How to retrieve a returned string by C function in Lua script?

Question

I have a Lua script which is calling a C function. Currently this function is returning nothing. I want to change this function to return a string, so at the end of this function in C I will push the string into Stack. Inside the calling Lua script I need to get back the pushed string value.

C initialization and registration with Lua

void cliInitLua( void )
{
   void* ud = NULL;
   Task task;

   // Create a new Lua state
   L = lua_newstate(&luaAlloc, ud);

   /* load various Lua libraries */
   luaL_openlibs(L);

   /*Register the function to be called from LUA script to execute commands*/
   lua_register(L,"CliCmd",cli_handle_lua_commands);

   //lua_close(L);
   return;
}

This is my c Function to return a string:

static int cli_handle_lua_commands(lua_State *L){
   ...
   ...
   char* str = ....; /*Char pointer to some string*/
   lua_pushstring(L, str);
   retun 1;
}

This is my Lua script

cliCmd("Anything here doesn't matter");
# I want to retreive the string str pushed in the c function.

The code you have already would be a nice starting point for an answer to work off of. — Stephan van den Heuvel
– Stephan van den Heuvel, Commented Nov 5, 2013 at 17:53
It is Lua, not LUA. Lua is the Portuguese word for moon, not an acronym. — LorenzoDonati4Ukraine-OnStrike
– LorenzoDonati4Ukraine-OnStrike, Commented Nov 5, 2013 at 21:18

Stephan van den Heuvel · Accepted Answer · 2013-11-05 23:38:42Z

5

In C you have something like

 static int foo (lua_State *L) {
   int n = lua_gettop(L);
   //n is the number of arguments, use if needed

  lua_pushstring(L, str); //str is the const char* that points to your string
  return 1; //we are returning one value, the string
}

In Lua

lua_string = foo()

This assumes you have already registered your function with lua_register

Pleas see the great documentation for more examples on these sorts of tasks.

edited Nov 5, 2013 at 23:38

answered Nov 5, 2013 at 18:01

Stephan van den Heuvel

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3 Comments

Etan Reisner Over a year ago

Unless you have very specific requirements accepting more arguments than you need is good practice (and matches general lua conventions).

Stephan van den Heuvel Over a year ago

@EtanReisner That is a great point. I took the liberty of editing my answer per your suggestion. I still left the check in, because if you get more arguments you will have to clear them from the stack.

Etan Reisner Over a year ago

You don't need to clear them. lua handles the stack for you at the lua<->C transition points. You can simply push your string and return 1 and everything works. If you were calling this function from another C function you would need to maintain stack cleanliness though.

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