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POAccess Table passed from Lua in C without Copying the values
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  1. POAccess Table passed from Lua in C without Copying the values
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    copied!<p>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.</p> <p>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??</p> <p>Here is my C code:-</p> <pre><code>#include &lt;lua.h&gt; /* Always include this */ #include &lt;lauxlib.h&gt; /* Always include this */ #include &lt;lualib.h&gt; /* Always include this */ #include &lt;malloc.h&gt; #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&lt;=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&lt;=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 &lt;= j) { while (arr[i] &lt; pivot) i++; while (arr[j] &gt; pivot) j--; if (i &lt;= j) { tmp = arr[i]; arr[i] = arr[j]; arr[j] = tmp; i++; j--; } }; /* recursion */ if (left &lt; j) sort(arr, left, j); if (i &lt; right) sort(arr, i, right); } int luaopen_power(lua_State *L){ lua_register(L,"quicksort",iquicksort); return 0; } </code></pre> <p>I generated a shared library by compiling this program using the following command:-</p> <pre><code>gcc -Wall -shared -fPIC -o power.so -I/usr/local/include/lua5.1 -llua5.1 quicksort.c </code></pre> <p>Here is the Lua code for calling this:-</p> <pre><code>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 </code></pre> <p>Thanks</p>
 

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