Can the string.byte function in Lua return a negative value?

I'm debugging someone else's code. I don't know Lua well. I would like to know if a negative return value is 开发者_JAVA百科even possible from string.byte.

No. The range of string.byte( ) should be 0..255 inclusive; the documentation does not specify, but the source code is clear:

static int str_byte (lua_State *L) {
  size_t l;
  const char *s = luaL_checklstring(L, 1, &l);
  ptrdiff_t posi = posrelat(luaL_optinteger(L, 2, 1), l);
  ptrdiff_t pose = posrelat(luaL_optinteger(L, 3, posi), l);
  int n, i;
  if (posi <= 0) posi = 1;
  if ((size_t)pose > l) pose = l;
  if (posi > pose) return 0;  /* empty interval; return no values */
  n = (int)(pose -  posi + 1);
  if (posi + n <= pose)  /* overflow? */
    luaL_error(L, "string slice too long");
  luaL_checkstack(L, n, "string slice too long");
  for (i=0; i<n; i++)
    lua_pushinteger(L, uchar(s[posi+i-1]));
  return n;
}

from lua-5.1.4/src/strlib.c, (C) 1994-2008 Lua.org; by BSD license

The important line is the call to lua_pushinteger, which is used to return an integer value to the calling function, and uchar which coerces the value to one in the range of 0..255.

It doesn't appear that it can return negative values - however I haven't worked with it much.

Here are some documentation links that might be helpful:

http://lua-users.org/wiki/StringLibraryTutorial

http://www.lua.org/manual/5.1/manual.html

if you have ANY questions regarding lua inner workings, its always easiest to just check the source: http://www.lua.org/source/5.1/lstrlib.c.html#str_byte (and yes, I do realize that understanding this requires a bit of work ;) )