
}
Supplementary information below:
The structs involved:
struct Address {
int id;
int set;
char *name;
char *email;
};
struct Database {
int MAX_DATA;
int MAX_ROWS;
struct Address *rows;
};
struct Connection {
FILE *file;
struct Database *db;
};
Creating the Database:
void Database_create(struct Connection *conn, const int MAX_ROWS, const int MAX_DATA)
{
int i = 0;
conn->db->MAX_ROWS = MAX_ROWS;
conn->db->MAX_DATA = MAX_DATA;
conn->db->rows = (struct Address *)malloc(sizeof(struct Address)*MAX_ROWS);
for(i = 0; i < MAX_ROWS; i++) {
struct Address addr;
addr.id = i;
addr.set = 0;
addr.name = (char *)malloc(sizeof(char)*MAX_DATA);
addr.email = (char *) malloc(sizeof(char)*MAX_DATA);
conn->db->rows[i] = addr;
}
}
Finally, cleaning up:
void Database_close(struct Connection *conn)
{
int i = 0;
struct Address *cur_row = NULL;
if(conn) {
if(conn->file) fclose(conn->file);
if(conn->db) {
if(conn->db->rows) {
for(i = 0; i < conn->db->MAX_ROWS; i++) {
cur_row = &conn->db->rows[i];
if(cur_row) {
free(cur_row->name);
free(cur_row->email);
}
}
free(conn->db->rows);
}
free(conn->db);
}
free(conn);
}
}

Answer 1

You know you have to allocate for your rows based on your comment. So you can do that:
conn->db->rows = malloc(MAX_ROWS * sizeof(struct Address));
Then, after you read in a row, you need to allocate memory for the data of each row before you read the data.
for (i = 0; i < ROWS; ++i) {
rc = fread(&conn->db->rows[i], sizeof(struct Address),
1 , conn->file);
if(rc != 1) die("Failed to load rows.", conn);
conn->db->rows[i].name = malloc(MAX_DATA);
conn->db->rows[i].email = malloc(MAX_DATA);
rc = fread(&conn->db->rows[i]->name, sizeof(char),
MAX_DATA, conn->file);
if(rc != MAX_DATA) die("Failed to load name.", conn);
rc = fread(&conn->db->rows[i]->email, sizeof(char),
MAX_DATA, conn->file);
if(rc != MAX_DATA) die("Failed to load email.", conn);
}
You will have to make sure you are reading in the data in the same order that it was written out when you created the database.

Answer 2

fread only deals with contiguous memory, so you can only read one struct or array at a time. Your strings are not contiguous with your Address struct. You can solve this 2 ways:
1) fread the initial struct, then fread each string separately. Then update the struct with pointers to the strings. (If you want to be efficient, you'll have to write the string sizes down, so you can just store the 'valid' parts of the string instead of max-size strings.)
2) You could make the strings part of the structure by declaring them as fixed-size arrays (instead of pointers to arrays).
struct Address {
int id;
int set;
char name[MAX];
char email[MAX];
};