PCL(3) Portable Coroutine Library PCL(3)NAME
co_create, co_call, co_resume, co_delete, co_exit_to, co_exit, co_cur‐
rent - C coroutine management
SYNOPSIS
#include <pcl.h>
coroutine_t co_create(void *func, void *data, void *stack, int stacksize);
void co_delete(coroutine_t co);
void co_call(coroutine_t co);
void co_resume(void);
void co_exit_to(coroutine_t co);
void co_exit(void);
coroutine_t co_current(void);
DESCRIPTION
The Portable Coroutine Library (PCL) implements the low level function‐
ality for coroutines. For a definition of the term coroutine see The
Art of Computer Programming by Donald E. 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 corou‐
tines and switching between them. Higher level functionality (sched‐
uler, etc.) is not covered.
Functions
The following functions are defined:
coroutine_t co_create(void *func, void *data, void *stack, int stack‐
size);
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 pos‐
sible 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.
void co_delete(coroutine_t co);
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.
void co_call(coroutine_t co);
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. Call‐
ing oneself returns immediately.
void co_resume(void);
This function passes execution back to the coroutine which
either initially started this one or restarted it after a prior
co_resume.
void co_exit_to(coroutine_t co);
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.
void co_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_t co_current(void);
This function returns the currently running coroutine.
Notes
Some interactions with other parts of the system are covered here.
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 corou‐
tine, 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).
DIAGNOSTICS
Some fatal errors are caught by the library. If one occurs, a short
message is written to file descriptor 2 (stderr) and a segmentation
violation is generated.
[PCL]: Cannot delete itself
A coroutine has called co_delete with it's own handle.
[PCL]: Resume to deleted coroutine
A coroutine has deleted itself with co_exit or co_exit_to and
the coroutine that was activated by the exit tried a co_resume.
[PCL]: Stale coroutine called
Someone tried to active a coroutine that has already been
deleted. This error is only detected, if the stack of the
deleted coroutine is still resident in memory.
[PCL]: Context switch failed
Low level error generated by the library in case a context
switch between two coroutines failes.
SEE ALSO
Original coroutine library at
http://www.goron.de/~froese/coro/coro.html . GNU Pth library at
http://www.gnu.org/software/pth/ .
AUTHOR
Developed by Davide Libenzi < davidel@xmailserver.org >. Ideas and man
page base source taken by the coroutine library developed by E. Toernig
< froese@gmx.de >. Also some code and ideas comes from the GNU Pth
library available at http://www.gnu.org/software/pth/ .
BUGS
There are no known bugs. But, this library is still in development
even if it results very stable and pretty much ready for production
use.
Bug reports and comments to Davide Libenzi < davidel@xmailserver.org >.
GNU 1.6 PCL(3)
