Lua co-routines - c++

Question

I'm trying to get an understanding of how I can use co-routines to "pause" a script and wait until some processing is done before resuming.
Perhaps I'm looking at co-routines in the wrong way. But my attempt is structured similar to the example given in this answer.
The loop in loop.lua never reaches a second iteration, and hence never reaches the i == 4 condition required to exit the running loop in the C code. If I do not yield in loop.lua, then this code performs as expected.
main.cpp
#include <lua/lua.hpp>
bool running = true;
int lua_finish(lua_State *) {
running = false;
printf("lua_finish called\n");
return 0;
}
int lua_sleep(lua_State *L) {
printf("lua_sleep called\n");
return lua_yield(L,0);
}
int main() {
lua_State* L = lua_open();
luaL_openlibs(L);
lua_register(L, "sleep", lua_sleep);
lua_register(L, "finish", lua_finish);
luaL_dofile(L, "scripts/init.lua");
lua_State* cL = lua_newthread(L);
luaL_dofile(cL, "scripts/loop.lua");
while (running) {
int status;
status = lua_resume(cL,0);
if (status == LUA_YIELD) {
printf("loop yielding\n");
} else {
running=false; // you can't try to resume if it didn't yield
// catch any errors below
if (status == LUA_ERRRUN && lua_isstring(cL, -1)) {
printf("isstring: %s\n", lua_tostring(cL, -1));
lua_pop(cL, -1);
}
}
}
luaL_dofile(L, "scripts/end.lua");
lua_close(L);
return 0;
}
loop.lua
print("loop.lua")
local i = 0
while true do
print("lua_loop iteration")
sleep()
i = i + 1
if i == 4 then
break
end
end
finish()
EDIT: Added a bounty, to hopefully get some help on how to accomplish this.

Answer 1

Return code 2 from lua_resume is a LUA_ERRRUN. Check the string on the top of the Lua stack to find the error message.
A similar pattern has worked for me, though I did use coroutine.yield instead of lua_yield and I'm in C rather than C++. I don't see why your solution wouldn't work
On your resume call, it's not clear if you're over simplifying for an example, but I'd make the following changes in your while loop:
int status;
status=lua_resume(cL,0);
if (status == LUA_YIELD) {
printf("loop yielding\n");
}
else {
running=false; // you can't try to resume if it didn't yield
// catch any errors below
if (status == LUA_ERRRUN && lua_isstring(cL, -1)) {
printf("isstring: %s\n", lua_tostring(cL, -1));
lua_pop(cL, -1);
}
}
Edit 2:
For debugging, add the following before you run your resume. You've got a string getting pushed on the stack somewhere:
int status;
// add this debugging code
if (lua_isstring(cL, -1)) {
printf("string on stack: %s\n", lua_tostring(cL, -1));
exit(1);
}
status = lua_resume(cL,0);
Edit 3:
Oh good grief, I can't believe I didn't see this already, you don't want to run luaL_dofile when you're going to yield because you can't yield a pcall directly as best I know, which is what happens in the dofile (5.2 will pass it through, but I think you still need the lua_resume). Switch to this:
luaL_loadfile(cL, "scripts/loop.lua");

Answer 2

Last time i was messing with Lua coroutines I ended with such code
const char *program =
"function hello()\n"
" io.write(\"Hello world 1!\")\n"
" io.write(\"Hello world 2!\")\n"
" io.write(\"Hello world 3!\")\n"
"end\n"
"function hate()\n"
" io.write(\"Hate world 1!\")\n"
" io.write(\"Hate world 2!\")\n"
" io.write(\"Hate world 3!\")\n"
"end\n";
const char raw_program[] =
"function hello()\n"
" io.write(\"Hello World!\")\n"
"end\n"
"\n"
"cos = {}\n"
"\n"
"for i = 0, 1000, 1 do\n"
" cos[i] = coroutine.create(hello)\n"
"end\n"
"\n"
"for i = 0, 1000, 1 do\n"
" coroutine.resume(cos[i])\n"
"end";
int _tmain(int argc, _TCHAR* argv[])
{
lua_State *L = lua_open();
lua_State *Lt[1000];
global_State *g = G(L);
printf("Lua memory usage after open: %d\n", g->totalbytes);
luaL_openlibs(L);
printf("Lua memory usage after openlibs: %d\n", g->totalbytes);
lua_checkstack(L, 2048);
printf("Lua memory usage after checkstack: %d\n", g->totalbytes);
//lua_load(L, my_lua_Reader, (void *)program, "code");
luaL_loadbuffer(L, program, strlen(program), "line");
printf("Lua memory usage after loadbuffer: %d\n", g->totalbytes);
int error = lua_pcall(L, 0, 0, 0);
if (error) {
fprintf(stderr, "%s", lua_tostring(L, -1));
lua_pop(L, 1);
}
printf("Lua memory usage after pcall: %d\n", g->totalbytes);
for (int i = 0; i < 1000; i++) {
Lt[i] = lua_newthread(L);
lua_getglobal(Lt[i], i % 2 ? "hello" : "hate");
}
printf("Lua memory usage after creating 1000 threads: %d\n", g->totalbytes);
for (int i = 0; i < 1000; i++) {
lua_resume(Lt[i], 0);
}
printf("Lua memory usage after running 1000 threads: %d\n", g->totalbytes);
lua_close(L);
return 0;
}
It seems you could not load file as coroutine? but use function instead And it should be selected to the top of the stack.
lua_getglobal(Lt[i], i % 2 ? "hello" : "hate");