Lua unprotected error using lua_pcall - lua

Question

I'm using a Lua Protected call, but I'm getting an unprotected error:
lua_getfield( m_L, LUA_GLOBALSINDEX, "startIteration" ); // 1
if( lua_isfunction( m_L, -1 ) ){
lua_pushnumber( m_L, n ); // 2
auto ret = lua_pcall( m_L, 1, 0, 0 ); // 0
checkLuaReturn( m_L, ret );
}else{
lua_pop( m_L, 1 ); // 0
}
And the error is:
PANIC: unprotected error in call to Lua API (../example/ex01.lua:31: attempt to index global 'raster' (a nil value))
And the Lua code is:
function startIteration( num )
io.write( "Start iteration " .. num .. "\n" )
raster.grass.save( "output.png" )
end
How can I solver it, to get a REAL protected call?
update: fixed extra pop

Answer 1

It seems like this has nothing to do with your lua_pcall() but rather with your lua code.
The line raster.grass.save( "output.png" ) is where the error is.
Try channging that to something like this:
if raster and type(raster) == 'table' then
if raster.grass and type(raster.grass) then
raster.grass.save()
end
end
But, that's not going to solve your issue, the issue is that you're calling a member of the variable raster before raster was even created.

Answer 2

I had the same problem, and it stumped me for a while. The answer on this thread misses the point of the question. OP is using lua_pcall, yet is still receiving an unprotected error.
In my case, the Lua stack was corrupted. I ran a function that did not return a value, but popped a value off the stack anyway. Later, when I ran lua_pcall on an unrelated function, I got an unprotected error. Correcting the error of popping off the stack fixed it.