CODEWITHSUNDEEP
lua
rfeak
rfeak

Reputation: 8204

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 even possible from string.byte.

Upvotes: 1

Views: 1060

Answers (3)

Łukasz Gruner
Łukasz Gruner

Reputation: 3219

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 ;) )

Upvotes: 0

swdunlop
swdunlop

Reputation: 124

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.

Upvotes: 3

Rion Williams
Rion Williams

Reputation: 76577

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

Upvotes: 1

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