[英]Access Table passed from Lua in C without Copying the values
我想將表從Lua傳遞到C,然后訪問C中的所有值,而不將值從Lua地址空間復制到C堆棧。 有辦法嗎? 我想最小化值的副本。
我嘗試使用gettable(),但在那種情況下,該值被復制到堆棧的頂部。 因此,正在生成一個副本。 我不要這個 還有其他辦法嗎?
這是我的C代碼:
#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;
}
我通過使用以下命令編譯該程序來生成共享庫:
gcc -Wall -shared -fPIC -o power.so -I/usr/local/include/lua5.1 -llua5.1 quicksort.c
這是用於調用此代碼的Lua代碼:-
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
謝謝
Lua的C API僅復制低級C類型,例如數字和布爾值。 對於所有其他字符串(包括字符串),它使用指向內部Lua數據的指針。
我不確定gettable()
將值復制到Lua堆棧中,我認為它會復制值的引用或指針...(尤其是當該值是表本身時,不會復制該表的內容) 。
考慮到Lua可能會進行魔術處理,我相信您的回答是否定的。
Lua是免費軟件,您可以下載它並查看源代碼 。 對於lua-5.2.0-rc4
, lua_gettable
函數位於文件src/lapi.c
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);
}
因此實際工作是通過src/lvm.c
文件中的luaV_gettable
完成的
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");
}
所以我認為答案是否定的。 但是,您可以修補或增強代碼。 我不明白為什么這個問題困擾您。 除非發生魔術(魔術(即metatable是Lua語義的重要組成部分)),否則僅復制(非常快速)簡單數據。 匯總數據內容未復制。
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