The PortableCoroutineLibrary(PCL) implements the low level functionality for coroutines. For a
definition of the term coroutine see TheArtofComputerProgramming by DonaldE.Knuth. Coroutines are
a very simple cooperative multitasking environment where the switch from one task to another is done
explicitly by a function call. Coroutines are a lot faster than processes or threads switch, since there
is no OS kernel involvement for the operation. This document defines an API for the low level handling of
coroutines i.e. creating and deleting coroutines and switching between them. Higher level functionality
(scheduler, etc.) is not covered.
Functions
The following functions are defined:
intco_thread_init(void);
If the PCL library is built in multi-thread mode, and if multi threads are actually used, this
function should be called before calling any PCL function. If the PCL library is built in multi-
thread mode, but it is used only from one thread (the main one, likely), then it is possible to
avoid to call co_thread_init(). Returns 0 in case of success, or an negative error code in case
of error.
voidco_thread_cleanup(void);
If the PCL library is built in multi-thread mode, and if multi threads are actually used, this
function should be called before the thread exits, or whenever the thread decides it won't call
the PCL functions anymore. A failure in calling co_thread_cleanup() will result in resource
leakage by the calling application.
coroutine_tco_create(void*func,void*data,void*stack,intstacksize);
This function creates a new coroutine. func is the entry point of the coroutine. It will be
called with one arg, a void*, which holds the data passed through the data parameter. If func
terminates, the associated coroutine is deleted. stack is the base of the stack this coroutine
will use and stacksize its size in bytes. You may pass a NULL pointer for stack in which case the
memory will be allocated by co_create itself. Both, stack and stacksize are aligned to system
requirements. A stacksize of less then 4096 bytes will be rejected. You have to make sure, that
the stack is large enough for your coroutine and possible signal handlers (see below). The stack
will not grow! (Exception: the main coroutine uses the standard system stack which may still
grow) On success, a handle (coroutine_t) for a new coroutine is returned, otherwise NULL.
voidco_delete(coroutine_tco);
This function deletes the given coroutine co. If the stack for this coroutine was allocated by
co_create it will be freed. After a coroutine handle was passed to co_delete it is invalid and
may not be used any more. It is invalid for a coroutine to delete itself with this function.
voidco_call(coroutine_tco);
This function passes execution to the given coroutine co. The first time the coroutine is
executed, its entry point func is called, and the data parameter used during the call to co_create
is passed to func. The current coroutine is suspended until another one restarts it with a
co_call or co_resume call. Calling oneself returns immediately.
voidco_resume(void);
This function passes execution back to the coroutine which either initially started this one or
restarted it after a prior co_resume.
voidco_exit_to(coroutine_tco);
This function does the same a co_delete(co_current()) followed by a co_call would do. That is, it
deletes itself and then passes execution to another coroutine co.
voidco_exit(void);
This function does the same a co_delete(co_current()) followed by a co_resume would do. That is,
it deletes itself and then passes execution back to the coroutine which either initially started
this one or restarted it after a prior co_resume.
coroutine_tco_current(void);
This function returns the currently running coroutine.
void*co_get_data(coroutine_tco);
This function returns the data associated with the co
coroutine. The data associated with a coroutine is the data parameter passed to co_create().
void*co_set_data(coroutine_tco,void*data);
Sets the data associated with the co coroutine, and returns the previously associated data.
Notes
Some interactions with other parts of the system are covered here.
Threads
If the PCL library has been built in multi-thread mode, then it is possible to use it in multi-
thread software. A thread should call co_thread_init() before using the PCL APIs, and call
co_thread_cleanup() before exiting, or when it has done using the PCL APIs.
WARNING: For no reason should two different threads run the same coroutine at the same time.
Signals
First, a signal handler is not defined to run in any specific coroutine. The only way to leave the
signal handler is by a return statement.
Second, the signal handler may run with the stack of any coroutine, even with the stack of library
internal coroutines which have an undefined stack size (just enough to perform a kernel call).
Using and alternate stack for signal processing (see sigaltstack(2)) is recommended!
Conclusion: avoid signals like a plague. The only thing you may do reliable is setting some
global variables and return. Simple kernel calls may work too, but nowadays it's pretty hairy to
tell, which function really is a kernel call. (Btw, all this applies to normal C programs, too.
The coroutines just add one more problem)
setjmp/longjmp
The use of setjmp(2)/longjmp(2) is limited to jumping inside one coroutine. Never try to jump
from one coroutine to another with longjmp(2).
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