Trying to pass maximum number of characters read by scanf through #define

Question

I'm trying to pass a number to scanf of how many characters I want to read from the input stream. However, I can't get it to work.

My code works when I just put the number in the code, but I want to pass the number through #define to change it easily.

This works:

#include <stdio.h>

int main(void)
{
    char tab[11];

    printf("Podaj tekst: ");
    scanf("%10[^\n]", tab);

    printf("%s", tab);
}

This doesn't work (it only gets characters up until first whitespace character):

#include <stdio.h>

#define size 10

int main(void)
{
    char tab[size+1];

    printf("Podaj tekst: ");
    scanf("%size[^\n]", tab);

    printf("%s", tab);
}

It seems strange to me that it doesn't work. Is there any workaround to do what I want (besides using fgets)?

@chux That makes sense, any idea how I can escape that 'size' into the scanf? — kunek, Jan 11 '19 at 02:11
See [Can I use scanf to capture a directive with a width specified by a variable?](https://stackoverflow.com/q/1549906/2410359) — chux - Reinstate Monica, Jan 11 '19 at 02:11
kunek, An even better approach is to use `fgets()` and ditch `scanf()`. — chux - Reinstate Monica, Jan 11 '19 at 02:14
@chux Looks like `fgets()` is a better approach here, thanks! — kunek, Jan 11 '19 at 02:18
See also [How to prevent `scanf()` causing a buffer overflow in C?](https://stackoverflow.com/questions/1621394/how-to-prevent-scanf-causing-a-buffer-overflow-in-c) — Jonathan Leffler, Jan 11 '19 at 06:03

Eric Postpischil · Accepted Answer · 2019-01-11T02:58:42.623

5

You can convert numerals or other text to strings with the preprocessor # operator. This requires using two macros, one to expand the size macro and another to apply the operator:

#define size 10

#define StringifyHelper(x)  #x
#define Stringify(x)        StringifyHelper(x)

...

char tab[size+1];
scanf("%" Stringify(size) "[^\n]", tab);

edited Jan 11 '19 at 02:58

answered Jan 11 '19 at 02:45

Eric Postpischil

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Trying to pass maximum number of characters read by scanf through #define

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