Creating Processes, Threads, etc.

Kinds of Program Control Flows

1. User-level threads, where you save and restore processor registers yourself, usually via an explicit "swap" subroutine call.  Also called "coroutines" or "user/userspace threads", or (by Sun Java folks) "green threads".  Typical examples include "minithread" (above), Netscape Threads, or any of a thousand tiny projects mostly relying on setjmp/longjmp or "makecontext/swapcontext".  Each thread has its own stack, usually just allocated with "malloc", but shares the rest of memory with other user-level threads.
   
2. Kernel-level threads, or just "threads", are where the OS saves and restores processor registers to switch threads at interrupt time.  Also called "native threads". The most common example is pthreads.  There's actually a hybrid of kernel and user-level threads where you save and restore processor registers yourself (not in the OS), but you do it inside a signal handler during an interrupt.
   
3. Processes, where the OS swaps out all of memory and all I/O connections in addition to the registers.  This is what you usually think of as a "program".
   
4. Virtual Machines, where the OS itself and everything in it is swapped out. QEMU is a pretty typical example.  A virtual machine is usually treated as an ordinary process inside the outer ("host") OS.

Creating Different Program Control Flows