Raspberry Pi and I2C in c++ with wiringPi for PCF8591

I'm trying to understand I2C bus for controlling a PCF8591 D/A Converter with the wiringPi C library on my Raspberry Pi B+.

I wrote a test code (in c++) which work fine, but I don't know if I'm doing it the right way. The program reads and display the 4 analog input values each 0.5 seconds, and set the output accordingly to the average value of the 4 inputs pins.

#include <iostream>
#include <wiringPi.h>
#include <wiringPiI2C.h>

using namespace std;

int readInput(int fd, int reg)
{
    wiringPiI2CReadReg8(fd, reg);
    return wiringPiI2CReadReg8(fd, reg);
}

int main()
{
    wiringPiSetupGpio();
    int dacModule = wiringPiI2CSetup(0x48);
    if (dacModule < 0)
    {
        cout << "I2C Setup Error" << endl;
        return 0;
    }

    int i;
    int A[4]        = {0,       0,      0,      0};
    int A_Reg[4]    = {0x40,    0x41,   0x42,   0x43};

    while (1)
    {
        for (i=0;i<4;i++) A[i] = readInput(dacModule, A_Reg[i]);

        wiringPiI2CWriteReg8(dacModule, 0x40, (A[0]+A[1]+A[2]+A[3])/4);

        cout << endl;
        for (i=0;i<4;i++) cout << i << " : " << A[i] << endl;
        delay(500);
    }
    return 0;
}

My questions are :

• I noticed I need to call the wiringPiI2CReadReg8 function twice in order to read the correct input value, why ? (If I don't do so, the function return the previous input value).


• I read there is a special module of the wiringPi library specially made for the PCF8591 (This one), is there any performance differences between the two approach (special lib vs generic I2C lib) ?

Thanks a lot for your help / feedbacks.