diff --git a/lstrlib.c b/lstrlib.c
index dde868c0..41ebc523 100644
--- a/lstrlib.c
+++ b/lstrlib.c
@@ -60,23 +60,50 @@ static int str_len (lua_State *L) {
 }
 
 
-/* translate a relative string position: negative means back from end */
-static lua_Integer posrelat (lua_Integer pos, size_t len) {
-  if (pos >= 0) return pos;
-  else if (0u - (size_t)pos > len) return 0;
-  else return (lua_Integer)len + pos + 1;
+/*
+** translate a relative initial string position
+** (negative means back from end): clip result to [1, inf).
+** The length of any string in Lua must fit in a lua_Integer,
+** so there are no overflows in the casts.
+** The inverted comparison avoids a possible overflow
+** computing '-pos'.
+*/
+static size_t posrelatI (lua_Integer pos, size_t len) {
+  if (pos > 0)
+    return (size_t)pos;
+  else if (pos == 0)
+    return 1;
+  else if (pos < -(lua_Integer)len)  /* inverted comparison */
+    return 1;  /* clip to 1 */
+  else return len + (size_t)pos + 1;
+}
+
+
+/*
+** Gets an optional ending string position from argument 'arg',
+** with default value 'def'.
+** Negative means back from end: clip result to [0, len]
+*/
+static size_t getendpos (lua_State *L, int arg, lua_Integer def,
+                         size_t len) {
+  lua_Integer pos = luaL_optinteger(L, arg, def);
+  if (pos > (lua_Integer)len)
+    return len;
+  else if (pos >= 0)
+    return (size_t)pos;
+  else if (pos < -(lua_Integer)len)
+    return 0;
+  else return len + (size_t)pos + 1;
 }
 
 
 static int str_sub (lua_State *L) {
   size_t l;
   const char *s = luaL_checklstring(L, 1, &l);
-  lua_Integer start = posrelat(luaL_checkinteger(L, 2), l);
-  lua_Integer end = posrelat(luaL_optinteger(L, 3, -1), l);
-  if (start < 1) start = 1;
-  if (end > (lua_Integer)l) end = l;
+  size_t start = posrelatI(luaL_checkinteger(L, 2), l);
+  size_t end = getendpos(L, 3, -1, l);
   if (start <= end)
-    lua_pushlstring(L, s + start - 1, (size_t)(end - start) + 1);
+    lua_pushlstring(L, s + start - 1, (end - start) + 1);
   else lua_pushliteral(L, "");
   return 1;
 }
@@ -149,11 +176,10 @@ static int str_rep (lua_State *L) {
 static int str_byte (lua_State *L) {
   size_t l;
   const char *s = luaL_checklstring(L, 1, &l);
-  lua_Integer posi = posrelat(luaL_optinteger(L, 2, 1), l);
-  lua_Integer pose = posrelat(luaL_optinteger(L, 3, posi), l);
+  lua_Integer pi = luaL_optinteger(L, 2, 1);
+  size_t posi = posrelatI(pi, l);
+  size_t pose = getendpos(L, 3, pi, l);
   int n, i;
-  if (posi < 1) posi = 1;
-  if (pose > (lua_Integer)l) pose = l;
   if (posi > pose) return 0;  /* empty interval; return no values */
   if (pose - posi >= INT_MAX)  /* arithmetic overflow? */
     return luaL_error(L, "string slice too long");
@@ -171,8 +197,8 @@ static int str_char (lua_State *L) {
   luaL_Buffer b;
   char *p = luaL_buffinitsize(L, &b, n);
   for (i=1; i<=n; i++) {
-    lua_Integer c = luaL_checkinteger(L, i);
-    luaL_argcheck(L, uchar(c) == c, i, "value out of range");
+    lua_Unsigned c = (lua_Unsigned)luaL_checkinteger(L, i);
+    luaL_argcheck(L, c <= (lua_Unsigned)UCHAR_MAX, i, "value out of range");
     p[i - 1] = uchar(c);
   }
   luaL_pushresultsize(&b, n);
@@ -695,16 +721,15 @@ static int str_find_aux (lua_State *L, int find) {
   size_t ls, lp;
   const char *s = luaL_checklstring(L, 1, &ls);
   const char *p = luaL_checklstring(L, 2, &lp);
-  lua_Integer init = posrelat(luaL_optinteger(L, 3, 1), ls);
-  if (init < 1) init = 1;
-  else if (init > (lua_Integer)ls + 1) {  /* start after string's end? */
+  size_t init = posrelatI(luaL_optinteger(L, 3, 1), ls) - 1;
+  if (init > ls) {  /* start after string's end? */
     lua_pushnil(L);  /* cannot find anything */
     return 1;
   }
   /* explicit request or no special characters? */
   if (find && (lua_toboolean(L, 4) || nospecials(p, lp))) {
     /* do a plain search */
-    const char *s2 = lmemfind(s + init - 1, ls - (size_t)init + 1, p, lp);
+    const char *s2 = lmemfind(s + init, ls - init, p, lp);
     if (s2) {
       lua_pushinteger(L, (s2 - s) + 1);
       lua_pushinteger(L, (s2 - s) + lp);
@@ -713,7 +738,7 @@ static int str_find_aux (lua_State *L, int find) {
   }
   else {
     MatchState ms;
-    const char *s1 = s + init - 1;
+    const char *s1 = s + init;
     int anchor = (*p == '^');
     if (anchor) {
       p++; lp--;  /* skip anchor character */
@@ -777,11 +802,14 @@ static int gmatch (lua_State *L) {
   size_t ls, lp;
   const char *s = luaL_checklstring(L, 1, &ls);
   const char *p = luaL_checklstring(L, 2, &lp);
+  size_t init = posrelatI(luaL_optinteger(L, 3, 1), ls) - 1;
   GMatchState *gm;
-  lua_settop(L, 2);  /* keep them on closure to avoid being collected */
+  lua_settop(L, 2);  /* keep strings on closure to avoid being collected */
   gm = (GMatchState *)lua_newuserdatauv(L, sizeof(GMatchState), 0);
+  if (init > ls)  /* start after string's end? */
+    init = ls + 1;  /* avoid overflows in 's + init' */
   prepstate(&gm->ms, L, s, ls, p, lp);
-  gm->src = s; gm->p = p; gm->lastmatch = NULL;
+  gm->src = s + init; gm->p = p; gm->lastmatch = NULL;
   lua_pushcclosure(L, gmatch_aux, 3);
   return 1;
 }
@@ -1572,7 +1600,7 @@ static int str_unpack (lua_State *L) {
   const char *fmt = luaL_checkstring(L, 1);
   size_t ld;
   const char *data = luaL_checklstring(L, 2, &ld);
-  size_t pos = (size_t)posrelat(luaL_optinteger(L, 3, 1), ld) - 1;
+  size_t pos = posrelatI(luaL_optinteger(L, 3, 1), ld) - 1;
   int n = 0;  /* number of results */
   luaL_argcheck(L, pos <= ld, 3, "initial position out of string");
   initheader(L, &h);
diff --git a/lvm.c b/lvm.c
index 652095dc..23e7ff70 100644
--- a/lvm.c
+++ b/lvm.c
@@ -991,7 +991,8 @@ void luaV_finishOp (lua_State *L) {
 
