segmentation fault in a thread

Hi there,
After going throug a lot of info on the internet I have this question:
if I do a outb() in a thread I get a segmentation fault. ioperm() is in that routine. Is that due to the fact that the created thread doesn't run under root? And is that normal? If I understand correctly child processes with fork() run under another user, but pthread runs under the same user (root).
Or is this not correct.
My program has now 3 parts: the main program does the file i/o work and has a timer interrupt routine (sigalrm) which triggers a thread that handles all time related things and a 3rd one which does the i/o to the par.port where the a/d converter is connected.
The latest routine now gives the seg.fault. It worked when the par.port i/o was in the main program, but then I had other problems.
Can anybody help me with this?

This is from the fork man page: fork creates a child process that differs from the parent process only in its PID and PPID. That would mean that the child process would run as the same user as the parent does.

So as far as I understand you have three different processes running and not three different threads. One of the processes triggers the other to start doing stuff by means of sending a signal at a timer event. After moving the code from the original main to a new process it started segfaulting. Is there any parts of code that initialized certain variables in the main loop which you haven't copied over to the seperate process? Maybe I am understanding it wrong but how is the data passed from the process that is reading/writing to file to the process that is reading/writing to the serial port?