Help with programming in C

[mzkkbprmt]

Hi guys. I've recently started learning C. I have borrowed a good book from the library, and, as an exercise, I had to make a program to show the relative mass of an object at speeds close to c, for a rest mass of 1 kg. This is my program:

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

#define START 0.9*3e8
#define END 0.99*3e8
#define INC 0.01*3e8

double convert(double v)
{
	double m, m0, c;
	m0 = 1.0;
	c = 3.0e8;

	m = m0 / sqrt(1 - (v / c) * (v / c));
	return m;
}

int main(void)
{
	double v;
	double mass;
	
	for (v = START; v <= END; v += INC)
	{
		mass = convert(v);
		printf("At velocity %f%% of c, an object of rest mass 1 kg weighs %f kg", v, mass);
	}
	return 0;
}

However, when trying to compile I get this error.

andrew@andrew-laptop:~/Documents/C$ gcc relmass.c -o relmass
/tmp/cc4LgTLT.o: In function `convert':
relmass.c:(.text+0x50): undefined reference to `sqrt'
collect2: ld returned 1 exit status

As far as I understanding it it is saying that sqrt() is undefined. However I know that sqrt() is defined in math.h which I did include. So can anyone tell me what is going wrong.

If it helps I am running ubuntu 9.04 with gcc version 4.3.3 (Ubuntu 4.3.3-5ubuntu4).

Can anyone tell me why this program won't compile, or spot any mistakes?

Thanks for your help.

[element]

You need to link the math shared library in when you compile. Try:

gcc relmass.c -lm -o relmass

The -lm flag tells GCC to link in the libm math library which has the sqrt (and other assorted math functions defined in math.h) function defined in it.

[mzkkbprmt]

Thanks for that. Solved it.

Note to self - find time to read through man gcc