Question

I am programming a c project which must use the pow function defined in math.h.
And when I tried to make the project, gcc gave the following link error:
undefined reference to `pow’.

I know the -lm option must be added into my link instruction, but there are still several questions puzzling me.

Q1:When I pass two constants into pow function, link is successful without -lm. Why?

Q2:-lm being at the end or at the start of link instruction makes different results. gcc -lm $(OBJS) -o exbin is wrong, but gcc $(OBJS) -o exbin -lm is correct. Why?

I use ubuntu 11.10 and gcc 4.4.4.

Thanks! Please excuse my pool English.

Answer 1

I cannot answer question 1 (that seems odd), but in regards to question 2 the reason gcc -lm $(OBJS) -o exbin does not work is because you must link things in order of useage. This is best explained by example:

/* File func_a.h */
/* Declare func_a */
void func_a();

/* File func_a.c */
#include "func_a.h"
void func_a()
{
    /* do stuff */
}

/* File func_b.c */
#include "func_a.h"
void func_b()
{
    /* Call func_a */
    func_a();
}

To properly link func_a and func_b into an executable, you must link them as gcc func_b func_a -o exec because func_b uses func_a. In short, you always want to link your library functions last.