%g

%g

%g

%g

%g

#include <math.h>
double x;
...
if (floor(x) == x && x != 0.0) { // x is an non-zero integer value
    int digits = floor(log10(fabs(x)+0.5)) + 1;
    printf("x is %.*fn", digits, x);
}

x          log10(x+0.5)
         1  0.176091259055681000
         9  0.977723605288848000
        10  1.021189299069940000
        99  1.997823080745730000
       100  2.002166061756510000
       999  2.999782798454140000
      1000  3.000217092972230000
     10000  4.000021714181250000
    100000  5.000002171466980000
   1000000  6.000000217147190000
  10000000  7.000000021714720000
 100000000  8.000000002171470000
1000000000  9.000000000217150000