How to print out errors in the console with the C API for Lua

user_6115615 0
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I have been searching high and low for a way to handle errors with the C API for Lua, and simply print them out in the console. Though, I'm unable to find a single working example. What I would like to do is simply something like:

static int test(lua_State *L)
{
  if(!lua_isstring(L, 1))
     return luaL_error(L, "This is not a valid string");    
}

or 

static int test(lua_State *L)
{
    try
    {
        if (!lua_isstring(L, 1))
        {
            throw luaL_error(L, "This is not a valid string");
        }
    }
    catch (int e)
    {
        std::cout << "Error " << e << std::endl;
    }
}

But nothing works as up to this point. What would be the correct way to do the error handling with the LUA C API and show messages in the console?

lua-c++-connection error-handling lua c c++
2 Answers

I had the exact same question, and catwell's answer didn't fully answer it for me. What I originally did in my program was simply call

luaL_dofile(L, "example.lua");

Nothing happened when an error occurred.

"If you call it from a standard Lua interpreter..." was a good clue. So I looked at the main in luac.c to see what it does:

if (lua_pcall(L,2,0,0)!=LUA_OK) fatal(lua_tostring(L,-1));

Note that luaL_dofile also expands to lua_pcall. So, if an error is raised while that file is ... done, the only thing that happens is that the error message is pushed onto L's stack. It is your responsibility to do something with it.

So I modified my code accordingly:

#include <stdio.h> // fprintf, stderr
#include <stdlib.h> // exit, EXIT_FAILURE
static void fatal(const char* message) {
  fprintf(stderr,"%s\n", message);
  exit(EXIT_FAILURE);
}
int main(void) {
  ...
  if (luaL_dofile(L, "example.lua") != LUA_OK) {
    fatal(lua_tostring(L,-1));
  }
  ...
}
user_8726160
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