Am I right to read a nested lua table as argument from C function?

Question

I'm going to implement a function with C language and which will be called by Lua script.

This function should receive a lua table(which even contains an array) as the argument, so I should read the fields in the table.I try to do like below, but my function is crashing when I run it. Can anyone help my find the problem?


/*
 function findImage(options)
    imagePath = options.imagePath
    fuzzy = options.fuzzy
    ignoreColors = options.ignoreColor;
    ...
 end

 Call Example:

 findImage {
              imagePath="/var/image.png", 
              fuzzy=0.5,
              ignoreColors={
                             0xffffff, 
                             0x0000ff, 
                             0x2b2b2b
                           }
            }

 */

static int findImgProxy(lua_State *L)
{
    lua_settop(L, 1);
    luaL_checktype(L, 1, LUA_TTABLE);

    lua_getfield(L, -1, "imagePath");
    lua_getfield(L, -2, "fuzzy");
    lua_getfield(L, -3, "ignoreColors");

    const char *imagePath = luaL_checkstring(L, -3);
    double fuzzy    = luaL_optint(L, -2, -1);

    int count  = lua_len(L, -1); // how to get the member count of ignoreColor array

    int colors[count];
    for (int i=0; i count; i++) {
        lua_rawgeti(L, 4, i);
        colors[i] = luaL_checkinteger(L, -1);
        lua_pop(L, 1);
    }

    lua_pop(L, 2);

    ...
    return 1;
}

mpeterv · Accepted Answer · 2013-09-05 14:13:55Z

1

lua_len doesn't return anything, it only pushes the length on the stack. Use this snippet to get the table length:

lua_len(L, -1);
int count = luaL_checkinteger(L, -1);
lua_pop(L, 1);

answered Sep 5, 2013 at 14:13

mpeterv

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2 Comments

Suge Over a year ago

I tried use int count = lua_rawlen, which works.Am I right?

mpeterv Over a year ago

Yes, it works, too. Unlike lua_rawlen, lua_len has to push its result on stack because it may trigger __len metamethod, which may return anything from nil to userdata.

greatwolf · Accepted Answer · 2013-09-05 14:15:47Z

0

int count  = lua_len(L, -1); // how to get the member count of ignoreColor array

int colors[count];
for (int i=0; i count; i++)
{
    colors[i] = luaL_checkinteger(L, -1-i);
}

The code segment for this looks incorrect (never mind the missing comparision operator in the loop). The correct function for getting the table length is lua_objlen. It looks like you're trying to get the numbers out of 'ignoreColor' but you haven't place them on the stack first. As a result luaL_checkinteger(L, -1-i); ends up accessing the wrong indices on the stack.

You probably want something closer to this for example:

int count  = lua_objlen(L, -1);
std::vector<int> colors(count);
for (int i = 0; i < count; lua_pop(L, 1))
{
  lua_rawgeti(L, 4, ++i);
  colors.push_back( luaL_checkinteger(L, -1) );
}

If you're using Lua 5.2, replace lua_objlen with:

int count  = lua_rawlen(L, -1);

Make sure there's enough space on the stack if you're moving a lot of elements from the table. eg. lua_checkstack

edited Sep 5, 2013 at 14:15

answered Sep 5, 2013 at 13:32

greatwolf

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4 Comments

mpeterv Over a year ago

lua_len is the correct function for getting the table length in Lua 5.2, though it performs an unnecessary metatable check.

greatwolf Over a year ago

@peterm arrrg they had to go and change the api up :( so annoying

Suge Over a year ago

@greatwolf, thank you for your help. I've edited my code above, could you have a look?

greatwolf Over a year ago

@Suge Note that variable length array's is an extension of gcc which is why I'm using vector in my example.

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