Thanks Andy,
thats not quite correct. it's like that:

void kb_init(void)
{
pcf8574_write();
}

void ext0_irq(void)
{
pcf8574_write();
pcf8574_read();
}

void main(void)
{
kb_init();
while(1){
...
}
}

Am i not allowed to call functions that i am using outside? What about an I2C ADC that i have to read out after a conversion? Do i have to duplicate the whole I2C code, or only the function for reading the ADC (or in my case PCF8574)? Do i have to guarantee, that if i am writing to another I2C IC, during this writing no IRQ can appear that has to write on I2C too?

regards,
Markus