pow doesn't accept the second parameter to be a variable on gcc

Question

pow doesn't accept the second parameter to be a variable on gcc

The following code works fine on VC++10

// file test.cc
#include "stdafx.h"
#include <stdio.h>
#include <math.h>

int main(void)
{   
    double x = 10;
    int y = 20;
    printf("%f\n", pow(x, y));
    return 0;
}

But the following code doesn't not work on gcc:

// test.c
#include <stdio.h>
#include <math.h>

int main(void)
{   
    double x = 10;
    int y = 20;
    printf("%f\n", pow(x, y)); // error here, says no such function, however when pass the second argument in `pow` for the code runs by gcc, It works fine!
    return 0;
}

Answer 1

You're mistaken. It has nothing to do with the second parameter.

In POSIXish systems pow() is in libm, whereas in win32ish systems it is part of the standard C library. That means instead of this:

$ gcc program.c
/tmp/ccTw1gCA.o: In function `main':
program.c:(.text+0x30): undefined reference to `pow'

you need to do this:

$ gcc program.c -lm

Answer 2

The reason it may appear that the second parameter works as a constant but not as a variable is that gcc has a built-in implementation of pow(). If the second parameter is a constant it might be using that where if it's a variable it's falling back on the glibc pow() function. See:

http://gcc.gnu.org/onlinedocs/gcc-4.5.0/gcc/Other-Builtins.html#Other-Builtins

If you pass -fno-builtin to gcc you should see consistent behavior--in this case error messages no matter what you pass to pow(). As others have mentioned whenever you use anything out of math.h you need to link with -lm.