"while (pushb != 0); // wait until button is
// pressed "

So if push button port is never operated, will your MCU wait just there endlessley ?

The proper approach would be :

1. Read Push button port.
2. Is it low ? If NO quit this routine.
3. If it is low, then start a timer for debounce_delay.
4. After debounce_delay is over, read push button port.
5. If it is still low, then you have a real signal. Process it as required.
6. If it is not low, then it is a false signal or is still debouncing.

For the debounce_delay, you can start with about 20 ms delay. Should suffice mostly but can stretch upto 40ms or more at times depending on the condition of relay contacts / pusbutton contacts.

PS: The above is one of several such methods and goes by the name of polling.

Raghu