How to properly print error messages from `luaL_dostring()`? - error-handling

Question

Here's my code that runs a Lua script as a string.
bool doString(const char *s)
{
const int ret = luaL_dostring(L, s);
if (ret)
{
if (ret == LUA_ERRSYNTAX)
printf("Error: %s", lua_tostring(L, -1));
else if (ret == LUA_ERRMEM)
printf("Error: memory error");
else
printf("Error: syntax error");
return false;
}
return true;
}
But in many cases, my code just prints Error: syntax error without any further description about where the error is happening.
How can I properly print the error messages so it can be more descriptive?

Answer 1

luaL_dostring returns LUA_OK /* 0 */ on success, and 1 on failure [1]. When it fails, it will push an error message on the stack describing the problem. This can be determined by following luaL_loadstring to lua_load in the docs [2]. Also, luaL_dostring invokes lua_pcall, which also pushes an error object on the stack when it fails (with no handler set) [3]
When you get a nonzero result, you can check the top of the stack for the error message and print that:
bool doString(const char *s) {
const int ret = luaL_dostring(L, s);
if (ret != LUA_OK) {
printf("Error: %s\n", lua_tostring(L, -1));
lua_pop(L, 1); // pop error message
return false;
}
return true;
}
[1] https://www.lua.org/manual/5.3/manual.html#luaL_dostring
[2] https://www.lua.org/manual/5.3/manual.html#lua_load
[3] https://www.lua.org/manual/5.3/manual.html#lua_pcall