stroage size of 'variable' isn't known

Question

I have a problem about "stroage size of a isn't known". I checked the stackoverflow and i found some answers. But, these solutions did not work or i could not use these answers very well.

Please help me to solve my problem. Thanks for your help and have a nice day.

main.c file

#include <avr/io.h>
#include <PORT.h>

int main(void)
{

    // Insert code
    PORT a;

    while(1)
    ;

    return 0;
}

PORT.h file

#ifndef PORT_H_INCLUDED
#define PORT_H_INCLUDED

#include <config.h>

    typedef enum PORT_TYPE{
    // All port in ATmega328P
        PORT_B,
        PORT_C,
        PORT_D
    }PORT_TYPE;

    typedef enum PORT_PIN{
    // All pins in ATmega328P
        PIN_0,
        PIN_1,
        PIN_2,
        PIN_3,
        PIN_4,
        PIN_5,
        PIN_6,
        PIN_7
    }PORT_PIN;

    typedef struct PORT PORT;

    void PORT_init(PORT * const me,
                   void (* setDirectionFunction)(PORT * const me, PORT_PIN pinNumber),
                   void (* setStatusFunction)(PORT * const me, PORT_PIN pinNumber),
                   void (* enablePullResistorFunction)(PORT * const me, PORT_PIN pinNumber),
                   void (* disablePullResistorFunction)(PORT * const me, PORT_PIN pinNumber),
                   void (* notifyPinChangeFunction)(PORT * const me, PORT_PIN pinNumber),
                   unsigned char (*readPINFunction)(PORT * const me, PORT_PIN pinNumber),
                   unsigned char (*readPortFunction)(PORT * const me));

    void PORT_setDirection(PORT * const me, PORT_PIN pinNumber);
    void PORT_setStatus(PORT * const me, PORT_PIN pinNumber);
    void PORT_enablePullResistor(PORT * const me, PORT_PIN pinNumber);
    void PORT_disablePullResistor(PORT * const me, PORT_PIN pinNumber);
    void PORT_notifyPinChange(PORT * const me, PORT_PIN pinNumber);
    unsigned char PORT_readPIN(PORT * const me, PORT_PIN pinNumber);
    unsigned char PORT_readPort(PORT * const me);

    PORT * PORT_create(PORT_TYPE whichPort);
    void PORT_destroy(PORT * const me);



#endif // PORT_H_INCLUDED

PORT.c file

#include <avr/io.h>
#include <PORT.h>

#define ADDR_PORTB (0x0023)
#define ADDR_PORTC (0x0026)
#define ADDR_PORTD (0x0029)

typedef volatile struct{
// it is used for manipulating registers.
    unsigned char pin;
    unsigned char ddr;
    unsigned char port;
}PORT_hw;

struct PORT{
    unsigned char changePIN;
    PORT_hw volatile *p_hw;
    void (* setDirection)(PORT * const me, PORT_PIN pinNumber);
    void (* setStatus)(PORT * const me, PORT_PIN pinNumber);
    void (* enablePullResistor)(PORT * const me, PORT_PIN pinNumber);
    void (* disablePullResistor)(PORT * const me, PORT_PIN pinNumber);
    void (* notifyPinChange)(PORT * const me, PORT_PIN pinNumber);
    unsigned char (*readPIN)(PORT * const me, PORT_PIN pinNumber);
    unsigned char (*readPort)(PORT * const me);
};

static PORT g_PORT[3];
g_PORT[0].p_hw = (PORT_hw volatile *)ADDR_PORTB;
g_PORT[1].p_hw = (PORT_hw volatile *)ADDR_PORTC;
g_PORT[2].p_hw = (PORT_hw volatile *)ADDR_PORTD;

void PORT_init(PORT * const me,
                   void (* setDirectionFunction)(PORT * const me, PORT_PIN pinNumber),
                   void (* setStatusFunction)(PORT * const me, PORT_PIN pinNumber),
                   void (* enablePullResistorFunction)(PORT * const me, PORT_PIN pinNumber),
                   void (* disablePullResistorFunction)(PORT * const me, PORT_PIN pinNumber),
                   void (* notifyPinChangeFunction)(PORT * const me, PORT_PIN pinNumber),
                   unsigned char (*readPINFunction)(PORT * const me, PORT_PIN pinNumber),
                   unsigned char (*readPortFunction)(PORT * const me)){
    me->changePIN = 0x00;
    me->pinStatus = 0x00;
    me->setDirection = setDirectionFunction;
    me->setStatus = setStatusFunction;
    me->enablePullResistor = enablePullResistorFunction;
    me->disablePullResistor = disablePullResistorFunction;
    me->notifyPinChange = notifyPinChangeFunction;
    me->readPIN = readPINFunction;
    me->readPort = readPortFunction;
}

void PORT_setDirection(PORT * const me, PORT_PIN pinNumber){}
void PORT_setStatus(PORT * const me, PORT_PIN pinNumber){}
void PORT_enablePullResistor(PORT * const me, PORT_PIN pinNumber){}
void PORT_disablePullResistor(PORT * const me, PORT_PIN pinNumber){}
void PORT_notifyPinChange(PORT * const me, PORT_PIN pinNumber){}
unsigned char PORT_readPIN(PORT * const me, PORT_PIN pinNumber){}
unsigned char PORT_readPort(PORT * const me){}


PORT * PORT_create(PORT_TYPE whichPort){
    PORT *p_PORT = &(g_PORT[whichPort]);
    PORT_init(p_PORT, PORT_setDirection, PORT_setStatus, PORT_enablePullResistor, PORT_disablePullResistor, PORT_notifyPinChange, PORT_readPIN, PORT_readPort);
    return p_PORT;
}
void PORT_destroy(PORT * const me){}

Answer 1

The PORT.h header file declares that struct PORT exists, but doesn't define it:

typedef struct PORT PORT;

The actual definition is in PORT.c, which is not visible in main.c.

As a result, you can't create a variable of type PORT. You can however create one of type PORT *. Because the definition is not known, this is referred to as an opaque pointer.

Judging by the functions defined in PORT.h, you would use the PORT_create function to return a PORT * that can be subsequently passed to other functions.