Question

Is it possible to write a Macro (using token concatenation) that returns format for printf?
E.g.

#define STR_FMT(x) ...code-here...

STR_FMT(10) expands to "%10s"

STR_FMT(15) expands to "%15s"

…
etc.

So that I can use this macro inside a printf:

printf(STR_FMT(10), "*");

Answer 1

You can, but I think it might be better to use the capability printf() has to specify the field size and/or precision dynamically:

#include <stdio.h>

int main(int argc, char* argv[])
{
    // specify the field size dynamically
    printf( ":%*s:\n", 10, "*");
    printf( ":%*s:\n", 15, "*");

    // specify the precision dynamically
    printf( "%.*s\n", 10, "******************************************");
    printf( "%.*s\n", 15, "******************************************");

    return 0;
}

This has the advantage of not using the preprocessor and also will let you use variables or functions to specify the field width instead of literals.

---

If you do decide to use macros instead, please use the # operator indirectly (and the ## operator if you use it elsewhere) like so:

// macros to allow safer use of the # and ## operators
#ifndef STRINGIFY
#define STRINGIFY2( x) #x
#define STRINGIFY(x) STRINGIFY2(x)
#endif

#define STR_FMTB(x) "%" STRINGIFY(x) "s"

Otherwise if you decide to use macros to specify the field width, you’ll get undesired behavior (as described in What are the applications of the ## preprocessor operator and gotchas to consider?).