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>