本文探讨了在Lua 5.3中使用table.insert时出现的positionoutofbounds错误原因。通过对比Lua 5.1与Lua 5.3的行为差异，分析了源代码实现的不同之处，并解释了为何某些插入操作会导致错误。
Why statement "table.insert(touches, touch.id, touch)" return error in current Lua(Codea)?
Introduction
When I tried to run some Codea code on the new version, I found that some code about touch always return error. First the error is float without integer representation, after modified it with math.floor, the error changed to position out of bounds. It made me confuse(I had a post in the forum [SOLVED] Should touch.id be a int or a float?). I tried to look for the reason, below is the whole record.
Test code
The code can run on the old version Codea which using the lua-5.1, and the current Codea is using lua-5.3, so I guess the bug is because of the different version of lua. For the easy testing, I do the debug on my Raspberry Pi which have two versions of lua. the test code is below:
Test Code 1
touches = {}
touch={id=100}
table.insert(touches, math.floor(touch.id), touch)
return error in Lua-5.3.2:
pi@rpi /opt/software/lua-5.3.2 $ lua
Lua 5.3.2 Copyright (C) 1994-2015 Lua.org, PUC-Rio
>
> touches = {}
> touch={id=100}
> table.insert(touches, math.floor(touch.id), touch)
stdin:1: bad argument #2 to 'insert' (position out of bounds)
stack traceback:
[C]: in function 'table.insert'
stdin:1: in main chunk
[C]: in ?
>
return success in Lua-5.1.5:
pi@rpi /opt/software $ lua5.1
Lua 5.1.5 Copyright (C) 1994-2012 Lua.org, PUC-Rio
>
> touches = {}
> touch={id=100}
> table.insert(touches, math.floor(touch.id), touch)
>
Test Code 2
return error in Lua-5.3.2:
kano@kano ~ $ lua
Lua 5.3.2 Copyright (C) 1994-2015 Lua.org, PUC-Rio
> my={}
> table.insert(my,123,12)
stdin:1: bad argument #2 to 'insert' (position out of bounds)
stack traceback:
[C]: in function 'table.insert'
stdin:1: in main chunk
[C]: in ?
> table.insert(my,1,12)
> table.insert(my,2,12)
> table.insert(my,4,12)
stdin:1: bad argument #2 to 'insert' (position out of bounds)
stack traceback:
[C]: in function 'table.insert'
stdin:1: in main chunk
[C]: in ?
> my[123]=123
> #my
2
> unpack(my)
stdin:1: attempt to call a nil value (global 'unpack')
stack traceback:
stdin:1: in main chunk
[C]: in ?
> table.unpack(my)
12 12
> for k,v in pairs(my) do print(k,v) end
1 12
2 12
123 123
>
return success in Lua-5.1.5:
pi@rpi /opt/software/lua-5.3.2/src $ lua
Lua 5.1.5 Copyright (C) 1994-2012 Lua.org, PUC-Rio
> my={}
> table.insert(my,123,12)
> #my
stdin:1: unexpected symbol near '#'
> table.length(my)
stdin:1: attempt to call field 'length' (a nil value)
stack traceback:
stdin:1: in main chunk
[C]: ?
> table.len(my)
stdin:1: attempt to call field 'len' (a nil value)
stack traceback:
stdin:1: in main chunk
[C]: ?
> table.insert(my,1,12)
> table.insert(my,2,12)
> table.insert(my,4,12)
> my[123]=123
> print(#my)
4
> table.unpack(my)
stdin:1: attempt to call field 'unpack' (a nil value)
stack traceback:
stdin:1: in main chunk
[C]: ?
> for k,v in pairs(my) do print(k,v) end
1 12
2 12
4 12
123 123
>
Conclusion
It is easy to see:
- Use
table.inserton a empty table must start with position1 123only be treat with the hash key inmy, not array index.- The table length will not include the elements which stored with hash.