 /*
 ** Protect code that will finish the loop (returns) or can only raise
-** errors.
+** errors. (That is, it will not return to the interpreter main loop
+** after changing the stack or hooks.)
 */
 #define halfProtect(exp)  (savepc(L), (exp))
 
@@ -1607,7 +1608,7 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
           L->top = ra;
           halfProtect(luaD_poscall(L, ci, 0));  /* no hurry... */
         }
-        else {
+        else {  /* do the 'poscall' here */
           int nres = ci->nresults;
           L->ci = ci->previous;  /* back to caller */
           L->top = base - 1;
@@ -1621,7 +1622,7 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
           L->top = ra + 1;
           halfProtect(luaD_poscall(L, ci, 1));  /* no hurry... */
         }
-        else {
+        else {  /* do the 'poscall' here */
           int nres = ci->nresults;
           L->ci = ci->previous;  /* back to caller */
           if (nres == 0)
@@ -1652,8 +1653,8 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
         lua_Integer ilimit, initv;
         int stopnow;
         if (unlikely(!forlimit(plimit, &ilimit, 1, &stopnow))) {
-            savestate(L, ci);  /* for the error message */
-            luaG_forerror(L, plimit, "limit");
+          savestate(L, ci);  /* for the error message */
+          luaG_forerror(L, plimit, "limit");
         }
         initv = (stopnow ? 0 : ivalue(init));
         setivalue(plimit, ilimit);
@@ -1717,8 +1718,7 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
         vmbreak;
       }
       vmcase(OP_TFORPREP) {
-        /* is 'toclose' not nil? */
-        if (!ttisnil(s2v(ra + 3))) {
+        if (!ttisnil(s2v(ra + 3))) {  /* is 'toclose' not nil? */
           /* create to-be-closed upvalue for it */
           halfProtect(luaF_newtbcupval(L, ra + 3));
         }
diff --git a/manual/manual.of b/manual/manual.of
index b9ab1ebe..421d04de 100644
--- a/manual/manual.of
+++ b/manual/manual.of
@@ -83,25 +83,10 @@ it usually represents the absence of a useful value.
 The type @emph{boolean} has two values, @false and @true.
 Both @nil and @false make a condition false;
 any other value makes it true.
-The type @emph{number} represents both
-integer numbers and real (floating-point) numbers.
-The type @emph{string} represents immutable sequences of bytes.
-@index{eight-bit clean}
-Lua is 8-bit clean:
-strings can contain any 8-bit value,
-including @x{embedded zeros} (@Char{\0}).
-Lua is also encoding-agnostic;
-it makes no assumptions about the contents of a string.
 
