How can I make the input not longer then a specified length? - c

Question

I am currently learning C and I am working on a task to improve some C-Code that might lead to a program crash.
Here is the code:
int main()
{
// Define buffers to store username and password
char username[16];
char password[16];
// Read username and password from user input
printf("Enter your name: ");
scanf("%s", username);
printf("Enter your password: ");
scanf("%s", password);
printf("[SHOUTING OUT LOUD] Hello, %s!\n", username);
return 0;
}
How can I make sure, that the input is not longer than 15 chars? Otherwise, the program could accidentally print out the password or overwrite the return address on the stack, which was subject of my other question:
Is it possible to crash this program somehow?
I already thought about putting the variables on the heap, but in the beginning, I don't know how long the input is. So I don't know, how much space I shall allocate.
Can somebody help me with this?
Thanks :)

Answer 1

Use a length modifier with the amount of bufferlength - 1 for the %s conversion specifier.
In this case, as both arrays are consisted of 16 char elements, use %15s.
Note: Always check the return value of input functions, if an error occurred at consuming the input.
printf("Enter your name: ");
if ( scanf("%15s", username)) != 1 )
{
fputs("Error at input - username!", stderr);
exit(1);
}
// No explicit need for a call to getchar() to catch the newline from stdin
// in your program. %s skips leading white space.
printf("Enter your password: ");
if ( scanf("%15s", password)) != 1 )
{
fputs("Error at input - password!", stderr);
exit(1);
}
or use fgets() instead:
printf("Enter your name: ");
if ( fgets(username, sizeof(username), stdin) == NULL )
{
fputs("Error at input - username!", stderr);
exit(1);
}
// fgets can catch the newline with. If this is the case, we need or want to
// remove this newline from the string by replacing it with '\0'.
// Else, we need to catch the left newline from stdin so that it is not
// catched by fetching the password.
size_t len = sizeof(username);
for ( size_t i = 0; i < len; i++ ) // Iterate through the buffered string.
{
if ( username[i] == '\n' )
{
username[i] = '\0'; // Replace newline with NUL.
break;
}
if ( i == len - 1 )
{
getchar(); // If no newline was in the string,
} // catch left newline from stdin.
}
printf("Enter your password: ");
if ( fgets(password, sizeof(password, stdin) == NULL )
{
fputs("Error at input - password!", stderr);
exit(1);
}
password[strcspn(password, "\n")] = '\0'; // Replace newline with NUL.
Note that although the latter use of fgets() seems more complicated, fgets() is general more safe than scanf() and the preferred choice to read strings.
Take a look at:
Disadvantages of scanf