Compile-time 'Undefined reference' error for functions defined in a .cpp source file that are called in my main project file

Question

I'm currently working with the NodeMCU ESP12E and MPU6050 IMU using the Arduino IDE and wanted to create a project that has a .h file that declares the variables and functions. In the .cpp file, I have various function definitions. The main.ino file just calls some of these functions. Please find some excerpts of code from each file and the errors I encountered. The errors say the three functions called in the .ino file are undefined even though they are defined in the .cpp file

Algo.h file:

extern int activity_state; // activity_state=1 for RUN and activity_state=0 for WALK
extern float * speedarray; // Dynamic array for storing speed
extern int speedindex;
extern float * timearray; // Dynamic array for storing time
extern int timeindex;
extern unsigned int localPort; 

Algo.cpp:

static void pedometerInit() // Initialising all pedometer algorithm variables
{
 int i = 0;
 //pedometerAlgo varibles
 pedi_window_high = 2000;
 pedi_window_low = 200;
 pedi_sampling_counter = 0;
 pedi_refresh_frequency = 50;
 validZone = 1;
 jatin_index_1 = 0;
 jatin_mag = 0;
 pedi_max = 0;
.
.

All the functions work on the variables declared in the header file.

Ped_algo.ino:

void setup() {
 Serial.begin(115200);
 WiFi.mode(WIFI_STA);
 WiFi.begin(STASSID, STAPSK); // Setting up WiFi station
 while (WiFi.status() != WL_CONNECTED) {
 Serial.print('.');
 delay(500);
 }
 Udp.begin(localPort); // Setting up UDP connection
 MPU6050_Init(); // Initial configuration of the MPU6050
 pedometerInit();
}
void loop() {
 pedometerAlgo();
}

Error messages:

Ped_Algo.ino.cpp.o:(.text.loop+0x0): undefined reference to pedometerAlgo()' in functionloop': undefined reference to pedometerInit()' undefined reference toMPU6050_Init()' undefined reference to `pedometerInit()'

Error compiling for board NodeMCU 1.0 (ESP-12E Module).

• KIIV: Well, for example in the static void pedometerInit() that static means linker is not allowed to use it outside its own compilation unit Algo.cpp (Algo.o)
• Michel Keijzers: Also, it seems you don't have the constructor pedometerAlgo::pedometerAlgo() implemented.
• Juraj: did you include Algo.h in .ino?
• dkr: I have included Algo.h in the .ino file. Also, I'm not using classes here. And removing the static, gives me the undefined reference error to all the variables used in the functions.
• Majenko: As well as extern in the header for your variables you have to actually create those variables somewhere. extern just t tells the compiler that they exist somewhere else. You have to ensure that they actually do exist somewhere.

Answer 1

Firstly, static keeps a function from ever being exported globally:

static void pedometerInit() // Initialising all pedometer algorithm variables

should not be static.

Secondly, extern does not define a variable, it merely tells the compiler that "this variable exists somewhere. Find it at link time." - however you're not actually defining those variables anywhere. In addition to the extern hints in the .h file you need to have the variables in the .cpp file for the compiler to be able to find them.

int activity_state; // activity_state=1 for RUN and activity_state=0 for WALK
float * speedarray; // Dynamic array for storing speed
int speedindex;
float * timearray; // Dynamic array for storing time
int timeindex;
unsigned int localPort; 
void pedometerInit() // Initialising all pedometer algorithm variables
{
 ... your code ...
}