How to determine button press duration with wiringpi in C

For a aspberry pi project I have buttons connected to GPIO and I want to take different actions depending on how long a button has been pressed. The following python code (extract) is working as expected:

on_button2(channel):
    t1 = time.time()
    # Wait for button release
    while GPIO.input(channel) == 0:
        pass
    duration = time.time() - t1
    print "duration: %f" % (duration)
    if duration > 0.75:
        mpd_client.previous()
    else:
        mpd_client.next()

GPIO.add_event_detect(BUTTON2_PIN, GPIO.FALLING, callback=on_button2, bouncetime=700);

I would like to convert this into a C program (don't ask why, I really don't like python and I'm much more familiar with C, so I'd like to do it in C)

Trying to convert this to C with wiringPi, I came up with this, but it is not working as expected:

unsigned long btn2_t0;

static void on_button_2_pressed() {
    unsigned long duration, t1;
    int level;

    level = digitalRead(BUTTON_2_PIN);

    // Debounce button
    t1 = millis();
    if (t1 - btn2_t0 < 700) {
        return;
    }
    btn2_t0 = t1;

    // Wait for button being released
    while (digitalRead(BUTTON_2_PIN) == LOW) {
        delay(100);
    }

    duration = millis() - t1;

    if (duration > 5000) {
        printf("Self destruction sequence initiated!\n");
    }
    else if (duration > 700) {
        player_previous();
    }
    else {
        player_next();
    }
}

int main() {
    // Setup WiringPi Lib
    wiringPiSetupGpio();
    pinMode(BUTTON_2_PIN, INPUT);

    // Register callbacks on button press
    wiringPiISR(BUTTON_2_PIN, INT_EDGE_FALLING, on_button_2_pressed);

    for (;;) {
        delay(500);
    }
    return 0;
}

It seems that the loop that should wait for button release is not being executed or that the while-condition is always true, and thus, duration always is zero.

Is the digitalRead(BUTTON_2_PIN) function equivalent to the GPIO.input(channel) in the python code at all?

If somebody could point me in the right direction on how to detect button press (software-debounced) and measure the duration of the button press in C.

Thanks a lot.

EDIT: Working solution

After playing around a lot and with the help of Francesco Boi I came around with a working solution, although I do not really understand why the compare logic with HIGH / LOW is swapped compared to the python code (I thought that the button press will cause the pin to FALL to LOW and releasing it would RAISE it to HIGH ...)

static void on_button_2_pressed() {

    unsigned long duration;
    static unsigned long button_pressed_timestamp;
    int level = digitalRead(BUTTON_2_PIN);

    if (level == HIGH) { // Why HIGH ?!?
        button_pressed_timestamp = millis();
    }
    else {
        duration = millis() - button_pressed_timestamp;
        button_pressed_timestamp = millis();

        if (duration < 100) {
            // debounce...
            return;
        }
        else if (duration < 700) {
            syslog(LOG_NOTICE, ">> NEXT\n");
        }
        else if (duration < 3000) {
            syslog(LOG_NOTICE, "<< PREV\n");
        }
        else {
            syslog(LOG_NOTICE, "!! REBOOT\n");
        }
    }
}

int main() {
    ...
    wiringPiISR(BUTTON_2_PIN, INT_EDGE_BOTH, on_button_2_pressed);
    ...
}

Solution

First of all you are reading the value of the pin into level then in the while loop you are re-reading it: why is that? Can't you do something like:

// Wait for button being released
while (level == LOW) {
    delay(100);
}

Also don't you want to reassign the btn2_t0 = t1; even when the time was smaller than 700 ms? Like:

t1 = millis();
if (t1 - btn2_t0 < 700) {
  btn2_t0 = t1;  
  return;
}

The behaviour depends on how you built your electronic circuit: is the button-press suppose to make the pin high or low? Be sure about the behaviour is the one you are expecting by either connecting a led and a resistor or using a Voltmeter.

However since your Python code worked I am assuming the electronic is correct and your algorithm too.

Be sure of course you are pressing the button long enough. Add some prints to your code to understand which branches it is executing because when there is electronic involved it is difficult to understand only by the code.

While waiting for news from you, in my opinion is better to do the following: define the callback as: INT_EDGE_BOTH so that it got called when the bottom is pressed and when the button is released. You can keep the time elapsed with a static variable.

void yourCallback()
{
    unsigned long ela, t;
    int level = digitalRead(BUTTON_2_PIN);
    static unsigned long button_pressed_timestamp;
    //if button pressed:
    if(level==LOW)
    {
      //start counting
      button_pressed_timestamp = millis();
    }
    else //button released
    {
       duration = millis()-button_pressed_timestamp;
       button_pressed_timestamp = millis(); //just to be sure....
       if (duration > 5000) {
          printf("Self destruction sequence initiated!\n");
       }  
       else if (duration > 700) {
          player_previous();
       }
       else {
          player_next();
       }
    }


}