C float or double in printf format specifier and pow function

Question

#include <stdio.h>
#include <math.h>

int main(void){

   printf("%lf\n", pow(1.0, 2.0));
   printf("%f\n", pow(1.0, 2.0));

   return 0;    
}

First printf() gives output 0.000000 but second printf() gives output 1.000000. Why?

Using Codeblocks on Windows 7 64 bit.

Using gcc command to compile give me an .exe that outputs 1.000000 for both statements.

If I compile pressing F9 on Codeblocks, I get 0.000000 for the first statement and 1.000000 for the second.

Finally, if I remove #include <stdio.h> from source code in Codeblocks, all give me 1.000000 (no warnings or errors).

Answer 1

Your code should print the same value for both lines on a C99 or C11 compiler. All float arguments to printf are always converted to double, and %lf and %f both do the same thing (print a double).

In the C89 standard, the %lf specifier is undefined behavior and doubles should only be printed with %f. So perhaps you're using an old compiler which doesn't support the C99 standard.

Relevant sections of the standards regarding %lf:

C99, 7.19.6.1/7:

l (ell) [...] has no effect on a following a, A, e, E, f, F, g,or G conversion specifier.

C89, 4.9.6.1:

[...] an optional l (ell) specifying that a following d , i , o , u , x , or X conversion specifier applies to a long int or unsigned long int argument; an optional l specifying that a following n conversion specifier applies to a pointer to a long int argument; [...] If an h , l , or L appears with any other conversion specifier, the behavior is undefined.