Problems with limiting a string length in C

Question

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);

You need to account for the extra null on the end which is needed to terminate a string. A string of length 15 characters will need to be 16 bytes in length. — Lee Taylor, Aug 13 '20 at 16:09
I hope you're not using `scanf_s()` because your compiler told you `scanf()` is "deprecated". Because `scanf()` is **not** deprecated. — Andrew Henle, Aug 13 '20 at 16:10
`scanf_s` needs the buffer size. Look it up in the documentation: "_If nothing helps, read the manual_" — Paul Ogilvie, Aug 13 '20 at 16:17

RobertS supports Monica Cellio · Accepted Answer · 2020-08-13T17:04:26.043

7

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).

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.
}

edited Aug 13 '20 at 17:04

answered Aug 13 '20 at 16:19

RobertS supports Monica Cellio

14,524
7
33
80

1

Size type can be a problem as [MS](https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/scanf-s-scanf-s-l-wscanf-s-wscanf-s-l?view=vs-2019) expects `unsigned`, yet `sizeof()` returns `size_t`. MS promotes `(unsigned)_countof(s)` – chux - Reinstate Monica Aug 13 '20 at 16:46
1

"scanf_s is Microsoft-specific" --> `scanf_s()` is specified in the annex of C11,C18 needing a `size_t` agreement. Many C11 compilers do not implement this function. – chux - Reinstate Monica Aug 13 '20 at 16:50
Microsoft's calling convention would probably work exactly equally with `size_t`... but they just want to force people to write as unportable code as possible. – Antti Haapala -- Слава Україні Aug 13 '20 at 16:55
@AnttiHaapala It breaks when `size_t` differs in size from `unsigned` - restricts what a future MS compiler can do. MS C variation on C11 and non-compilation (even in 2020) with C99 certainly looks inconsistent. – chux - Reinstate Monica Aug 13 '20 at 17:02
@chux-ReinstateMonica `size_t` already does differ from unsigned, but the calling convention might be compatible nevertheless... – Antti Haapala -- Слава Україні Aug 13 '20 at 18:38

foggynight · Answer 2 · 2020-08-13T20:12:03.167

3

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.

edited Aug 13 '20 at 20:12

answered Aug 13 '20 at 16:21

foggynight

73
6

Vlad from Moscow · Answer 3 · 2020-08-13T16:56:42.587

2

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'.

edited Aug 13 '20 at 16:56

answered Aug 13 '20 at 16:31

Vlad from Moscow

301,070
26
186
335

Isn't the precision redundant in the case of `scanf_s`? – RobertS supports Monica Cellio Aug 13 '20 at 17:08
Vlad, What advantage do you see using a _width_ in the format versus `scanf_s("%s", word, ( unsigned_or_size_t) sizeof word);`? – chux - Reinstate Monica Aug 13 '20 at 17:08
@chux-ReinstateMonica Same place. Same time. Same thought. :-) – RobertS supports Monica Cellio Aug 13 '20 at 17:09
@RobertSsupportsMonicaCellio It allows to read exactly 14 characters and the call will be successfull. – Vlad from Moscow Aug 13 '20 at 17:13

score 1 · Answer 4 · answered Aug 13 '20 at 23:57

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!

Problems with limiting a string length in C

4 Answers4

Strings