Simple CRUD in C to understand how one connect to DATABASE

crud.c

#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>

#define MAX_DATA 512
#define MAX_ROWS 100

struct Address {
  int id;
  int set;
  char name[MAX_DATA];
  char email[MAX_DATA];
};

struct Database {
  struct Address rows[MAX_ROWS];
};

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

void die(const char *message)
{
  if(errno) {
    perror(message);
  } else {
    printf("ERROR: %s\n", message);
  };
  exit(1);
}

void Address_print(struct Address *addr)
{
  printf("%d , %s %s\n",
          addr->id, addr->name, addr->email);
}

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.");
}

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

  if(mode == 'c') {
    conn->file = fopen(filename, "w");
  } else {
    conn->file = fopen(filename, "r+");
    if(conn->file) {
      Database_load(conn);
    }
  }
  return conn;
};

void Database_close(struct Connection *conn)
{
  if(conn){
    if(conn->file) fclose(conn->file);
    if(conn->db) free(conn->db);
    free(conn);
  }
}

void Database_write(struct Connection *conn)
{
  rewind(conn->file);

  int rc = fwrite(conn->db, sizeof(struct Database), 1, conn->file);
  if(rc != 1) die ("Failed to write database");

  rc = fflush(conn->file);
  if(rc == -1) die ("Cannot flush database");
}

void Database_create(struct Connection *conn)
{
  int i = 0;
  for(i = 0; i < MAX_ROWS; i++) {
    //make a prototype to initialize it
    struct Address addr = {.id = i, .set = 0};
    //then just assign it
    conn->db->rows[i] = addr;
  }
}

void Database_set(struct Connection *conn, int id, const char *name, const char *email)
{
  struct Address *addr = &conn->db->rows[id];
  if(addr->set) die ("Already set, delete it first");
  
  addr->set = 1;
  // WARNING: bug, read the "How to BREAK IT" and fix this
  char *res = strncpy(addr->name, name, MAX_DATA);
  // demonstrate the strncpy bug
  if(!res) die ("Name copy failed!");
  
  res = strncpy(addr->email, email, MAX_DATA);
  if(!res) die ("Email copy failed!");
}

void Database_get(struct Connection *conn, int id) 
{
  struct Address *addr = &conn->db->rows[id];

  if(addr->set) {
    Address_print(addr);
  } else {
    die("ID is not set");
  }
}

void Database_delete(struct Connection *conn, int id)
{
  struct Address addr = {.id = id, .set = 0};
  conn->db->rows[id] = addr;
}

void Database_list(struct Connection *conn)
{
  int i = 0;
  struct Database *db = conn->db;
  
  for(i = 0; i < MAX_ROWS; i++) {
    struct Address *cur = &db->rows[i];

    if(cur->set) {
      Address_print(cur);
    }
  }
}

int main(int argc, char *argv[])
{
  if(argc <3 ) die ("USAGE: ex17 <dbfile> <action> [action params]");
  
  char *filename = argv[1];
  char action = argv[2][0];
  struct Connection *conn = Database_open(filename, action);
  int id = 0;
  
  if(argc > 3) id = atoi(argv[3]);
  if(id >= MAX_ROWS) die("There is not that many records.");

  switch(action) {
    case 'c':
      Database_create(conn);
      Database_write(conn);
      break;
    case 'g':
      if(argc != 4) die("Need an id to get");

      Database_get(conn, id);
      break;
    case 's':
      if(argc != 6) die("Need id, name, email to set");
      Database_set(conn, id, argv[4], argv[5]);
      Database_write(conn);
      break;
    case 'd':
      if(argc != 4) die("Need an id to delete");
      
      Database_delete(conn, id);
      Database_write(conn);
      break;
    case 'l':
      Database_list(conn);
      break;
    default:
      die("Invalid action, only: c=create, g=get, s=set, d=del, l=list");
  }

  Database_close(conn);
  return 0; 
}