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Reentrant coroutines in C++
Posted by Erik on Jan 09 2006 @ 16:14 :: 4588 unique visits
My previous post Simple coroutines in C showed some macro's to turn normal C/C++ funtions into coroutines. A disadvantage of these coroutines was that they weren't reentrant. I have found two ways to fix this problem:- Adding an extra context parameter to the function. This would require some modification in the body of the function, e.g. context->i instead of i. It would also require the caller of the function to create and remember the context. An advantage is that this method works in both C and C++.
- Overloading the function application operator of a class and using the class as the context. This only works in C++ but requires minimal code modifications.
Here I will only discuss the second method because I find it a nicer way to solve the problem.
First thing is that the coroutine macro's need a little modification.
The cr_start macro is divided into 3 seperate macro's:
CODE: CPP
// old cr_start
#define cr_start() static int __s = 0; switch (__s) { case 0:
// new macro's
#define cr_context int __s;
#define cr_init() __s = 0;
#define cr_start() switch (__s) { case 0:
#define cr_return(x) { __s = __LINE__; return x; case __LINE__: ; }
#define cr_end() { break; default: for (;;) ; } } __s = 0;
#define cr_start() static int __s = 0; switch (__s) { case 0:
// new macro's
#define cr_context int __s;
#define cr_init() __s = 0;
#define cr_start() switch (__s) { case 0:
#define cr_return(x) { __s = __LINE__; return x; case __LINE__: ; }
#define cr_end() { break; default: for (;;) ; } } __s = 0;
Here is an example of the foobar() function from my previous post about coroutines as a reentrant coroutine:
CODE: CPP
class foobar_f {
private:
cr_context;
// place the variables which need to be remembered between calls here.
int i;
public:
foobar_f(void) {
cr_init();
}
// overloading the function application operator "()"
int operator () (void) {
cr_start();
for (i = 0; i < 1337; i++) {
cr_return(i);
}
cr_end();
}
};
private:
cr_context;
// place the variables which need to be remembered between calls here.
int i;
public:
foobar_f(void) {
cr_init();
}
// overloading the function application operator "()"
int operator () (void) {
cr_start();
for (i = 0; i < 1337; i++) {
cr_return(i);
}
cr_end();
}
};
The usage of the coroutine requires a little modification from the caller's side:
CODE: CPP
// create a function object, sometimes (incorrectly) called functor.
foobar_f foobar;
printf("%d\n", foobar()); // prints 0
printf("%d\n", foobar()); // prints 1
printf("%d\n", foobar()); // prints 2
foobar_f foobar;
printf("%d\n", foobar()); // prints 0
printf("%d\n", foobar()); // prints 1
printf("%d\n", foobar()); // prints 2
One important note I forgot in my previous post
You can't use a the cr_return macro inside the case of one of your own switch statements!
This because coroutines are implemented using a switch statement and cr_return uses a case statement of it's own.
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2. On Jan 21 2009 @ 06:24 mike wrote:
oh, this is trickier than I thought. if __COUNTER__ is being invoked twice in the same line, as is done in the example, then it will return different values. you could do some integer arithmetic (eg. __COUNTER__/2) to get a number which goes up every other time it's invoked. also, counter starts at 0, and zero's already in use, so you could try something like (__COUNTER__ + 2)/2.3. On Feb 14 2009 @ 19:48 guest wrote:
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1. On Jan 21 2009 @ 06:06 mike wrote:
One downside of using __LINE__ is that the numbers will typically be very large, and this has implications for how efficiently the switch is compiled. If the case values are small integers, then typically the compiler will render the switch as a jump to something indicated in an address table it generates, which executes faster than rendering it as a bunch of if statements. even if the numbers are small enough to cause it to compile this way, the larger they are, the more space is wasted on this table (not a big deal, maybe).The point is, in all major C++ compilers you can use the macro __COUNTER__, which expands to an integer which is one larger than the last time it expanded.
Generally I love this coroutine implementation, though. It's the best of all the ones I've seen.