Undefined reference with extern C

问题

I am trying to compile a program with Netbeans (g++) that inlcudes Aquila, an open source libary. I followed the installation instructions.

But when trying to compile a small test program, I get this error

In function `Aquila::OouraFft::fft(double const*)':OouraFft.cpp:(.text+0x24f):
undefined reference to `cdft'

OouraFft.h:

#include "Fft.h"

extern "C" {
    void cdft(int, int, double *, int *, double *); //prototypes of offending function
    void rdft(int, int, double *, int *, double *); //second one.
}

The C file in the libs folder contains the definition of the functions.

The line that cause the actual error in OouraFft.cpp:

// let's call the C function from Ooura's package
    cdft(2*N, -1, a, ip, w);

So I am thinking the external C file is not being linked with the project but included all the directories in the project directory. I have been googling for hours and can't figure it out.

回答1:

Up to git version 6f5d6b, Aquila's CMake config will generate static library by default. You should copy /path-to-your-build-tree/lib/libOoura_fft.a to /path-to-your-install-dir/lib/, and link it to your test program.

回答2:

As Answeror said you need to put libOoura_fft.a in the right place (/usr/local/lib is the default for libAquila.a).

You also need to link to it as Chris Vandevelde said, but you should link to -lOoura_fft (first "O" capital). You should be linking with -lAquila as well

Just wanted to clarify.

来源：https://stackoverflow.com/questions/22033417/undefined-reference-with-extern-c