luaL_prepbuffsize
char *b = luaL_prepbuffsize(B, l);
return luaL_prepbuffsize(B, sz);
((void)((B)->n < (B)->size || luaL_prepbuffsize((B), 1)), \
LUALIB_API char *(luaL_prepbuffsize) (luaL_Buffer *B, size_t sz);
#define luaL_prepbuffer(B) luaL_prepbuffsize(B, LUAL_BUFFERSIZE)
char *buff = luaL_prepbuffsize(&b, MAX_ITEM); /* to put formatted item */