Question

I want to pass a table from Lua to C and then access all the values in C without copying the values from Lua address space to C stack. Is there a way of doing this?? I want to minimize the copy of values.

I tried using gettable() but in that case, the value is copied onto top of stack. So a copy is getting generated. I don’t want this. Is there any other way??

Here is my C code:-

#include <lua.h>                               /* Always include this */
#include <lauxlib.h>                           /* Always include this */
#include <lualib.h>                            /* Always include this */
#include <malloc.h>

#define EXCEPTION_IS_NUMBER (-2)    //Passed a custom error no. to be returned in 
                                    //case of error
#define SUCCESS (0)

static int iquicksort(lua_State *L) {
    int k,len=0;
    len=lua_tointeger(L,-2);        //-2 specifies second element from top of stack.
                                    //So I have passed 2 elements from Lua to C,  first
                                    //is size of table and second table. So when they 
                                    //are pushed to stack, the size is second element
                                    //from top.So here I am storing it in variable len.
    int *q;
    int *p=(int *)malloc(len*sizeof(int));
    q=p;
    for(k=1;k<=len;k++)
    {
            lua_pushinteger(L,k);    //if I want to access a[2], where a is my table 
                                     //and 2 is the index, then '2' needs to be at top
                                     //of the stack and I need to pass the location of
                                     //'a' in stack as second argument to gettable().
                                     //So here Address of table was at top, I pushed 
                                     //the index on top, now address is second element
                                     //from top. So I passed it as '-2' in gettable
                                     //below. What gettable() does is that it fetches 
                                     //and copies that value at stack top. So I can 
                                     //use it from there.
            lua_gettable(L,-2);
            if(lua_isnumber(L,-1))   //Checking top value replaced by fxn is number...
            {
                    *p++=lua_tointeger(L,-1);   //Storing the values in array
            }
            else
            {
                    lua_pushinteger(L,EXCEPTION_IS_NUMBER);
                    return 1;
            }
            lua_pop(L,1);
    }
    p=q;
    sort(p,0,len-1);
    for(k=1;k<=len;k++)   //This fxn changes the value at prescribed location of table.
                          //here I am changing the values at Table's location...
                          //i.e. storing the sorted values in table.....
    {
            lua_pushinteger(L,k);
            lua_pushinteger(L,*p++);
            lua_settable(L,-3);
    }
    lua_pushinteger(L,SUCCESS);
    return 1;
}

//Simple quicksort of values.....
void sort(int *arr, int left,int right){
    int i = left, j = right;
    int tmp;
    int pivot = arr[(left + right) / 2];

  /* partition */
    while (i <= j) {
            while (arr[i] < pivot)
                    i++;
            while (arr[j] > pivot)
                    j--;
            if (i <= j) {
                    tmp = arr[i];
                    arr[i] = arr[j];
                    arr[j] = tmp;
                    i++;
                    j--;
            }
    };

  /* recursion */
    if (left < j)
            sort(arr, left, j);
    if (i < right)
            sort(arr, i, right);
}

int luaopen_power(lua_State *L){
    lua_register(L,"quicksort",iquicksort);
    return 0;
}

I generated a shared library by compiling this program using the following command:-

gcc -Wall -shared -fPIC -o power.so -I/usr/local/include/lua5.1 -llua5.1 quicksort.c

Here is the Lua code for calling this:-

require("power")
x={5,4,6,5,3,2,3,9}
print("Before quicksort call....")
t=quicksort(#x,x)
if t==0 then
        for i,v in ipairs(x) do print(i,v) end
else
        print(string.format("%s %d","Error occurred. Errorcode is:: ",t))
end

Thanks

Answer 1

The C API for Lua only copies low-level C types, such as numbers and booleans. For all others, including strings, it uses pointers to internal Lua data.