How to use a define inside a format string?

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Say I have a character array:

#define LEN 10
char arr[LEN + 1];

Lets do some scanf operation to it:

scanf("Name: %s", arr);

This could be dangerous if someone is typing a name that is longer than 10 characters. So better use this:

scanf("Name: %10s", arr);

Well now I would run into trouble if LEN is changed. I would have to go through the whole code to correct every line where I used the 10 in context of arr. So I thought about somehting like this:

scanf("Name: %LENs", arr);

But this will not work.LEN is not resolved by the preprocessor beacuse it is used inside a string.

How to use a define inside a format string?

format-specifiers c-preprocessor c
1 Answers

C joins adjacent string literals and you can stringify a preprocessor parameter with #, so the following should do the trick:

#define LEN 10

// this converts to string
#define STR_(X) #X

// this makes sure the argument is expanded before converting to string
#define STR(X) STR_(X)

[...]

scanf("Name: %" STR(LEN) "s", arr);

The macros are needed because with just #LEN, you'd end up with LEN expanded to 10, and with only one macro applying # to its argument, the result would be "LEN" (the argument wouldn't be expanded).

The preprocessor / compiler will transform this in the following steps:

1. scanf("Name: %" STR_(10) "s", arr);
2. scanf("Name: %" "10" "s", arr);
3. scanf("Name: %10s", arr);

In the last step, the string literals are joined into a single one.

On a side note, your scanf() format string would require the user to literally enter

Name: xyz

to actually match. I doubt this is what you wanted. You probably want something like this:

fputs("Name: ", stdout);
fflush(stdout);
scanf("%" STR(LEN) "s", arr);

Also consider not using scanf() at all. With e.g. fgets(), this whole preprocessor magic is obsolete. For reasons why you shouldn't use scanf(), see my beginners' guide away from scanf().

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