scanf() variable length specifier

Question

How can I use a variable to specify the max number of chars scanf() should read in?

For example using printf() you can use the * like so

#define MAXVAL 5
printf("Print at maximum MAXVAL chars: %.*s\n", MAXVAL, "myStringHere");

This will only print 5 chars, how can I make scanf only read in MAXVAL? MAXVAL must be used as the length specifier. I cannot simply do

scanf("%5s", string);

Right now I can only think of reading into a large array using scanf then using ssprintf to store the string into my length limited string. Using a length specifier would be so much easier however.

Size reduce of `-1` that amount because `scanf` add a NUL character reading for that string(`%s`). E.g. `scanf("%4s", string);` — BLUEPIXY, Aug 20 '14 at 17:19
@user3121023 I think he wants to use `MAXVAL` instead of hardcoding a value in the format string. — Filipe Gonçalves, Aug 20 '14 at 17:19
@mafso `fgets` breaks on newlines, `scanf` breaks on whitespace. @CSStudent you can create format specifier in code. — clcto, Aug 20 '14 at 17:21
@clcto "scanf breaks on whitespace" ignores the effect of the format parameter. The function `scanf()` does not necessarily break on white-space as it depends on the format. `scanf("%5s")` will consume leading white-space (not break on w-s), then scan non-white-space characters until 1) white-space (does break on w-s) 2) 5 `char` read 3) EOF or 4) IOError. — chux - Reinstate Monica, Aug 20 '14 at 21:04
possible duplicate of [Using C preprocessor to construct a string literal for scanf?](http://stackoverflow.com/questions/2740039/using-c-preprocessor-to-construct-a-string-literal-for-scanf) — Joshua Taylor, Aug 31 '14 at 22:37

score 25 · Accepted Answer · answered Aug 20 '14 at 17:22

25

You can use the C preprocessor to help you with that.

#define STR2(x) #x
#define STR(X) STR2(X)
scanf("%" STR(MAXVAL) "s", string);

The processor combines "%" STR(MAXVAL) "s" to "%5s"

answered Aug 20 '14 at 17:22

R Sahu

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Thats quite a clever method. Interesting use of the preprocessor – CS Student Aug 20 '14 at 17:26
1

I've just tried this method, whats the need in using 2 `#defines`? Can't I just use `#define STR(x) #x` and then use `scanf("%" STR(MAXVAL) "s", string);` ? – CS Student Aug 20 '14 at 17:53
3

@CSStudent, please take a look at http://stackoverflow.com/questions/16989730/c-stringify-how-does-it-work. – R Sahu Aug 20 '14 at 17:57
After reading through the answers to that question I'm still confused. I don't see why 2 `defines` are needed. Am I missing something obvious? – CS Student Aug 20 '14 at 18:04
If you don't use two levels, `"%" STR(MAXVAL) "s"` will be transformed to `%MAXVALs`, not `%5s`. – R Sahu Aug 20 '14 at 18:11
It seemed to have worked using gcc with only one define, to test I used: `#define STR(x) #x #define MAX 50` and `printf("one define: %%" STR(MAX) "s");` and it output `one define: %50s` I used `%%` so the % char would be printed to stdout. – CS Student Aug 20 '14 at 18:20
I don't know how you are getting the above output. Checkout this example at http://ideone.com/aaw2bD. – R Sahu Aug 20 '14 at 18:49
7

@CSStudent: The two definitions are necessary so that the argument is expanded properly. That is, if `STR(X)` was simply defined as `#x`, then `STR(MAXVAL)` would expand to `#MAXVAL`, meaning the `scanf` call would expand to `scanf("%" "MAXVAL" "s", string );`, which is not what you want. By using the second define, `MAXVAL` will expand to 5 before being stringized. – John Bode Aug 20 '14 at 19:07
@JohnBode Cheers John once again – CS Student Aug 20 '14 at 20:21
The same approach (but perhaps with better macro names was used in the answer to a very similar question from 2010: [Using C preprocessor to construct a string literal for scanf?](http://stackoverflow.com/q/2740039/1281433) – Joshua Taylor Aug 31 '14 at 22:39

BLUEPIXY · Answer 2 · 2014-08-20T17:51:04.773

5

#include <stdio.h>

#define MAXLEN 5
#define S_(x) #x
#define S(x) S_(x)

int main(void){
    char string[MAXLEN+1];

    scanf("%" S(MAXLEN) "s", string);
    printf("<%.*s>\n", MAXLEN, string);
    return 0;
}

edited Aug 20 '14 at 17:51

answered Aug 20 '14 at 17:26

BLUEPIXY

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Daniel Trugman · Answer 3 · 2022-10-17T22:49:20.017

4

Kernighan and Pike recommend using snprintf() to create the format string. I developed this idea into a method that safely reads strings:

void scan_string(char *buffer, unsigned length) {
    char format[16]; // Max int (10) + %, s & null-terminator (3)
                     // Rounded up to the closes power of 2
    snprintf(format, sizeof(format), "%%%ds", length - 1);
    scanf(format, buffer);
}

int main() {
  char str[5];
  scan_string(str, sizeof(string));
  printf("%s\n", str);
}

edited Oct 17 '22 at 22:49

answered Oct 20 '17 at 11:46

Daniel Trugman

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Shouldn't the format string be null-terminated, hence the size being 12+1? – sspathare97 Oct 16 '22 at 02:43
1

@sspathare97, you are right. Was a long time ago. Updated the length and improved some of the comments as well. – Daniel Trugman Oct 17 '22 at 22:50
Thanks for the confirmation and update! – sspathare97 Oct 18 '22 at 23:12

score 2 · Answer 4 · edited May 23 '17 at 12:19

2

You can't. You need to use something other than scanf(). A good and popular choice is fgets(), although its semantics are slightly different: fgets() will read a line of input, whereas scanf() with %s will read whitespace separated sequences of characters.

To use fgets(), you'd want something like:

fgets(string, MAXVAL, stdin);

If for some reason you really want to use scanf(), have a look at this question: How to prevent scanf causing a buffer overflow in C?

edited May 23 '17 at 12:19

Community

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1

answered Aug 20 '14 at 17:18

Filipe Gonçalves

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This is probably the best option to maintain good readability but I think I will go with the preprocessor trickery. So I can still use `scanf()` – CS Student Aug 20 '14 at 17:30

scanf() variable length specifier

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