Reallocating/resizing Lua 5.1 userdata in C - c

Question

How do you resize a Lua 5.1 userdata object in C during runtime?
I would like to change the size of the NumArray structure described in Roberto Ierusalimschy's book Programming in Lua, 2nd edition, pp. 260, from the Lua 5.1 console.
My NumArray userdata can either store unsigned chars or lua_Numbers. I played around with calling the unmodified luaM_realloc_ function defined in lmem.c, but finally any call to C's realloc
(via l_alloc) function just returns NULL, so I get a not enough memory error message.
Would somebody please help me ?
--- lapi.c -------------------------------------------------------------------------------------------------
LUA_API void *agn_resizeud (lua_State *L, void *block, size_t osize, size_t nsize) {
Udata *u = (Udata *)luaM_realloc_(L, block, osize + sizeof(Udata), nsize + sizeof(Udata));
u->uv.len = nsize;
if (u == NULL)
luaL_error(L, "Error in " LUA_QS ": failed to allocate memory.", "API/agn_resizeud");
return u;
}
--- numarray.c ---------------------------------------------------------------------------------------------
#define checkarray(L, n) (NumArray *)luaL_checkudata(L, n, "numarray")
[...]
/* Auxiliary C function to finally call luaM_realloc_ via the Lua C API */
void *reallocud (lua_State *L, void *block, size_t o, size_t n, size_t sizeofelem, size_t sizeofnumarray) {
if (n + 1 > MAX_SIZET/sizeofelem)
luaL_error(L, "Error in " LUA_QS ": memory allocation error: block too big.", "(reallocud)", n);
else
return agn_resizeud(L, block, sizeofnumarray + o*sizeofelem, sizeofnumarray + n*sizeofelem);
}
/* function to resize a NumArray */
static int numarray_resize (lua_State *L) {
size_t i;
global_State *g = G(L);
NumArray *a = checkarray(L, 1);
int n = luaL_checkinteger(L, 2);
if (n < 1)
luaL_error(L, "Error in " LUA_QS ": new size %d is non-positive.", "numarray.resize", n);
if (n == a->size) { /* do nothing and do not complain */
lua_pushinteger(L, a->size);
return 1;
}
if (n > a->size) { /* extend */
a = reallocud(L, a, a->size, n, a->isnumber ? sizeof(lua_Number) : sizeof(char), sizeof(NumArray));
if (a->isnumber) {
for (i=a->size; i < n; i++) a->data.n[i] = 0;
} else {
for (i=a->size; i < n; i++) a->data.c[i] = 0;
}
} else { /* reduce */
if (a->isnumber) {
for (i=a->size - 1; i > n - 1; i--) a->data.n[i] = 0;
} else {
for (i=a->size - 1; i > n - 1; i--) a->data.c[i] = 0;
}
a = reallocud(L, a, a->size, n, a->isnumber ? sizeof(lua_Number) : sizeof(char), sizeof(NumArray));
}
a->size = n;
lua_pushnumber(L, a->size);
return 1;
}
[...]
typedef struct NumArray {
size_t size; /* number of slots */
char isnumber;
union data {
lua_Number n[1]; /* pointer to the various lua_Number values */
unsigned char c[1];
} data;
} NumArray;

Answer 1

There is no need to mess with the internals of Lua.
But you need to change how the userdata is created: instead of allocating the struct and the data in the same memory block, allocate them separately, using lua_newuserdata to allocate the struct and malloc to allocate the data.
Then, to resize the data part, just update the size in the struct and realloc the data part.

Answer 2

you have:
static int numarray_resize (lua_State *L) {
[...]
if (n > a->size) { /* extend */
a = realloc(L, a, a->size, n, a->isnumber ? sizeof(lua_Number) : sizeof(char), sizeof(NumArray));
if (a->isnumber) {
[...]
But realloc takes 2 arguments, you want reallocud.
reallocing L with size a is likely to be a giant number and L may not have ever been allocated by the alloc family so there a couple reasons to get NULL unexpectedly.