Implementing a User-Level Threads Package

Question

I have been tasked in a class to create a user level thread library in C. I was wondering if anyone could give me a list of things to read up on to accomplish this. I have a good idea as to where to start, but any resources on user level threads and some applicable aspects of the C language that might help would be extremely valuable.

I am very unclear as to how I would implement a scheduler for such. Assume that I have a pretty good understanding of the C language and some of its more helpful library functions.

Answer 1

I’ve done this for a homework assignment without writing any assembler at all. The thread switch mechanism was setjmp/longjmp. What this involved was allocating memory for each thread’s stack, then very carefully massaging the values in the jmp_buff so execution jumps to the next thread’s stack.

See also Russ Cox’s pretty readable libtask.

Edit in response to OP’s comment: In deciding when to switch threads there are two main directions: preemptive & cooperative. In the preemptive model, you’ll have something like a timer signal that causes execution flow to jump to a central dispatcher thread, which chooses the next thread to run. In a cooperative model, threads “yield” to each other, either explicitly (e.g., by calling a yield() function you’ll provide) or implicitly (e.g., requesting a lock held by another thread).

Take a look at the API of libtask for an example of the cooperative model, particularly the description of the function taskyield(). That’s the explicit yield I mentioned. There are also the non-blocking I/O functions which include an implicit yield—the current “task” is put on hold until the I/O completes, but the other tasks get their chance to run.