Lua C API: Stack empty after pcall error

Question

I have a problem using Lua C API. When pcall (C API function) fail, the error is pushed on the stack. lua_tostring shows an error on the stack but lua_gettop shows says the stack is empty.

#include <lua5.2/lauxlib.h>
#include <lua5.2/lua.h>
#include <lua5.2/lualib.h>

int main()
{
    lua_State *L = luaL_newstate();
    lua_pcall(L, 0, 0, 0);
    printf("%d\n", lua_gettop(L)); // outputs 0, indicating empty stack
    printf("%s\n", lua_tostring(L, -1)); // outputs "attempt to call a nil value", indicating non-empty stack
}

Compile with: gcc main.c `pkg-config --cflags lua5.2` `pkg-config --libs lua5.2`

This program display: 0 attempt to call a nil value

lua_gettop(L) return the stack size. Here I get 0. How can I get a string from an empty stack ?

The behavior is the same with the 5.1 version.

lhf lhf · Accepted Answer · 2012-05-15T18:36:36

This has been answered in the Lua mailing list. The behavior is correct: you need to push a function onto the stack to call it. The stack after luaL_newstate is empty.

Edit: The OP said "How can I get a string from an empty stack?". My answer is: Why do you want to get something from an empty stack, when you know it is empty since lua_gettop returned 0?

Bottom line:

The stack was empty before calling lua_pcall. This is an error. Lua recovered, but you can't count on it.
The stack was empty after lua_pcall. Lua thinks so and tells you so via lua_gettop.
You should not try to get a value from an empty stack. The string that is printed is just garbage left over by lua_pcall, but you can't count on it.

Lua C API: Stack empty after pcall error

2 Answers