Missing function names in Lua stack traceback

Question

I'm trying to improve error handling in my program that runs a Lua script. Simplified version:

function _errH(msg)

    print(msg .. "\n" .. debug.traceback(nil, 1))

end

function _main()

  -- cause some random error
  print(a-2)

end

function main()

    xpcall(_main, errH)
end

main()

Gives me the following call stack (please ignore the wrong line numbers):

stack traceback:
    [string ""]:7428: in function '__sub'
    [string ""]:7651: in function <[string ""]:7432>
    [C]: in function 'xpcall'
    [string ""]:7658: in function 'main'
    [string ""]:7928: in main chunk

Now I'm wondering why there is [string ""] everywhere.

Is there any way to influence these missing texts? Do I have to name the functions in addition to defining them? Why do I see in function '__sub' but not in function '_main' for example?

Answer 1

Generally you can assign the chunk name when loading. The auxiliary function use lua_load and most will set the chunk name based on the type of content being loaded:

• lua_load accepts a chunk name
• luaL_loadfile will use the filename
• luaL_dofile uses luaL_loadfile
• luaL_loadstring will use the string contents
• luaL_dostring uses luaL_loadstring
• luaL_loadbuffer accepts a chunk name

If you look at the source code for luaL_loadstring:

LUALIB_API int luaL_loadstring (lua_State *L, const char *s) {
  return luaL_loadbuffer(L, s, strlen(s), s);
}

Notice that its really just a convenience function that calls luaL_loadbuffer with the string as the chunk name. So, what is needed is to load your code from strings like:

luaL_loadbuffer(L, s, strlen(s), "=my_chunk");

Where "=my_chunk" is the name of the chunk that will appear in the debug information.

The equal sign before the name changes the trace back:

my_chunk:1: in main chunk

Where omitting the equal sign will show:

[string "my_chunk"]:1: in main chunk