Undefined reference to `pow' and `floor'

Question

I'm trying to make a simple fibonacci calculator in C but when compiling gcc tells me that I'm missing the pow and floor functions. What's wrong?

Code:

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

int fibo(int n);

int main() {
        printf("Fib(4) = %d", fibo(4));
        return 0;
}

int fibo(int n) {
        double phi = 1.61803399;

        return (int)(floor((float)(pow(phi, n) / sqrt(5)) + .5f));
}

Output:

gab@testvm:~/work/c/fibo$ gcc fib.c -o fibo
/tmp/ccNSjm4q.o: In function `fibo':
fib.c:(.text+0x4a): undefined reference to `pow'
fib.c:(.text+0x68): undefined reference to `floor'
collect2: ld returned 1 exit status

Answer 1

You need to compile with the link flag -lm, like this:

gcc fib.c -lm -o fibo

This will tell gcc to link your code against the math lib. Just be sure to put the flag after the objects you want to link.

Answer 2

Add -lm to your link options, since pow() and floor() are part of the math library:

gcc fib.c -o fibo -lm

Answer 3

For the benefit of anyone reading this later, you need to link against it as Fred said:

gcc fib.c -lm -o fibo

One good way to find out what library you need to link is by checking the man page if one exists. For example, man pow and man floor will both tell you:

Link with -lm.

An explanation for linking math library in C programming - Linking in C

Answer 4

In regards to the answer provided by Fuzzy:

I actually had to do something slightly different.

Project -> Properties -> C/C++ Build -> Settings -> GCC C Linker -> Libraries

Click the little green add icon, type m and hit ok. Everything in this window automatically has -l applied to it since it is a library.

Answer 5

To find the point where to add the -lm in Eclipse-IDE is really horrible, so it took me some time.

If someone else also uses Edlipse, here's the way how to add the command:

Project -> Properties -> C/C++ Build -> Settings -> GCC C Linker -> Miscelleaneous -> Linker flags: in this field add the command -lm