Writing Data from an Array of Classes to a Binary File C++

Question

In my code I have an Array of class User as a private variable in class UserManager. When UserManager::Shutdown() is called it save the Array of Users to a file. Yet I get an access violation error. The Access violation error occurs in fwrite.c

I am compiling on Visual Studio 2010 running Windows 7.

User Class

class User {
public:
User() {
    memset(username, 0, 255);
    memset(password, 0, 255);
}

int userId;
char username[255];
char password[255];
};

MAX_USERS Definition

#define MAX_USERS 2048

User Manager Constructor

UserManager::UserManager() {
memset( users, 0, MAX_USERS);
}

Shutdown Function

void UserManager::Shutdown( void) {
FILE *userDB = fopen( "users.bin", "wb");
int k=1;
for(k=1;k<MAX_USERS-1;k++){
    if (users[k]!=NULL) {
        fwrite((const void*)&users[k]->userId, sizeof(int), 1, userDB);
        fwrite((const void*)users[k]->username, 255, 1, userDB);
        fwrite((const void*)users[k]->password, 255, 1, userDB);
    } else {
        fpos_t skip = 255 + 255 + sizeof(int);
        fsetpos( userDB, &skip);
    }
}

fclose( userDB);
}

The array 'users' is memset to zero at the constructor.

Answer 1

Theory 1

Your loop iteration starts at 1, but array indexing starts at 0. Without seeing the definition of MAX_USERS or the actual error my guess is your k value is overrunning your users array boundary.

Theory 2

You declared users as

User *users[MAX_USERS] = {};

Then you didn't new up each user when you created them. So, instead of

users[index] = new User();

you did

  User user1;
  users[index] = &user1;

The second version would probably go out of scope. Then the memory for user1 would be reallocated somewhere else and when you tried to access it via users[k]-> you get the access violation.

My version

Below is the code I used to try figure out the problem. It's quick and dirty with me filling in the gaps but it runs to completion. Hopefully it will help.

#define MAX_USERS 2048
class User { 
    public: User() {     
    memset(username, 0, 255);     
    memset(password, 0, 255); }  
    int userId; 
    char username[255]; 
    char password[255]; 
}; 

void writefile( User **users)
{
    FILE *userDB = fopen( "users.bin", "wb"); 
    int k=0; for(k=0;k<MAX_USERS-1;k++){     
        if (users[k]!=NULL) {         
            fwrite((const void*)&users[k]->userId, sizeof(int), 1, userDB);         
            fwrite((const void*)users[k]->username, 255, 1, userDB);         
            fwrite((const void*)users[k]->password, 255, 1, userDB);     
            fprintf(stdout, "UserID %d\n", users[k]->userId);
            fprintf(stdout, "%s\n", users[k]->username);
            fprintf(stdout, "%s\n", users[k]->password);
            fflush(stdout);
        } else {         
            fpos_t skip = 255 + 255 + sizeof(int);         
            fsetpos( userDB, &skip);     
        } 
    }  fclose( userDB); 
} 

int _tmain(int argc, _TCHAR* argv[])
{
    User *users[MAX_USERS] = {};
    memset(users, 0, MAX_USERS);
    users[0] = new User();
    users[0]->userId = 1;
    memcpy(users[0]->username,"BLah", strlen("BLah"));
    memcpy(users[0]->password,"something:", strlen("something:"));
    users[2] = new User();
    users[2]->userId = 1;
    memcpy(users[2]->username,"BLah2", strlen("BLah2"));
    memcpy(users[2]->password,"something:", strlen("something:"));
    writefile(users);

    return 0;
}

Answer 2

Are you initializing all variables ?
The suspicion is that the array is the cause.