Guard scanf from reading too many characters with a define?

Question

Usually, I use a define for the size of a string, but when I use scanf(), I want to guard the function from reading too many characters (and reserve space for the null-terminator). I was wondering whether I could do this using my define, instead of a hardcoded magic number...

#include <stdio.h>

#define MAXLEN 4

int main(void) {
    char a[MAXLEN];
    scanf("%3s", a); // Can I do that with 'MAXLEN' somehow?
}

Is it possible? If yes, how?

I have linked to the duplicate, but why don't you use `fgets()` and `sscanf()`? — Iharob Al Asimi, Sep 16 '17 at 19:27
I could, but I was wondering about `scanf()` @IharobAlAsimi, thanks! — gsamaras, Sep 16 '17 at 19:28
@gsamaras I have posted another solution in the original question (I was posting here but it got closed) — Jean-François Fabre, Sep 16 '17 at 19:35

score 9 · Accepted Answer · edited Sep 16 '17 at 19:28

9

Use defines to stringify:

#define LENSTR_(x) #x
#define LENSTR(x) LENSTR_(x)

then you can use:

#define MAXLEN 3

char a[MAXLEN + 1];
scanf("%" LENSTR(MAXLEN) "s", a);

edited Sep 16 '17 at 19:28

gsamaras

71,951
46
188
305

answered Sep 16 '17 at 19:11

xing

2,125
2
14
10

1

@Jean-FrançoisFabre because that's how one stringifies the values of macro arguments in C... – Antti Haapala -- Слава Україні Sep 16 '17 at 19:16
I don't get how this works but it does so I'm upvoting – savram Sep 16 '17 at 19:19
1

@AnttiHaapala: Yes, but most only copy that pattern without knowing **why** this is necessary. And that does not work as expected for variable fields. What if you have a VLA? – too honest for this site Sep 16 '17 at 19:19
2

This only works if `MAXLEN` is a decimal integer literal. It would fail for `#define MAXLEN 0x10` – chqrlie Sep 16 '17 at 19:20
This seems a nice approach to me, but I would be concerned that any attempt to protect the buffer from over-run will (I believe) leave unread input in the input buffer if the limit is reached. It's always seemed nicer to me to have an input routine that allocates space, if possible. – Kevin Boone Sep 16 '17 at 19:20
@AnttiHaapala I forgot to type "macro". Why does it need 2 macros instead of one? – Jean-François Fabre Sep 16 '17 at 19:21
5

@Jean-FrançoisFabre: Using just one macro will expand to `"MAXLEN"`. The two level approach expands to `LENSTR_(3)` first, which will then stringify to `"3"`. – M Oehm Sep 16 '17 at 19:25
That "#x" is necessary? – savram Sep 16 '17 at 19:30
2

@savram: The `#` is the "stringification" or "stringizing" operator in macro definitions. It turns the argument into a C string literal. – M Oehm Sep 16 '17 at 19:32
I see. I think I understand it now. Somebody said it doesn't work with 0x10, why? – savram Sep 16 '17 at 19:36
@savram: Because the `scanf` format string expects the maximum length in decimal format. – M Oehm Sep 16 '17 at 19:42

Guard scanf from reading too many characters with a define?

1 Answers1

Linked