Initializing a nested structure

Question

This is a problem from learn C the hardway. This is database management system in C I have three structures :-

 struct Address {
         int id;
         int set;
         char *name;
         char *email;
 };

 struct Database {
int rows;
    struct Address *row;
 };

 struct Connection {
    FILE *file;
    struct Database *db;
 };

I trying to initialize the database structure. However I am getting a segfault

    void Database_create(struct Connection *conn, int no_of_rows)
    {
      int i = 0;
  conn->db->row_num = no_of_rows; 

      for(i = 0; i < conn->db->row_num; i++) {

      // make a prototype to initialize it
      struct Address addr;
      addr.id = i;
      addr.set = 0;

      // then just assign it
      conn->db->rows[i] = addr;
      }
  }

I've made another function which allocates memory to these structures.

     struct Connection *Database_open(const char *filename, char mode)
      {   
        struct Connection *conn = malloc(sizeof(struct Connection));
        if(!conn) die("Memory error");

    int number = conn->db->rows;

        conn->db = malloc(sizeof(struct Database));
    if(!conn->db) die("Memory error");

        conn->db->row = malloc(sizeof(*conn->db->row) * number);
        if(!conn->db->row) die("Memory error");

        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");

      return conn;
   }

valgrind says "Use of uninitialized value of size 4" in Database_open()

Could anyone suggest what I might be doing wrong here?

Answer 1

db in Connection and row in Database are uninitialised pointers. You need to initialise them and provide storage for the structs they point to.

You can probably save some dynamic allocation by changing Connection to have its Database as a member rather than a pointer

struct Connection {
    FILE *file;
    struct Database db;
 };

You need to allocate memory for the database rows

conn->db.row = malloc(no_of_rows * sizeof(*conn->db.row));

Database_create would then look something like

int Database_create(struct Connection *conn, int no_of_rows)
{
    int i = 0;
    conn->db.rows = no_of_rows;
    conn->db.row = malloc(no_of_rows * sizeof(*conn->db.row));
    if (conn->db.row == NULL) {
        /* out of memory */
        return 1;  /* indicate failure to caller */
    }
    for(i = 0; i < conn->db->rows; i++) {
        conn->db.row[i].id = i;
        conn->db.row[i].set = 0;
    }
    return 0; /* indicate success to caller */
}

Note that this assumes memory is already allocated for the Connection