How to store multiple data using file and compare a specific data from it to user data in c programming?

Question

I created 2 function. One for login and another for signup. It's working fine for only one user but for multiple user it's not working. It's only read the first two string from the file. How can I fix it and make it to read second or more set of data from file? Here is my code:

//login (reading data)
login() {
    system("cls");
    system("color 3");
    char username[10], password[10], un[10], pwd[10];
    FILE *fp;
    fp = fopen("stored_data.txt","r");
    fscanf(fp, "%s%s", username, password);
    printf("Enter your username: ");
    fflush(stdin);
    gets(un);
    printf("Enter your password: ");
    fflush(stdin);
    gets(pwd);
    if ((strcmp(username,un)==0)&&(strcmp(password,pwd)==0)) {
        printf("Login successful, welcome %s.\n",un);
    }
    else {
        system("cls");
        system("color 4");
        printf("Invalid username or password.");
    }
    fclose(fp);
}
//Sign up (store data)
signup() {
    system("cls");
    system("color 3");
    char username[10], un[10], pwd[10];
    FILE *fp;
    fp = fopen("stored_data.txt","r");
    fscanf(fp, "%s", username);
    fp = fopen("stored_data.txt","a");
    printf("Enter your username: ");
    fflush(stdin);
    gets(un);
    if((strcmp(username,un)==0)) {
        printf("User already existed.\n");
    }
    else {
        printf("Enter your password: ");
        fflush(stdin);
        gets(pwd);
        fprintf(fp,"%s %s",un,pwd);
        printf("New user's information added successfully.\n");
    }
    fclose(fp);
}

[`fflush(stdin);` invokes *undefined behavior*.](https://stackoverflow.com/questions/2979209/using-fflushstdin) — MikeCAT, Mar 19 '21 at 11:16
No return types in the function definitions? that's not very common these days :) — Morten Jensen, Mar 19 '21 at 14:11

score 0 · Accepted Answer · answered Mar 19 '21 at 13:39

You should look for the name after the user entered his name and password. By this line

fscanf(fp, "%s%s", username, password);

You are reading the first username and password in your file. Instead, you should move this into a while loop.

...
printf("Enter your username: ");
fflush(stdin);
gets(un);
printf("Enter your password: ");
fflush(stdin);
gets(pwd);
while(!feof(fp)){
    fscanf(fp, "%s%s", username, password);
    if (strcmp(username,un) == 0){
        if(strcmp(password, pwd) == 0)
            printf("Login successful, welcome %s.\n",un);
        else break;
    }
}
...

How to store multiple data using file and compare a specific data from it to user data in c programming?

1 Answers1