Analyze the lua source code
Use the command git grep -n "error info", we can find the relational function source code:
pi@rpi /opt/software/lua-5.3.2 $ git grep -n "position out of bounds"
src/ltablib.c:90: luaL_argcheck(L, 1 <= pos && pos <= e, 2, "position out of bounds");
src/ltablib.c:110: luaL_argcheck(L, 1 <= pos && pos <= size + 1, 1, "position out of bounds");
pi@rpi /opt/software/lua-5.3.2 $
The result is clear, we know that the code is in file src/ltablib.c, line 90 and 110, the code is below:
79 static int tinsert (lua_State *L) {
80 lua_Integer e = aux_getn(L, 1, TAB_RW) + 1; /* first empty element */
81 lua_Integer pos; /* where to insert new element */
82 switch (lua_gettop(L)) {
83 case 2: { /* called with only 2 arguments */
84 pos = e; /* insert new element at the end */
85 break;
86 }
87 case 3: {
88 lua_Integer i;
89 pos = luaL_checkinteger(L, 2); /* 2nd argument is the position */
90 luaL_argcheck(L, 1 <= pos && pos <= e, 2, "position out of bounds");
91 for (i = e; i > pos; i--) { /* move up elements */
92 lua_geti(L, 1, i - 1);
93 lua_seti(L, 1, i); /* t[i] = t[i - 1] */
94 }
95 break;
96 }
97 default: {
98 return luaL_error(L, "wrong number of arguments to 'insert'");
99 }
100 }
101 lua_seti(L, 1, pos); /* t[pos] = v */
102 return 0;
103 }
104
105
106 static int tremove (lua_State *L) {
107 lua_Integer size = aux_getn(L, 1, TAB_RW);
108 lua_Integer pos = luaL_optinteger(L, 2, size);
109 if (pos != size) /* validate 'pos' if given */
110 luaL_argcheck(L, 1 <= pos && pos <= size + 1, 1, "position out of bounds");
111 lua_geti(L, 1, pos); /* result = t[pos] */
112 for ( ; pos < size; pos++) {
113 lua_geti(L, 1, pos + 1);
114 lua_seti(L, 1, pos); /* t[pos] = t[pos + 1] */
115 }
116 lua_pushnil(L);
117 lua_seti(L, 1, pos); /* t[pos] = nil */
118 return 1;
119 }
We can see the check 1 <= pos && pos <= e in function tinsert and the check 1 <= pos && pos <= size + 1 in function tremove. And we can find the define of e and size:
lua_Integer e = aux_getn(L, 1, TAB_RW) + 1; /* first empty element */
The relational code:
27 #define TAB_R 1 /* read */
28 #define TAB_W 2 /* write */
29 #define TAB_L 4 /* length */
30 #define TAB_RW (TAB_R | TAB_W) /* read/write */
31
32
33 #define aux_getn(L,n,w) (checktab(L, n, (w) | TAB_L), luaL_len(L, n))
34
35
36 static int checkfield (lua_State *L, const char *key, int n) {
37 lua_pushstring(L, key);
38 return (lua_rawget(L, -n) != LUA_TNIL);
39 }
40
41
42 /*
43 ** Check that 'arg' either is a table or can behave like one (that is,
44 ** has a metatable with the required metamethods)
45 */
46 static void checktab (lua_State *L, int arg, int what) {
47 if (lua_type(L, arg) != LUA_TTABLE) { /* is it not a table? */
48 int n = 1; /* number of elements to pop */
49 if (lua_getmetatable(L, arg) && /* must have metatable */
50 (!(what & TAB_R) || checkfield(L, "__index", ++n)) &&
51 (!(what & TAB_W) || checkfield(L, "__newindex", ++n)) &&
52 (!(what & TAB_L) || checkfield(L, "__len", ++n))) {
53 lua_pop(L, n); /* pop metatable and tested metamethods */
54 }
55 else
56 luaL_argerror(L, arg, "table expected"); /* force an error */
57 }
58 }
Now we know the meaning: check the insert position, if it is bigger than the current size of the table, then return error with info "position out of bounds".
