printf("%.16e", sqrt(2.0));

##</code><code>##

printf(MY_FORMAT, sqrt(2.0));

##</code><code>##

WRITEMAKEFILE(
  .... 
  DEFINE => '-DMY_FORMAT="%.16e"',
  ....
);

##</code><code>##

use strict;
use warnings;
use Config;

use Inline C => Config =>
  USING => 'ParseRegExp',
  CCFLAGSEX => '-DMY_FORMAT="%.16e"',
  BUILD_NOISY => 1,
;

use Inline C =><<'EOC';

SV * foo(void) {
  printf(MY_FORMAT, sqrt(2.0));
  printf("\n");
}

EOC

foo();

##</code><code>##

C:\_32\C>type try.c
#include <stdio.h>
#include <math.h>

int main(void) {
printf(MY_FORMAT, sqrt(2.0));
return 0;
}

C:\_32\C>gcc -o try.exe try.c -DMY_FORMAT=\"%.16e\"

C:\_32\C>try
1.4142135623730951e+000
C:\_32\C>