error function declared but never defined - c

Question

I have added a library to my c project in codeVision AVR. when I want to use it's functions receive this error:
function 'function name' is declared but never defined.
here is my code:
#include "pid.h"
#include <mega32.h>
PidType _pid;
void main(void)
{
//some uC hardware initializing codes which are removed here to simplify code
PID_Compute(&_pid);
while (1)
{
// Place your code here
}
}
in pid.h:
.
.
bool PID_Compute(PidType* pid);
.
.
and pid.c:
#include "pid.h"
.
.
bool PID_Compute(PidType* pid) {
if (!pid->inAuto) {
return false;
}
FloatType input = pid->myInput;
FloatType error = pid->mySetpoint - input;
pid->ITerm += (pid->ki * error);
if (pid->ITerm > pid->outMax)
pid->ITerm = pid->outMax;
else if (pid->ITerm < pid->outMin)
pid->ITerm = pid->outMin;
FloatType dInput = (input - pid->lastInput);
FloatType output = pid->kp * error + pid->ITerm - pid->kd * dInput;
if (output > pid->outMax)
output = pid->outMax;
else if (output < pid->outMin)
output = pid->outMin;
pid->myOutput = output;
pid->lastInput = input;
return true;
}
the ERROR:
function 'PID_Compute' declared, but never defined.
Where is the problem?
EDIT:
to add the library to my project I placed the .c and .h library files in the same folder that my main project file is:
and then #include "pid.h" in my main file:
#include "pid.h"
#include <mega32.h>
// Declare your global variables here
PidType _pid;
void main(void)
{
.
.
my error and warnings:
EDIT2:
I simplified the code and now can show you the entire code:
main code:
#include "pid.h"
PidType _pid;
void main(void)
{
PID_Compute(&_pid);
while (1)
{
}
}
pid.h:
#ifndef PID_H
#define PID_H
#include <stdbool.h>
typedef struct {
int i;
} PidType;
bool PID_Compute(PidType* pid);
#endif
pid.c:
#include "pid.h"
bool PID_Compute(PidType* pid) {
pid->i = 2;
return true;
}

Answer 1

thank you every body.
As you said, the pid.c was not added to the project.
for those who may face the same problem:
in codeVision AVR we have to add .c file to project from project->configure->files->input files->add
I addec .c file to project and the error went away.

Answer 2

From the screenshot with the tree view of your files it is clear that the file "pid.c" is not part of the project.
Move it into your project. Then it should build without that linker error.
This does not mean the location in the file system. I reference the "virtual" view of the IDE on your project.