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Your allocator function needs to handle the case where nsize is nonzero, but ptr is null.  This is the case for a new allocation, rather than a resize of an existing block.  In this case, osize represents something else (see below), and you don't want to do the "memsize +=" line.

From the manual:

"When ptr is NULL, osize encodes the kind of object that Lua is allocating. osize is any of LUA_TSTRING, LUA_TTABLE, LUA_TFUNCTION, LUA_TUSERDATA, or LUA_TTHREAD when (and only when) Lua is creating a new object of that type. When osize is some other value, Lua is allocating memory for something else."


On 24 November 2014 at 06:02, Santosh Kadam <sanmanik@yahoo.com> wrote:
Hi, 

Greetings....

I am doing a few experiments with Lua and Lua memory consumptions. I have following memory allocator function registered with Lua via lua_newstate

lua_State *L = lua_newstate(myl_alloc,NULL);

static void *myl_alloc (void *ud, void *ptr, size_t osize,
                                                size_t nsize)
   {
       (void)ud;  (void)osize;  /* not used */
       if (nsize == 0) {
        memsize-=osize;
//        printf ("Freeing [%ld] bytes, memory size = [%ld] \n",osize, memsize);
         free(ptr);
         return NULL;
       }
       else
        {
/*
        if (memsize > 31000)
        {
          printf ("Memory exceeded 32K \n");
          return NULL;
        }
*/
         memsize+=(nsize-osize);
//         printf ("Allocating new [%ld] bytes,old size = [%ld], total = [%ld] \n",nsize, osize, memsize);
         return realloc(ptr, nsize);
        }
     }

This function adds / subtracts the memory size used by Lua and stores the same in the global memsize variable. 

At the end of the execution i.e. When I call lua_close - I expected to get a "zero" printed by the memsize but I am getting a negative number. 
----------
The memory size = [-1165]
----------

Why is it that the memory count negative ? 

I am pasting the code .....

Thanks and Regards
Santosh



Below is my script ..

------------------
function add(a, b)
    a=10
    b=20
    print (a+b)
--    collectgarbage("collect")
    print(collectgarbage("count"))
    print ("Add - after garbage collect")
    return a+b
end

function subt(a, b)
    a=10
    b=20
    print (b-a)
    print ("memory in subt")
 --   collectgarbage("collect")
    print(collectgarbage("count"))
    return b-a
end

add (3,4)
---------------------

And the complete program is as below...

---------------
#include"lua.h"
#include"lauxlib.h"
#include"lualib.h"
#include<stdlib.h>
#include<stdio.h>

long memsize = 0;

static void *myl_alloc (void *ud, void *ptr, size_t osize,
                                                size_t nsize);
int main ()
{
  char str [80], *str1, *str2;
  int i;

  FILE *f = fopen("./add.lua", "rb");
  fseek(f, 0, SEEK_END);
  long fsize = ftell(f);
  fseek(f, 0, SEEK_SET);

  str1 = malloc(fsize + 1);
  fread(str1, fsize, 1, f);
  fclose(f);
  printf ("Add String = \n[%s]\n",str1);

  FILE *f1 = fopen("./subt.lua", "rb");
  fseek(f1, 0, SEEK_END);
  fsize = ftell(f1);
  fseek(f1, 0, SEEK_SET);

  str2 = malloc(fsize + 1);
  fread(str2, fsize, 1, f1);
  fclose(f1);

  printf ("Factorial String = \n[%s]\n",str2);
  //lua_State *L = luaL_newstate();
  lua_State *L = lua_newstate(myl_alloc,NULL);
  luaL_openlibs(L);
  luaL_loadstring(L, str1);
  
/*
  if (lua_pcall(L,0, 1, 0)) {
    printf ("Something went wrong during execution [%s\n",lua_tostring(L, -1)) ;
    lua_pop(L,1);
  }
*/


  if (lua_pcall(L,0, LUA_MULTRET, 0)) {
            printf ("Something went wrong during execution [%s\n",lua_tostring(L, -1)) ;
            lua_pop(L,1);
  }


  do
  {
     printf ("Enter the operation: ");
     scanf ("%79s",str);  
   //  printf ("Memory before loadString = [%ld]\n",memsize);
     if (strcmp (str, "add") == 0)
      {
   //  printf ("Memory after loadString = [%ld]\n",memsize);


          lua_getglobal(L, "add");
          lua_pushnumber(L, 5);
          lua_pushnumber(L, 5);

          if (lua_pcall(L,2, 1, 0)) {
            printf ("Something went wrong during execution [%s\n",lua_tostring(L, -1)) ;
            lua_pop(L,1);
          }
          lua_pop(L,1);
    // printf ("Memory after pcall = [%ld]\n",memsize);

      }

     if (strcmp (str, "subt") == 0)
      {
        luaL_loadstring(L, str2);
          lua_getglobal(L, "subt");
          lua_pushnumber(L, 5);
          lua_pushnumber(L, 5);
        printf ("Inside subt [%s]\n", lua_tostring(L, -1));
          if (lua_pcall(L,2, 1, 0)) {
            printf ("Something went wrong during execution [%s\n",lua_tostring(L, -1)) ;
            lua_pop(L,1);
          }
            lua_pop(L,1);

      }

     printf ("You have entered %s \n",str);
  } while (strcmp (str, "quit"));
  
  lua_close(L);
printf ("The memory size = [%ld]\n",memsize);
  return 0;
}


static void *myl_alloc (void *ud, void *ptr, size_t osize,
                                                size_t nsize)
   {
       (void)ud;  (void)osize;  /* not used */
       if (nsize == 0) {
        memsize-=osize;
//        printf ("Freeing [%ld] bytes, memory size = [%ld] \n",osize, memsize);
         free(ptr);
         return NULL;
       }
       else
        {
/*
        if (memsize > 31000)
        {
          printf ("Memory exceeded 32K \n");
          return NULL;
        }
*/
         memsize+=(nsize-osize);
     //   printf ("Allocating new [%ld] bytes,old size = [%ld], total = [%ld] \n",nsize, osize, memsize);
         return realloc(ptr, nsize);
        }
     }
--------------