Print stacktrace from C code with embedded lua

Question

If I understand this correctly, Lua by default will call the debug library "debug.traceback" when an error occurs.

However, when embedding Lua into C code like done in the example here: Simple Lua API Example

We only have available the error message on the top of the stack.

i.e.

if (status) {
    /* If something went wrong, error message is at the top of */
    /* the stack */
    fprintf(stderr, "Couldn't load file: %s\n", lua_tostring(L, -1));

    /* I want to print a stacktrace here. How do I do that? */
    exit(1);
}

How do I print the stack trace from C after the initial error?

Answer 1

Lua by default will call the debug library "debug.traceback" when an error occurs.

No, it won't. The Lua runtime (lua.exe) will do that, but the Lua library will not do that on its own. If you want a call-stack with your Lua errors, then you need to generate one.

The Lua runtime does this by using lua_pcall's error function. The stack has not been unwound when the error function is called, so you can get a stack trace there. The error function the runtime uses is this one:

static int traceback (lua_State *L) {
  if (!lua_isstring(L, 1))  /* 'message' not a string? */
    return 1;  /* keep it intact */
  lua_getfield(L, LUA_GLOBALSINDEX, "debug");
  if (!lua_istable(L, -1)) {
    lua_pop(L, 1);
    return 1;
  }
  lua_getfield(L, -1, "traceback");
  if (!lua_isfunction(L, -1)) {
    lua_pop(L, 2);
    return 1;
  }
  lua_pushvalue(L, 1);  /* pass error message */
  lua_pushinteger(L, 2);  /* skip this function and traceback */
  lua_call(L, 2, 1);  /* call debug.traceback */
  return 1;
}

Answer 2

Working off Nicol’s answer above here is a working example:

static int traceback(lua_State *L) {
    lua_getfield(L, LUA_GLOBALSINDEX, "debug");
    lua_getfield(L, -1, "traceback");
    lua_pushvalue(L, 1);
    lua_pushinteger(L, 2);
    lua_call(L, 2, 1);
    fprintf(stderr, "%s\n", lua_tostring(L, -1));
    return 1;
}

int main(int argc, char **argv) {
    lua_State *L = lua_open();
    luaL_openlibs(L);    
    lua_pushcfunction(L, traceback);
    int rv = luaL_loadfile(L, "src/main.lua");
    if (rv) {
        fprintf(stderr, "%s\n", lua_tostring(L, -1));
        return rv;
    } else {
        return lua_pcall(L, 0, 0, lua_gettop(L) - 1);
    }
}

Answer 3

I met the some question as you do,and I found this way work:

luaL_traceback(L, L, NULL, 1);
printf("%s\n", lua_tostring(L, -1));

SinceluaL_tracebackis exactlydebug.traceback() using to print the stack,so I think this may be a proper way,and you can read the API manual aboutluaL_traceback or just read the source code of Lua to figure out the what the params means.