How can fix this weird error in my simple C program?

Question

This is a simple C authentication program that went wrong. It's reading the strings from the console as a username and password as input correct, but the username input has becomes blank space string. What might be the mistake?

Code:

#include <stdio.h>

int main()
{
    char username[4];
    char password[8];

    printf("Enter username: ");
    scanf("%s", username);
    printf("Enter password: ");
    scanf("%s", password);

    if (username == "ojas" && password == "ojas1234")
    {
        printf("Access granted!");
    }
    else
    {
        printf("%s and %s\n", username, password);
        printf("Access denied");
    }
    return 0;
}

Output

Enter username: ojas
Enter password: ojas1234
 and ojas1234
Access denied

and both arrays need to be bigger. I would recommend at least 100. — user3386109, Nov 08 '20 at 05:27
Does this answer your question? [How do I properly compare strings in C?](https://stackoverflow.com/questions/8004237/how-do-i-properly-compare-strings-in-c) — Andy, Nov 08 '20 at 05:31

score 2 · Accepted Answer · answered Nov 08 '20 at 05:36

There are several problems with your code, including:

You don't compare C strings with "==". You must use strcmp() (or equivalent) instead.
The combination of small character arrays and unsafe usage of "scanf()" is likely to cause buffer overruns. You should define a larger buffer, and use a "safer" method like fgets() instead.

SUGGESTED CHANGES:

#include <stdio.h>
#include <string.h>

#define MAX_STRING 80

int main()
{
   char username[MAX_STRING];
   char password[MAX_STRING];

   printf("Enter username: ");
   fgets(username, MAX_STRING, stdin);
   strtok(username, "\n");
   printf("Enter password: ");
   fgets(password, MAX_STRING, stdin);
   strtok(password, "\n");

   if ((strcmp(username, "ojas") == 0) && (strcmp(password, "ojas1234") == 0)){
      printf("Access granted!");
   }
   else {
      printf("%s and %s\n", username, password);
      printf("Access denied");
   }

   return 0;
}

score 1 · Answer 2 · answered Nov 08 '20 at 06:55

To compare two strings, you can use strcmp() function. This will compare the two strings character by character. Don't forget to use the header file string.h You can't use equal sign to compare two strings because string literals are identical and a string in C is a character array and it refers to an address. So if you use "==" signs, you will end up comparing two addresses.
Though the above mistake is corrected, still you will get the output as Access Denied. That is because your buffer overflows. So, define a large number for your username and password char arrays.

The below code might help you to fix these issues.

#include<stdio.h>
#include<string.h>

#define LEN 50

int main()
{
   char username[LEN];
   char password[LEN];

   printf("Enter username: ");
   scanf("%s", username);
   printf("Enter password: ");
   scanf("%s", password);

   if ((strcmp(username,"ojas") == 0) && (strcmp(password,"ojas1234") == 0))
   {
      printf("Access granted!");
   }
   else
   {
      printf("%s and %s\n", username, password);
      printf("Access denied");
   }

return 0;

}

How can fix this weird error in my simple C program?

Code:

Output

2 Answers2