今天看到的诡异的C语言语法

converse

今天看内核中neitfilter部分的代码,在iptable_filter.c中有这样的变量:

1. 
   
2. static struct
   
3. {
   
4. struct ipt_replace repl;
   
5. struct ipt_standard entries[3];
   
6. struct ipt_error term;
   
7. } initial_table __initdata
   
8. = { { "filter", FILTER_VALID_HOOKS, 4,
   
9.       sizeof(struct ipt_standard) * 3 + sizeof(struct ipt_error),
   
10.       { [NF_IP_LOCAL_IN] 0,
    
11. [NF_IP_FORWARD] sizeof(struct ipt_standard),
    
12. [NF_IP_LOCAL_OUT] sizeof(struct ipt_standard) * 2 },
    
13.       { [NF_IP_LOCAL_IN] 0,
    
14. [NF_IP_FORWARD] sizeof(struct ipt_standard),
    
15. [NF_IP_LOCAL_OUT] sizeof(struct ipt_standard) * 2 },
    
16.       0, NULL, { } },
    
17.     {
    
18.      /* LOCAL_IN */
    
19.      { { { { 0 }, { 0 }, { 0 }, { 0 }, "", "", { 0 }, { 0 }, 0, 0, 0 },
    
20.   0,
    
21.   sizeof(struct ipt_entry),
    
22.   sizeof(struct ipt_standard),
    
23.   0, { 0, 0 }, { } },
    
24.        { { { { IPT_ALIGN(sizeof(struct ipt_standard_target)), "" } }, { } },
    
25.   -NF_ACCEPT - 1 } },
    
26.      /* FORWARD */
    
27.      { { { { 0 }, { 0 }, { 0 }, { 0 }, "", "", { 0 }, { 0 }, 0, 0, 0 },
    
28.   0,
    
29.   sizeof(struct ipt_entry),
    
30.   sizeof(struct ipt_standard),
    
31.   0, { 0, 0 }, { } },
    
32.        { { { { IPT_ALIGN(sizeof(struct ipt_standard_target)), "" } }, { } },
    
33.   -NF_ACCEPT - 1 } },
    
34.      /* LOCAL_OUT */
    
35.      { { { { 0 }, { 0 }, { 0 }, { 0 }, "", "", { 0 }, { 0 }, 0, 0, 0 },
    
36.   0,
    
37.   sizeof(struct ipt_entry),
    
38.   sizeof(struct ipt_standard),
    
39.   0, { 0, 0 }, { } },
    
40.        { { { { IPT_ALIGN(sizeof(struct ipt_standard_target)), "" } }, { } },
    
41.   -NF_ACCEPT - 1 } }
    
42.     },
    
43.     /* ERROR */
    
44.     { { { { 0 }, { 0 }, { 0 }, { 0 }, "", "", { 0 }, { 0 }, 0, 0, 0 },
    
45. 0,
    
46. sizeof(struct ipt_entry),
    
47. sizeof(struct ipt_error),
    
48. 0, { 0, 0 }, { } },
    
49.       { { { { IPT_ALIGN(sizeof(struct ipt_error_target)), IPT_ERROR_TARGET } },
    
50.    { } },
    
51. "ERROR"
    
52.       }
    
53.     }
    
54. };
    
55. 
    
56. 
    

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其中的结构体定义struct ipt_replace
是:

1. 
   
2. /* The argument to IPT_SO_SET_REPLACE. */
   
3. struct ipt_replace
   
4. {
   
5. /* Which table. */
   
6. char name[IPT_TABLE_MAXNAMELEN];
   
7. 
   
8. /* Which hook entry points are valid: bitmask.  You can't
   
9.            change this. */
   
10. unsigned int valid_hooks;
    
11. 
    
12. /* Number of entries */
    
13. unsigned int num_entries;
    
14. 
    
15. /* Total size of new entries */
    
16. unsigned int size;
    
17. 
    
18. /* Hook entry points. */
    
19. unsigned int hook_entry[NF_IP_NUMHOOKS];
    
20. 
    
