How to fix C++ "Undefined reference to ..." C function pointer?

I am trying to call a function (from an #included library) that takes as a parameter a function pointer, and passing to this function a pointer to a function located in a C file. The compiler throws an "undefined reference to [function name]" error.

I tried removing the code from the .c file and putting it directly into main.cpp file (see below the section marked 'THIS WORKS') - and the error is avoided. I know that I should be able to keep it in the .c file because I am following very closely to an example that compiles without errors.

/****************/
/*** MAIN.CPP ***/
/****************/

extern "C"
{
    #include "btntask.h"
}

using namespace touchgfx;

/* Kernel includes. */
#include "FreeRTOS.h"
#include "task.h"
#include "queue.h"

#define configGUI_TASK_PRIORITY                 ( tskIDLE_PRIORITY + 3 )
#define configGUI_TASK_STK_SIZE                 ( 1024 )

static void GUITask(void* params)
{
    /* STUFF */
}


/*********** THIS WORKS ************/

/*
void btn_tasked(void* params)/{
    /* STUFF */
}
*/

/*********** THIS WORKS ************/


int main(void)
{
    xTaskCreate(GUITask, "GUITask",
                configGUI_TASK_STK_SIZE,
                NULL,
                configGUI_TASK_PRIORITY,
                NULL);

    /* error undefined reference to btn_task */
    xTaskCreate(btn_task, "BTNTask",
               512,
               NULL,
               configGUI_TASK_PRIORITY+1,
               NULL);

    for (;;);
}

This is btntask.h

/****************/
/*** btntask.h ***/
/****************/
#ifndef BTNTASK_H
#define BTNTASK_H

    void btn_task(void* params);

#endif /* BTNTASK_H */

This is btntask.c

/****************/
/*** btntask.c ***/
/****************/

#include "btntask.h"

void btn_task(void* params)
{
  /* STUFF */
}

The compiler log is as follows:

Converting images
Compiling Core/Src/main.cpp
Linking TouchGFX/build/bin/target.elf
TouchGFX/build/ST/STM32F429IDISCO/Core/Src/main.o: In function `main':
d:\Dropbox\TouchGFXProjects\MiniGame\Project/Core/Src/main.cpp:116: undefined reference to `btn_task'
collect2.exe: error: ld returned 1 exit status
gcc/Makefile:363: recipe for target 'TouchGFX/build/bin/target.elf' failed
make[2]: *** [TouchGFX/build/bin/target.elf] Error 1
gcc/Makefile:359: recipe for target 'generate_assets' failed
make[1]: *** [generate_assets] Error 2
../gcc/Makefile:45: recipe for target 'all' failed
make: *** [all] Error 2

The compilation is performed by the software package I am using (TouchGFX). If it is helpful, this is the compilation commands reported in the log:

touchgfx update_project --project-file=simulator/msvs/Application.vcxproj && touchgfx update_project --project-file=../EWARM/application.ewp && touchgfx update_project --project-file=../EWARM6/project.ewp && touchgfx update_project --project-file=../MDK-ARM/application.uvproj

make -f ../gcc/Makefile -j8

* UPDATE * I noticed that TouchGFX populates a Debug folder of .obj files, one for each of the source code files in the application, and I can see that it is missing a btntask.obj. Clearly it is not linking the btntask.obj file. I need to figure out why that is. There is a makefile that details all of the linking, but it uses a lot of syntax I am not familiar with.

* SOLUTION * Turns out that the Makefile names a list of directories to include. The solution was to edit the list to add additional directories where my btntask files are located.

# Directories containing application-specific source and header files.
# Additional components can be added to this list. make will look for
# source files recursively in comp_name/src and setup an include directive
# for comp_name/include.
components := TouchGFX/gui target TouchGFX/generated/gui_generated

Thanks everyone for chiming in.