In version 5.1, we can insert in any position:
pi@rpi /opt/software $ lua5.1
Lua 5.1.5 Copyright (C) 1994-2012 Lua.org, PUC-Rio
> touches = {}
> touch={id=100}
> table.insert(touches, 1000000, touch)
>
Let us see the source code in version 5.1:
90 static int tinsert (lua_State *L) {
91 int e = aux_getn(L, 1) + 1; /* first empty element */
92 int pos; /* where to insert new element */
93 switch (lua_gettop(L)) {
94 case 2: { /* called with only 2 arguments */
95 pos = e; /* insert new element at the end */
96 break;
97 }
98 case 3: {
99 int i;
100 pos = luaL_checkint(L, 2); /* 2nd argument is the position */
101 if (pos > e) e = pos; /* `grow' array if necessary */
102 for (i = e; i > pos; i--) { /* move up elements */
103 lua_rawgeti(L, 1, i-1);
104 lua_rawseti(L, 1, i); /* t[i] = t[i-1] */
105 }
106 break;
107 }
108 default: {
109 return luaL_error(L, "wrong number of arguments to " LUA_QL("insert"));
110 }
111 }
112 luaL_setn(L, 1, e); /* new size */
113 lua_rawseti(L, 1, pos); /* t[pos] = v */
114 return 0;
115 }
116
117
118 static int tremove (lua_State *L) {
119 int e = aux_getn(L, 1);
120 int pos = luaL_optint(L, 2, e);
121 if (!(1 <= pos && pos <= e)) /* position is outside bounds? */
122 return 0; /* nothing to remove */
123 luaL_setn(L, 1, e - 1); /* t.n = n-1 */
124 lua_rawgeti(L, 1, pos); /* result = t[pos] */
125 for ( ;pos<e; pos++) {
126 lua_rawgeti(L, 1, pos+1);
127 lua_rawseti(L, 1, pos); /* t[pos] = t[pos+1] */
128 }
129 lua_pushnil(L);
130 lua_rawseti(L, 1, e); /* t[e] = nil */
131 return 1;
132 }
We can see that, it is a different dealing with the insert position argument.
if (pos > e) e = pos; /* `grow' array if necessary */
If the pos is bigger than the current max position(the e), then expand the table with the pos, It is a "grow" array, but it need alloc more memery. Maybe for the optimizing reason, in 5.3 this is not allowed, then the old code get error.
More details of source code
如果想了解更清楚, 可以在源代码里搜索一下函数(或者宏) luaL_argcheck:
pi@rpi /opt/software/lua-5.3.2 $ git grep -n "luaL_argcheck"
...
src/lauxlib.h:114:#define luaL_argcheck(L, cond,arg,extramsg) \
...
看样子是个宏, 打开 src/lauxlib.h, 查到如下宏定义:
114 #define luaL_argcheck(L, cond,arg,extramsg) \
115 ((void)((cond) || luaL_argerror(L, (arg), (extramsg))))
发现又调用了一个 luaL_argerror, 先在本文件里查一下, 发现有函数声明:
38 LUALIB_API int (luaL_argerror) (lua_State *L, int arg, const char *extramsg);
那么函数定义应该在 src/lauxlib.c 中, 再用 git grep -n 搜一把, 如下:
pi@rpi /opt/software/lua-5.3.2 $ git grep -n "luaL_argerror"
...
src/lauxlib.c:164:LUALIB_API int luaL_argerror (lua_State *L, int arg, const char *extramsg) {
...
很好, 打开看看具体代码:
164 LUALIB_API int luaL_argerror (lua_State *L, int arg, const char *extramsg) {
165 lua_Debug ar;
166 if (!lua_getstack(L, 0, &ar)) /* no stack frame? */
167 return luaL_error(L, "bad argument #%d (%s)", arg, extramsg);
168 lua_getinfo(L, "n", &ar);
169 if (strcmp(ar.namewhat, "method") == 0) {
170 arg--; /* do not count 'self' */
171 if (arg == 0) /* error is in the self argument itself? */
172 return luaL_error(L, "calling '%s' on bad self (%s)",
173 ar.name, extramsg);
174 }
175 if (ar.name == NULL)
176 ar.name = (pushglobalfuncname(L, &ar)) ? lua_tostring(L, -1) : "?";
177 return luaL_error(L, "bad argument #%d to '%s' (%s)",
178 arg, ar.name, extramsg);
179 }
看得出来, 我们的试验代码触发了最后一条判断语句:
...
175 if (ar.name == NULL)
176 ar.name = (pushglobalfuncname(L, &ar)) ? lua_tostring(L, -1) : "?";
177 return luaL_error(L, "bad argument #%d to '%s' (%s)",
178 arg, ar.name, extramsg);
...
--The End
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