系统技术非业余研究

看到erlang-china.org上有个帖子在问这个问题

一直不太明白iolist,跟list到底有什么区别？请各位大侠指教。。

我翻看了半天erlang的文档也没写的太明白,所以就看看源码:
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}

从源码可以看出来iolist是这样的定义的:
1. []
2. binary
3. 列表, 每个元素是int(0-255)或者binary或者iolist.
其中binary是指 bitsize % 8 == 0 .
int 是0-255

root@ubuntu:/usr/src/otp# erl
Erlang R13B04 (erts-5.7.5) [source] [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的作用是用于往port送数据的时候.由于底层的系统调用如writev支持向量写, 就避免了无谓的iolist_to_binary这样的扁平话操作, 避免了内存拷贝,极大的提高了效率.
建议多用.