How to display the full length of a double with printf in C

Question

How can I use printf in C to display the full length of a double.

Consider

int main(){

double x = 0.00000000000001;

printf("x: %f \n", x);

}

The result I get is "i: 0.000000", however from what I read %f is the correct format specifier and doubles should have 15-17 significant figures therefore I don't understand why I am getting a rounded result. I have also tried things like %15f and %lf.

I am using Ubuntu if that helps.

Answer 1

From http://linux.die.net/man/3/printf:

f, F

The double argument is rounded and converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the decimal-point character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is explicitly zero, no decimal-point character appears. If a decimal point appears, at least one digit appears before it.

Answer 2

If you want to output 15 digits after the decimal points, it should be %.15f instead of %15f:

printf("i: %.15f \n", i);