在C中使用非标准的数组声明 [英] Using non standard declaration of array in C

问题描述

我遇到了下面的代码，它以非标准的方式在 C 中声明 char * 数组：

  / *消息类型描述数组* / 
 char * msgType [M_LAST_MSG] = 
 {
 [M_INIT_MSG]Init，
 [M_RESET_MSG]Serdes Reset
}; 
  

M_INIT_MSG ， M_RESET_MSG 和 M_LAST_MSG 是相应值为0,1和2的枚举。
根据正式 C 文档中，这个数组中的变量是字符串（文字），那么以这种方式使用这些枚举的目的是什么？是否有任何文档支持它？

用ARM编译 gcc 编译器 gcc-arm-none-eabi 。

解决方案

此语法允许您通过索引初始化数组的特定元素。您可以使用 int 或 enum 值指定要初始化的数组元素。这样，你指定的值不需要是连续的。

如果你有这样的话：

  int x [5] = {[2] 3，[4] 7}; 
  

它相当于这个：

  int x [5] = {0，0，3，0，7}; 
  

在上例中，枚举值指定将数组的元素0和1初始化为Init和Serdes Reset。

从 C99标准：

18每个指示符列表都以
与最近的周围花括号
对相关联的当前对象。指示符列表中的每个项目（按顺序）指定
为其当前对象的特定成员，并将下一个指示符（如果有）的
当前对象更改为该成员。
在指定符列表结尾处生成的当前对象是
，子对象将由以下初始化程序初始化。

33示例9数组可以通过使用指定符来初始化为与枚举的
元素相对应：

  enum {member_one，member_two}; 
 const char * nm [] = {
 [member_two] =会员二，
 [会员一] =会员一，
}; 
  

编辑：

请注意，标准的语法包含一个 = ，而OP的例子没有。没有 = 的语法显然是GCC支持的旧语法。编译OP的例子给出了以下警告：

lockquote

警告：过期使用指定的初始值设定项而不使用'='

GCC文档 a>声明以下内容：

自GCC 2.5开始已经过时的另一种语法是GCC仍然接受的是[[index ]'之前的元素值，没有'='。

I came across the following code which declares char * array in C in a non-standard way:

    /* Message Type description array */ 
    char *msgType[M_LAST_MSG] = 
    {    
       [M_INIT_MSG]     "Init", 
       [M_RESET_MSG]    "Serdes Reset"
    };

M_INIT_MSG, M_RESET_MSG and M_LAST_MSG are enumerations with corresponding values of 0, 1 and 2. According to the formal C documentations, the variables inside this array are string (literals) so what is the purpose of using those enumerations in that manner and is there any documentation to back it up?

Compiled with ARM gcc compiler gcc-arm-none-eabi.

解决方案

This syntax allows you to initialize specific elements of an array by index. You can use either int or enum values to specify which array element to initialize. This way, the values you assign don't need to be consecutive.

If for example you had this:

int x[5] = { [2] 3, [4] 7 };

It would be equivalent to this:

int x[5] = { 0, 0, 3, 0, 7 };

In the above example, the enum values specify that elements 0 and 1 of the array are initialized to "Init" and "Serdes Reset".

From section 6.7.8 of the C99 standard:

18 Each designator list begins its description with the current object associated with the closest surrounding brace pair. Each item in the designator list (in order) specifies a particular member of its current object and changes the current object for the next designator (if any) to be that member. The current object that results at the end of the designator list is the subobject to be initialized by the following initializer.

33 EXAMPLE 9 Arrays can be initialized to correspond to the elements of an enumeration by using designators:

enum { member_one, member_two };
const char *nm[] = {
    [member_two] = "member two",
    [member_one] = "member one",
};

EDIT:

Note that the syntax from the standard includes a = while OP's example does not. The syntax without = is apparently an old syntax supported by GCC. Compiling OP's example gives the following warning:

warning: obsolete use of designated initializer without ‘=’

The GCC documentation states the following:

An alternative syntax for this that has been obsolete since GCC 2.5 but GCC still accepts is to write ‘[index]’ before the element value, with no ‘=’.

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