在 C 中访问从 Lua 传递的表，而不复制值

Question

我想将一个表从Lua传递到C，然后访问C中的所有值，而不将值从Lua地址空间复制到C堆栈。有办法做到这一点吗？我想最小化值的副本。

我尝试使用 gettable() 但在这种情况下，该值被复制到堆栈顶部。因此正在生成一个副本。我不想要这个。还有其他办法吗？？

这是我的 C 代码：-

#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;
}

我通过使用以下命令编译该程序生成了一个共享库：-

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

这是调用它的 Lua 代码：-

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

谢谢

Answer 1

Lua 的 C API 仅复制低级 C 类型，例如数字和布尔值。对于所有其他内容，包括字符串，它使用指向内部 Lua 数据的指针。

Answer 2

我不确定 gettable() 是否将值复制到 Lua 堆栈，我认为它复制了该值的引用或指针...（特别是当该值本身是一个表时，该表的内容不被复制）。

鉴于Lua可能会进行魔法处理，我相信你的答案是否定的。

Lua 是免费软件，您可以下载它并查看源代码。对于 lua-5.2.0-rc4，lua_gettable 函数位于文件 src/lapi.c 中，

LUA_API void lua_gettable (lua_State *L, int idx) {
  StkId t;
  lua_lock(L);
  t = index2addr(L, idx);
  api_checkvalidindex(L, t);
  luaV_gettable(L, t, L->top - 1, L->top - 1);
  lua_unlock(L);
}

因此实际工作由 完成luaV_gettable 来自文件 src/lvm.c，所以

void luaV_gettable (lua_State *L, const TValue *t, TValue *key, StkId val) {
  int loop;
  for (loop = 0; loop < MAXTAGLOOP; loop++) {
    const TValue *tm;
    if (ttistable(t)) {  /* `t' is a table? */
      Table *h = hvalue(t);
      const TValue *res = luaH_get(h, key); /* do a primitive get */
      if (!ttisnil(res) ||  /* result is not nil? */
          (tm = fasttm(L, h->metatable, TM_INDEX)) == NULL) { /* or no TM? */
        setobj2s(L, val, res);
        return;
      }
      /* else will try the tag method */
    }
    else if (ttisnil(tm = luaT_gettmbyobj(L, t, TM_INDEX)))
      luaG_typeerror(L, t, "index");
    if (ttisfunction(tm)) {
      callTM(L, tm, t, key, val, 1);
      return;
    }
    t = tm;  /* else repeat with 'tm' */
  }
  luaG_runerror(L, "loop in gettable");
}

我认为答案是否定的。但是，您可以修补或增强代码。我不明白为什么这个问题让你烦恼。仅复制简单数据（非常快），除非发生魔法（而魔法即元表是 Lua 语义的重要组成部分）；聚合数据内容不会被复制。