Read and write from RTC by ioctl

First problem: I use the RTC by I2C and ioctl. When I read and write something, for example:

while(1)
{
waitForInterrupt();
readFromRTC();
writeToRTC();
}

After writeToRTC() program stops.

How to protect from this?

Second problem: How to use readFromRTC() and writeToRTC() in 2 threads? Thanks for help.