I used pow() function in my C program but did not link to -lm it still worked. Why?

Question

I was reading a book on GCC. It said only the standard library is linked by default for any C program. Since the pow() is not in the standard library, I will have to link to it using the -lm flag. However, when I compiled, I simply used:

gcc hello.c -o hello

and it still worked..

And there is another similar problem, the book also said that if you have printf("%f\\n", 4); in your C program and if you compile WITHOUT -Wall option, no warning will be issued. However, I tried compiling it without the -Wall option but I still got a warning:

hello.c:6:2: warning: format '%f' expects argument of type 'double', but argument 2 has type 'int' [-Wformat]

Why is this? The book said I have to supply -lm and -Wall in order to make my program compiled and get the warning but I did not use either of them but I still got my program compiled and got the warning?

Thank you!

Answer 1

A bit more information based on some experimentation.

Consider this program:

#include <stdio.h>
#include <math.h>
int main(void) {
#ifdef CONSTANT
    double x = pow(2.0, 10.0);
#else
    double expon = 10.0;
    double x = pow(2.0, expon);
#endif
    printf("x = %f\n", x);
    return 0;
}

On Ubuntu, when I compile it with

$ gcc c.c -o c

it complains about an undefined reference to pow ; adding -lm corrects it.

But this:

$ gcc -DCONSTANT c.c -o c

compiles and links without error, replacing the pow() call with a constant 1024.0 . But this:

$ gcc -fno-builtin -DCONSTANT c.c -o c

complains again about the undefined reference to pow .

Conclusion: gcc uses the built-in implementation of pow only when the result can be determined at compile time. Otherwise it generates an explicit call to the pow() function, which requires linking with -lm .

Note also that this depends on the way the C library is organized (the library is provided separately; it's not part of gcc). On Cygwin, which uses newlib rather than the glibc used on Ubuntu, the -lm option is not needed; apparently the math routines are an integral part of the standard library rather than being provided separately. (gcc on Cygwin still accepts the -lm option.)

Answer 2

GCC supplies several standard library functions as built-ins :

GCC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and will not be documented here because they may change from time to time; we do not recommend general use of these functions.

The remaining functions are provided for optimization purposes.

If you look at the list of built-ins, you'll see that pow is one of them.

If you add -fno-builtin to your compiler options, you should get the linker error that you're expecting.