Undefined reference to function in custom library

Question

Hello I am making a private Arduino library but am having trouble understanding how it is linked. Why does this h file work, but when I move the definition of board_led to a matching c file, it throws an error at compilation: In function loop: undefined reference to board_led(bool).

I am using the following board manager to compile in the Arduino IDE: https://raw.githubusercontent.com/maximintegratedmicros/arduino-collateral/master/package_maxim_index.json

I am using the following board target: MAXIM ARM 32bit boards > MAX32630FTHR

#pragma once
#include "Arduino.h"
#include <stdbool.h>
#include <gpio.h>
extern const gpio_cfg_t s_board_led;
extern void board_led(bool on) {
 GPIO_OutPut( &s_board_led, on ? 0 : ~0 );
}

Answer 1

I suspect you're compiling from C++ code, and including that header.

When C++ links to C code, the C++ code needs to see extern "C" on the declaration, or surrounding the declaration (via extern "C" { /* function declarations */ })

If you are compiling C++ code against the C code (and linking them together), try making sure extern "C" is used, for example like this:

// arduino_lib.h
#pragma once
#include "Arduino.h"
#include <stdbool.h>
#include <gpio.h>
extern const gpio_cfg_t s_board_led;
extern "C" void board_led(bool on);

While nothing changes in the .c file

// arduino_lib.c
#include "arduino_lib.h"
const gpio_cfg_t s_board_led = /* some value */;
void board_leb(bool on) {
 GPIO_OutPut( &s_board_led, on ? 0 : ~0 );
}

Answer 2

To complement Bob's answer it is good practice to add header guards for detecting C++ specifically:

#ifdef __cplusplus
extern "C" {
#endif // __cplusplus
// your function definitions here
#ifdef __cplusplus
}
#endif // __cplusplus

That way the extern "C" is only applied if the header file is included into a c++ file but not if it's included into a c file.