-The type @emph{number} uses two internal representations,
-or two @x{subtypes},
-one called @def{integer} and the other called @def{float}.
-Lua has explicit rules about when each representation is used,
-but it also converts between them automatically as needed @see{coercion}.
-Therefore,
-the programmer may choose to mostly ignore the difference
-between integers and floats
-or to assume complete control over the representation of each number.
+The type @emph{number} represents both
+integer numbers and real (floating-point) numbers,
+using two @x{subtypes}: @def{integer} and @def{float}.
 Standard Lua uses 64-bit integers and double-precision (64-bit) floats,
 but you can also compile Lua so that it
 uses 32-bit integers and/or single-precision (32-bit) floats.
@@ -110,6 +95,22 @@ is particularly attractive
 for small machines and embedded systems.
 (See macro @id{LUA_32BITS} in file @id{luaconf.h}.)
 
+Lua has explicit rules about when each subtype is used,
+but it also converts between them automatically as needed @see{coercion}.
+Therefore,
+the programmer may choose to mostly ignore the difference
+between integers and floats
+or to assume complete control over the representation of each number.
+
+The type @emph{string} represents immutable sequences of bytes.
+@index{eight-bit clean}
+Lua is 8-bit clean:
+strings can contain any 8-bit value,
+including @x{embedded zeros} (@Char{\0}).
+Lua is also encoding-agnostic;
+it makes no assumptions about the contents of a string.
+The length of any string in Lua must fit in a Lua integer.
+
 Lua can call (and manipulate) functions written in Lua and
 functions written in C @see{functioncall}.
 Both are represented by the type @emph{function}.
@@ -6788,13 +6789,16 @@ the string argument should not contain @x{embedded zeros}.
 
 }
 
-@LibEntry{string.gmatch (s, pattern)|
+@LibEntry{string.gmatch (s, pattern [, init])|
 Returns an iterator function that,
 each time it is called,
 returns the next captures from @id{pattern} @see{pm}
 over the string @id{s}.
 If @id{pattern} specifies no captures,
 then the whole match is produced in each call.
+A third, optional numeric argument @id{init} specifies
+where to start the search;
+its default value @N{is 1} and can be negative.
 
 As an example, the following loop
 will iterate over all the words from string @id{s},
diff --git a/testes/pm.lua b/testes/pm.lua
index 1afaccf6..8cc8772e 100644
--- a/testes/pm.lua
+++ b/testes/pm.lua
@@ -297,6 +297,35 @@ for k,v in pairs(t) do assert(k+1 == v+0); a=a+1 end
 assert(a == 3)
 
 
+do   -- init parameter in gmatch
+  local s = 0
+  for k in string.gmatch("10 20 30", "%d+", 3) do
+    s = s + tonumber(k)
+  end
+  assert(s == 50)
+
+  s = 0
+  for k in string.gmatch("11 21 31", "%d+", -4) do
+    s = s + tonumber(k)
+  end
+  assert(s == 32)
+
+  -- there is an empty string at the end of the subject
+  s = 0
+  for k in string.gmatch("11 21 31", "%w*", 9) do
+    s = s + 1
+  end
+  assert(s == 1)
+
+  -- there are no empty strings after the end of the subject
+  s = 0
+  for k in string.gmatch("11 21 31", "%w*", 10) do
+    s = s + 1
+  end
+  assert(s == 0)
+end
+
+
 -- tests for `%f' (`frontiers')
 
 assert(string.gsub("aaa aa a aaa a", "%f[%w]a", "x") == "xaa xa x xaa x")
diff --git a/testes/strings.lua b/testes/strings.lua
index 587a0e06..88480924 100644
--- a/testes/strings.lua
+++ b/testes/strings.lua
@@ -94,6 +94,11 @@ assert(string.char(string.byte("\xe4l\0
 assert(string.char(string.byte("\xe4l\0ķu", 1, 0)) == "")
 assert(string.char(string.byte("\xe4l\0ķu", -10, 100)) == "\xe4l\0ķu")
 
+checkerror("out of range", string.char, 256)
+checkerror("out of range", string.char, -1)
+checkerror("out of range", string.char, math.maxinteger)
+checkerror("out of range", string.char, math.mininteger)
+
 assert(string.upper("ab\0c") == "AB\0C")
 assert(string.lower("\0ABCc%$") == "\0abcc%$")
 assert(string.rep('teste', 0) == '')
diff --git a/testes/tpack.lua b/testes/tpack.lua
index 4c5fc7f7..2b9953f8 100644
--- a/testes/tpack.lua
+++ b/testes/tpack.lua
@@ -314,9 +314,7 @@ do    -- testing initial position
   for i = 1, #x + 1 do
     assert(unpack("c0", x, i) == "")
   end
-  checkerror("out of string", unpack, "c0", x, 0)
   checkerror("out of string", unpack, "c0", x, #x + 2)
-  checkerror("out of string", unpack, "c0", x, -(#x + 1))
  
 end