Modifying Lua arguments in C function

Question

A Lua script is using one of my C defined functions as bellow:

function lua_func()

    local var = 5

    -- Do some stuff here, possibly using var.

    c_func(var)

    -- Do other stuff here, that must not use var.

end

This C function takes an argument that the caller has created and does what it needs.

This argument to the C function must be of single-use, i.e. after the C function has used it, I don't want it to be accessible any more to the rest of the Lua script.

I am looking for a way for the C function to "consume" this argument. To use it, and then set it to nil, so it is no longer usable.

Is this possible, and if so how?

I don’t know lua but does it support scopes? If yes then just create a scope around where var should be used. — Fredrik
– Fredrik, Commented Apr 2, 2019 at 10:18
The Lua script is not in my control. It is intended for the users to modify the system's behavior. So I am looking for a solution in C, that will "invalidate" the variable immediately after use. — Fotis Panagiotopoulos
– Fotis Panagiotopoulos, Commented Apr 2, 2019 at 10:21
The scope of var is controlled by the lua script, there is nothing you can do if you can’t change the lua script. The C function will always get a copy of the variable, in C everything is pass by value. — Fredrik
– Fredrik, Commented Apr 2, 2019 at 10:26
You could send in a pointer to var and then set var to some invalid value, like -1 or something that makes sense in your context. — Fredrik
– Fredrik, Commented Apr 2, 2019 at 10:27
Lua is pass-by-value, too. Isn't it the value 5 that you don't want used again? If not, there is no requirement that a function argument has anything to do with a variable. Syntactically, an argument is an expression. — Tom Blodget
– Tom Blodget, Commented Apr 5, 2019 at 1:20

Egor Skriptunoff · Accepted Answer · 2019-04-02 10:34:58Z

3

Variant 1:

function lua_func()

    do
        local var = 5

        -- Do some stuff here, possibly using var.

        c_func(var)
    end

    -- Do other stuff here, that must not use var.

end

Variant 2:

function lua_func()

    local var_container = {5}

    -- Do some stuff here, possibly using var.

    c_func(var_container)  -- it assigns nil to var_container[1] before exit

    -- Do other stuff here, that must not use var.

end

Variant 3:

function lua_func()

    local var = 5
    local destructor = function() var = nil end

    -- Do some stuff here, possibly using var.

    c_func(var, destructor)  -- it invokes destructor() before exit

    -- Do other stuff here, that must not use var.

end

answered Apr 2, 2019 at 10:34

Egor Skriptunoff

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3 Comments

Fotis Panagiotopoulos Over a year ago

Skriptunoff Variant 2 seems the most appealing for my case. Would it work in case var is a lightuserdata? If so can you please provide me with a C example on how C can set this lightuserdata to NULL?

Fotis Panagiotopoulos Over a year ago

I mean by directly passing a lightuserdata variable, not wrapping it in a table.

Egor Skriptunoff Over a year ago

@FotisPanagiotopoulos - Variant 2 means you must wrap your value into a table.

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