CODEWITHSUNDEEP
cscanfc-strings
BrandonCanCode
BrandonCanCode

Reputation: 41

Problems with limiting a string length in C

I'm relatively new to coding and do not understand why I'm receiving an Exception thrown with a simple scanf.

char word[15];

printf("\nEnter a word: ");
scanf_s("%15s", word);

Upvotes: 3

Views: 587

Answers (4)

Geronimo Buggiani
Geronimo Buggiani

Reputation: 11

You're almost there! As you may already know strings in C are stored and managed in memory using an array of chars that terminates with a NULL character ('\0').

In short, if you want to limit the length of your string to say, 15 characters, you just add 1 to that to account for the NULL terminating character and voila! Your code works. A coder's mnemotechnic device:

Desired MAX number of character = Desired MAX number of character + 1;
// or the shortened 
the Desired MAX number of character += 1;

Also, bear in mind that, as mentioned above, scanf_s requires an additional argument that specifies the size of the input buffer. So your code would look like:

scanf_s("%16s", word, (unsigned)sizeof( word ));

Hope it helps and happy coding!

Upvotes: 1

foggynight
foggynight

Reputation: 73

Strings

Strings in C are represented using an array of characters.

These arrays must be terminated using a null character \0, as the C standard library is written assuming you will be writing strings this way.

When you define the array char word[15] you are allocating enough memory to store 14 characters plus the null character.

Upvotes: 3

RobertS supports Monica Cellio
RobertS supports Monica Cellio

Reputation: 15052

scanf_s requires a third argument, the sizeof the buffer:

scanf_s("%s", word, sizeof(word));

scanf_s can either be Microsoft-specific or the one defined by the C standard but which isn't mandatory to be implemented in every C implementation. Dependent on which one you use you either need to cast the sizeof() argument to unsigned(Microsoft implementation) or cast it to rsize_t (C standard).

Related:

Note that you also should check the return value of scanf_s.

if ( scanf_s("%s", word, (unsigned) sizeof(word)) != 1 )
{
    // error routine.
}

If you want to write portable code, use fgets() instead:

if ( !fgets(word, sizeof(word), stdin) )
{
   // error routine.
}

Upvotes: 7

Vlad from Moscow
Vlad from Moscow

Reputation: 311048

The function scanf_s for the conversion specifier s requires an addition argument that specifiers the size of the input buffer.

You could write for example (if it is the Microsoft implementation of scanf_s)

scanf_s("%14s", word, ( unsigned )sizeof( word ) );

or (if it is the standard scanf_s)

scanf_s("%14s", word, ( rsize_t )sizeof( word ) );

Also the length should be specified equal to sizeof( word ) - 1 that is equal to 14. One character is reserved for the terminating zero character '\0'.

Upvotes: 2

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