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Two subprograms (functions, methods, or subroutines) are called co-routines if they are executed using fibers.  Each co-routine is executed under co-operative multitasking with the other co-routine(s).  At certain points co-routines perform I/O or explicit yields which allow the next co-routine to start.  The advantage of co-routines and fibers over threads is explicit control over context switching.  The disadvantage is the burden of implementing a "fair" and "acceptable" switching of tasks.

Author: Daniel Barbalace
Website: http://www.clearthought.info


[[fibers]]
A process is the "heaviest" unit of kernel scheduling. Processes own resources allocated by the operating system.  Resources include memory, file handles, sockets, device handles, and windows.  Processes do not share address spaces or file resources except through explicit methods (such as inheriting file handles or share mem segments, or mapping the same file in a shared way). Processes are typically pre-emptively multitasked. However, Windows 3.1 and older versions of Mac OS used co-operative or non-preemptive multitasking.

The term thread will refer only to kernel supplied threads. A thread is the "lightest" unit of kernel scheduling. At least one thread exists within each process. If multiple threads can exist within each process then they all share the same memory and file resources. Threads are pre-emptively multitasked if the operating system's schedule for processes is pre-emptive.  Threads do not own resources except for a stack and a copy of registers including the program counter.

A fiber is a user-level "thread". Fibers are co-operatively multitasked, with context switching occuring only at I/O points or at explicit yield points. A fiber can be scheduled to run in any thread in the same process. Typically fibers are implemented entirely by user-level libraries.  In Unix the I/O would be handled by using select() multiplexing. WIN32 supplies a fiber interface API. Subroutines or functions that use fibers are called "co-routines" because they are pseudo-mulitasked. SunOS 4.x implemented "light-weight processes" as fibers known as "green threads". SunOS 5.x and later implemented LWPs as threads. 

Author: Daniel Barbalace
Website: http://www.clearthought.info