GCC inline C function causes error

Question

This is strange to me.

inline complex_t complex_add(complex_t a, complex_t b)
{
    complex_t r = {a.re + b.re, a.im + b.im};
    return r;
}

inline complex_t complex_subtract(complex_t a, complex_t b)
{
    complex_t r = {a.re - b.re, a.im - b.im};
    return r;
}


inline complex_t complex_multiply(complex_t a, complex_t b)
{
    complex_t r = {a.re * b.re - a.im * b.im, a.re * b.im + a.im * b.re};
    return r;
}

inline complex_t complex_conjugate(complex_t a)
{
    complex_t r = {a.re, -a.im};
    return r;
}

inline complex_t complex_divide(complex_t a, complex_t b)
{
    float dem = b.im * b.im + b.re * b.re;

    complex_t r = complex_multiply(a, complex_conjugate(b));
    r.re /= dem;
    r.im /= dem;
    return r;
}

int divergence(double x, double y, int iters, complex_t* r)
{
    const complex_t c = {x, y};
    complex_t z = {0, 0};
    int i;
    for(i = 0; i < iters && complex_mag(z) < 2; i++)
    {
        z = complex_add(complex_multiply(z, z), c);
    }
    *r = z;
    return i;
}

When I compile this code I get the following error in the line z = complex_add(complex_multiply(z, z), c)...

home/chase/workspace/MandelbrotSet/Debug/../main.c:62: undefined reference to `complex_multiply'
makefile:44: recipe for target 'MandelbrotSet' failed
/home/chase/workspace/MandelbrotSet/Debug/../main.c:62: undefined reference to `complex_add'
collect2: error: ld returned 1 exit status
make: *** [MandelbrotSet] Error 1

However when I remove inline from the complex number functions the code compiles fine. Any ideas why? How come adding the inline makes it so the compiler can not find the function?

Which compiler flags are you using? (the behaviour of `inline` differs between C90, C99, and GNU89) — M.M, Aug 21 '15 at 00:07
Also you willl need to indicate where this code fits into your project structure. For example if these function bodies were in a `.c` file and you tried to call them from a different `.c` file you would get undefined reference errors — M.M, Aug 21 '15 at 00:12
inline doesn't happen at link time. where did you put your inline function? — Jason Hu, Aug 21 '15 at 00:15
Everything is in the same c file. I just tested it. It works with c90 but not c99. Why? — chasep255, Aug 21 '15 at 00:15
@chasep255 As Matt McNabb said, the behaviour of `inline` in gcc varies depending on the language you select. Specifically, there is the GNU behaviour and the C99 behaviour. Consult the gcc manual for more details. — fuz, Aug 21 '15 at 00:16
@BLUEPIXY: `static` [should preceed](http://port70.net/~nsz/c/c11/n1570.html#6.11.5) the function specifier actually. — too honest for this site, Aug 21 '15 at 00:34
Do you have any evidence to suggest that `inline`ing these functions will significantly improve the performance of your program? — autistic, Aug 21 '15 at 00:47
"Everything is in the same c file" - ok, so your code is the same structure as the example in [this question](http://stackoverflow.com/questions/6312597/is-inline-without-static-or-extern-ever-useful-in-c99) which has links to some very detailed explanations — M.M, Aug 21 '15 at 02:02

GCC inline C function causes error

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