Why libthread has threadcreate instead of something like fork? With threadcreate you should make struct to pass more than one argument and pass a pointer to it.
I also see no problem with recieving different values for each thread. Memory is shared, but return value is stored in register (accumulator register for x86) which is not shared. Maybe it was made for compatibility with Alef? In Alef it is possible to return mulitply values, maybe they are stored in memory. But there is rfork with RFMEM flag and it has no problem. And by the way when libthread should be used and when rfork(RFMEM) is better?
