c - 转义序列差异

Question

 #include<stdio.h>
 int main()
 {
          int i=0;
          printf("%d:%4d\n",++i,'\1');
          printf("%d:%4d\n",++i,'\2');
          printf("%d:%4d\n",++i,'\3');
          printf("%d:%4d\n",++i,'\4');
          printf("%d:%4d\n",++i,'\5');
          printf("%d:%4d\n",++i,'\6');
          printf("%d:%4d\n",++i,'\7');
          printf("%d:%4d\n",++i,'\8');
          printf("%d:%4d\n",++i,'\9');
          printf("%d:%4d\n",++i,'\10');
          printf("%d:%4d\n",++i,'\11');
          printf("%d:%4d\n",++i,'\12');
          printf("%d:%4d\n",++i,'\13');
          printf("%d:%4d\n",++i,'\14');
          printf("%d:%4d\n",++i,'\15');
          printf("%d:%4d\n",++i,'\16');
          printf("%d:%4d\n",++i,'\17');
          printf("%d:%4d\n",++i,'\18');
          printf("%d:%4d\n",++i,'\19');
          printf("%d:%4d\n",++i,'\20');
          printf("%d:%4d\n",++i,'\21');
          printf("%d:%4d\n",++i,'\22');
          printf("%d:%4d\n",++i,'\23');
          printf("%d:%4d\n",++i,'\24');
          printf("%d:%4d\n",++i,'\25');
          printf("%d:%4d\n",++i,'\26');
          printf("%d:%4d\n",++i,'\27');
          printf("%d:%4d\n",++i,'\28');
          printf("%d:%4d\n",++i,'\29');
          printf("%d:%4d\n",++i,'\30');
          return 0;
 }

输出:

1:   1
2:   2
3:   3
4:   4
5:   5
6:   6
7:   7
8:  56    
9:  57
10:   8
11:   9
12:  10
13:  11
14:  12
15:  13
16:  14
17:  15
18: 312
19: 313
20:  16
21:  17
22:  18
23:  19
24:  20
25:  21
26:  22
27:  23
28: 568
29: 569
30:  24

'\1'不就相当于ASCII值为1的字符吗？
为什么在输出中，8、9、18、19、28、29...的数字顺序不对？ http://codepad.org/I1N6A71j

Answer 1

从 2.14.3 开始:

The escape \ooo consists of the backslash followed by one, two, or three octal digits that are taken to specify the value of the desired character. The escape \xhhh consists of the backslash followed by x followed by one or more hexadecimal digits that are taken to specify the value of the desired character. There is no limit to the number of digits in a hexadecimal sequence. A sequence of octal or hexadecimal digits is terminated by the first character that is not an octal digit or a hexadecimal digit, respectively.

由于 \8 和 \18 不是有效的八进制序列，这些文字的含义取决于您的平台:

Escape sequences in which the character following the backslash is not listed in Table 7 are conditionally-supported, with implementation-defined semantics.