While loop in C giving Segmentation Fault when not using print statement

Question

I came across a strange behaviour of C with while loop today.

A very simple code :

int userLogin()
{
    struct User user;
       
    while (1)
    {
        printf("\n\nEnter username (or quit to return back) - ");
        char userName[50];
        scanf("%s", userName);

        if (strcmp(userName, "quit") == 0)
        {
            printf("\nReturning to Login menu...\n");
            return 2;
        }

        printf("\n\nEnter Password - ");
        char password[50];
        scanf("%s", password);

        user = login_user(userName, password);
        if (strcmp(user.userID, userName) != 0 ||
            strcmp(user.password, password) != 0)
        {
            printf("\nInvalid username or password !\n");
        }
        else
        {
            printf("\nLogin successful...Entering Library menu !\n");
            currentUser = user;

            if (currentUser.role == 1)
            {
                libraryOptions();
            }
            else
            {
                adminPanel();
            }

            break;
        }
    }
}

Now this gives out Segmentation fault (Core Dumped).

However, same code, I just insert a print statement before while(1) :

And suddenly it starts working...

Can somebody help me understand the reason for this ? What am I missing here ?

PS: I know what segmentation fault is, what I need to understand is what changes if I include a print statement.

PPS: struct User is in a header file

struct User {
        char *userName; //User name
        char *userEmail; //Usre email
        char *userID; // user id
        char *password; //password
        int role; //1 for regular user and 2 for admin
};

login_user is also in another C file which includes the user header file:

 struct User login_user(const char *userName, const char *password)
{
   

 int i;
    struct User user;
    user.userName = "";
    user.password="";

    if (allUsers.length == 0)
    {
        return user;
    }

    for (i = 0; i < allUsers.length; i++)
    {
        if (strcmp(userArray[i].userID, userName) == 0 &&
            strcmp(userArray[i].password, password) == 0)
        {
            user.userName = userArray[i].userName;
            user.password = userArray[i].password;
            user.userEmail = userArray[i].userEmail;
            user.userID = userArray[i].userID;
            user.role = userArray[i].role;
            return user;
            
        }

      
    }
    return user;
    
}

I still reiterate that I don't want the answer to main code or reason to segmentation fault (I am working on it), all I request is someone to explain the reason why the print statement makes the code work.

Answer 1

Nobody can explain why you encounter this issue as this is undefined behavior. You must fix your code instead of trying to understand something that cannot be understood.

In your login_user function you are allocating strings like this :

struct User user;
user.userName = "";
user.password="";

And then return the user. You cannot do that. This allocates a nul terminated string on the stack but is "destroyed" when the function is done. Maybe what you meant to do is = NULL ?

Use fixed size arrays in your struct instead :

#define MAX_BUFFER_SIZE 100

struct User {
    char userName[MAX_BUFFER_SIZE]; //User name
    char userEmail[MAX_BUFFER_SIZE]; //Usre email
    char userID[MAX_BUFFER_SIZE]; // user id
    char password[MAX_BUFFER_SIZE]; //password
    int role; //1 for regular user and 2 for admin
};

Also, as someone else pointed out in his answer, scanf functions used like you did can cause more undefined behavior as a buffer overflow could happen. There are ways to use scanf in a safe way but I suggest you take a look at fgets. The same applies to strcmp, you should use strncmp instead.

Answer 2

        char userName[50];
        scanf("%s", userName);

Very bad, you do not control length in input string

        char password[50];
        scanf("%s", password);

same here. Also, you do not change echoed symbols to "*".

        if (strcmp(user.userID, userName) != 0 ||
            strcmp(user.password, password) != 0)

You have not allocated memory for user.userID and user.password strings.

Also, after authentication, you have to fill password string with zeros so it will not remain in RAM