21. /* Underflow points. */
    
22. unsigned int underflow[NF_IP_NUMHOOKS];
    
23. 
    
24. /* Information about old entries: */
    
25. /* Number of counters (must be equal to current number of entries). */
    
26. unsigned int num_counters;
    
27. /* The old entries' counters. */
    
28. struct ipt_counters *counters;
    
29. 
    
30. /* The entries (hang off end: not really an array). */
    
31. struct ipt_entry entries[0];
    
32. };
    
33. 
    

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他对应的前面这一大段代码:

1. 
   
2. { "filter", FILTER_VALID_HOOKS, 4,
   
3.       sizeof(struct ipt_standard) * 3 + sizeof(struct ipt_error),
   
4.       { [NF_IP_LOCAL_IN] 0,
   
5. [NF_IP_FORWARD] sizeof(struct ipt_standard),
   
6. [NF_IP_LOCAL_OUT] sizeof(struct ipt_standard) * 2 },
   
7.       { [NF_IP_LOCAL_IN] 0,
   
8. [NF_IP_FORWARD] sizeof(struct ipt_standard),
   
9. [NF_IP_LOCAL_OUT] sizeof(struct ipt_standard) * 2 },
   
10.       0, NULL, { } }
    
11. 
    

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关键在于:

1. 
   
2. { [NF_IP_LOCAL_IN] 0,
   
3. [NF_IP_FORWARD] sizeof(struct ipt_standard),
   
4. [NF_IP_LOCAL_OUT] sizeof(struct ipt_standard) * 2 }
   
5. 
   

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对应的是数组

1. 
   
2. unsigned int hook_entry[NF_IP_NUMHOOKS];
   

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我试了一下,数组确实是可以这样来初始化的:

1. 
   
2. int main(int argc, char *argv[])
   
3. {
   
4. int array[5] = {[3]1, [1]1 , [2] 2, [4] 4};
   
5. int i;
   
6. 
   
7. for (i = 0; i < 5; ++i)
   
8. {
   
9.   printf("array = %d\n", array[i]);
   
10. }
    
11. 
    
12. return 0;
    
13. }
    
14. 
    

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yaj114777175

是可以这样初始化啊,你想表达什么??

converse

我试了一下,C89的标准都可以,gcc带-std=c89参数编译可以通过

converse

我想说我之前没有看过这样的语法...

converse

之前看到的,顶多是{[0]=0}这样的语法,不带等号的还真没见过....

albcamus

]$ gcc junk.c  -std=c89 -ansi -pedantic
junk.c: 在函数‘main’中:
junk.c:3: 警告：过时的用法，应使用‘=’来指定元素初始值
junk.c:3: 警告：过时的用法，应使用‘=’来指定元素初始值
junk.c:3: 警告：过时的用法，应使用‘=’来指定元素初始值
junk.c:3: 警告：过时的用法，应使用‘=’来指定元素初始值
junk.c:8: 警告：隐式声明与内建函数‘printf’不兼容


应该是超老的写法了

converse

哦,后面几个参数啥意思?我只写了std=89

koolcoy

不带等号的确没见过~

倒是加过c99的.foo = bar;的初始化语法的。

retuor

是很诡异。查了一下，是 gnu 的扩展：

Using and Porting the GNU Compiler Collection (GCC)

This manual documents how to run, install and port the GNU compiler, as well as its new features and incompatibilities, and how to report bugs. It corresponds to GCC version 2.95.

4.20 Labeled Elements in Initializers

Standard C requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.

In GNU C you can give the elements in any order, specifying the array indices or structure field names they apply to. This extension is not implemented in GNU C++.

To specify an array index, write `[index]' or `[index] =' before the element value. For example,

       

int a[6] = { [4] 29, [2] = 15 };

is equivalent to

       

int a[6] = { 0, 0, 15, 0, 29, 0 };

albcamus

-pedantic好像是为了警告使用gcc c扩展的选项，和-ansi或者-std=一起用，具体man一下吧