Read no more than size of string with scanf()

Edit: for my class I have to use scanf. So recommending other ways of input is not the solution I am looking for (if there is one that involves scanf).

If I am reading in user input for a small project (for example, a game). Lets say I ask would you like to play? This would accept a yes or no answer. So i write up some simple code like this:

#include <stdio.h>

int main(void)
{
     char string[3]; //The max number of letters for "yes".

     printf("Would you like to play?");
     scanf("%s", string);
}

So this code should simply ask them to input yes or no. I am setting the length of my char array to size 3. This way it is large enough to hold yes and also no. But if someone were to enter invalid input such as yesss, I know how to compare the string afterwards to handle such an event, but wouldn't this technically/possibly overwrite other local variables I have declared because it would extend outside the length of my array? If so, is there a way to handle this to restrict 3 input characters or something? And if not, why/how does it know to only input for the size of 3?

Solution 1:

Your array needs to be able to hold four chars, since it must also contain the 0-terminator. With that fixed, specifying a maximal length in the format,

scanf("%3s", string);

ensures that scanf reads no more than 3 characters.

Solution 2:

the safest way is to use

fgets(string, 4, stdin);

here you can store maximum 3 characters including one space reserved for NULL ('\0') character.

Solution 3:

You should use the width modifier of scanf() and set it to be one less than the size of your string, so that you ensure that space exists for the NULL terminator.

So, if you want to store "yes", you will firstly need a bigger array than the one you have; one with size 4, 3 characters plus 1 for the null terminator. Moreover, you should instruct scanf() to read no more than size - 1 characters, where size is the length of your array, thus 3 in this case, like this:

#include <stdio.h>

int main(void)
{
     char string[4];
     scanf("%3s", string);
}