看到erlang-china.org上有個帖子在問這個問題

一直不太明白iolist,跟list到底有什麼區別？請各位大俠指教。。

我翻看了半天erlang的文件也沒寫的太明白,所以就看看原始碼:
erts/emulator/beam/utils.c

3015int io_list_len(Eterm obj)
3016{
3017    Eterm* objp;
3018    Sint len = 0;
3019    DECLARE_ESTACK(s);
3020    goto L_again;
3021
3022    while (!ESTACK_ISEMPTY(s)) {
3023        obj = ESTACK_POP(s);
3024    L_again:
3025        if (is_list(obj)) {
3026        L_iter_list:
3027            objp = list_val(obj);
3028            /* Head */
3029            obj = CAR(objp);
3030            if (is_byte(obj)) {
3031                len++;
3032            } else if (is_binary(obj) && binary_bitsize(obj) == 0) {
3033                len += binary_size(obj);
3034            } else if (is_list(obj)) {
3035                ESTACK_PUSH(s, CDR(objp));
3036                goto L_iter_list; /* on head */
3037            } else if (is_not_nil(obj)) {
3038                goto L_type_error;
3039            }
3040            /* Tail */
3041            obj = CDR(objp);
3042            if (is_list(obj))
3043                goto L_iter_list; /* on tail */
3044            else if (is_binary(obj) && binary_bitsize(obj) == 0) {
3045                len += binary_size(obj);
3046            } else if (is_not_nil(obj)) {
3047                goto L_type_error;
3048            }
3049        } else if (is_binary(obj) && binary_bitsize(obj) == 0) { /* Tail was binary */
3050            len += binary_size(obj);
3051        } else if (is_not_nil(obj)) {
3052            goto L_type_error;
3053        }
3054    }
3055
3056    DESTROY_ESTACK(s);
3057    return len;
3058
3059 L_type_error:
3060    DESTROY_ESTACK(s);
3061    return -1;
3062}

從原始碼可以看出來iolist是這樣的定義的:
1. []
2. binary
3. 列表, 每個元素是int(0-255)或者binary或者iolist.
其中binary是指 bitsize % 8 == 0 .
int 是0-255

[email protected]:/usr/src/otp# erl
Erlang R13B04 (erts-5.7.5)  [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.7.5  (abort with ^G)
2> iolist_size(<<>>).
0
3> iolist_size(<<1:1>>).
** exception error: bad argument
 in function  iolist_size/1
 called as iolist_size(<<1:1>>)
4> iolist_size(<<1:8>>).
1
5> iolist_size([]).
0
6> iolist_size(<<1,2>>).
2
7> iolist_size([1,2]).
2
8> iolist_size([1,2, <<1,2>>]).
4
9> iolist_size([1,2, <<1,2>>, [2]]).
5
10> iolist_size([1,2, <<1,2>>, [2]]).
5
11> iolist_size([<<1:1>>]).
** exception error: bad argument
 in function  iolist_size/1
 called as iolist_size([<<1:1>>])
12> iolist_size([257]).
** exception error: bad argument
     in function  iolist_size/1
        called as iolist_size([257])

Iolist的作用是用於往port送資料的時候.由於底層的系統呼叫如writev支援向量寫, 就避免了無謂的iolist_to_binary這樣的扁平話操作, 避免了記憶體拷貝,極大的提高了效率.
建議多用.

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