在C语言中为什么你需要一个goto标签之后的声明？ [英] In C why do you need a statement after a goto label?

问题描述

我写一些C code和我的code，我有两个嵌套循环。在特定的条件下我想破出内部循环和继续外循环。我试图做到这一点用在标签上外环的code结束，条件，转到的标签。然而 GCC 给我不能在复合语句的结尾有一个标签错误。为什么不呢？

注1：这不是开关语句和是的问题已回答<一个href=\"http://stackoverflow.com/questions/3455208/why-do-you-need-to-put-break-after-the-last-label-of-a-switch-statement\">elsewhere.

注2：这不是一个关于风格，我是否应该或不应该使用转到陈述或条件变量而不是问题。

。

编辑：有人问了一个例子，我可以给检查的一个稍微浅显的例子，如果一个数组是另一个数组的一个子

  INT superArray [SUPER_SIZE] = {...}，子阵[SUB_SIZE] = {...};
    INT superIndex，分类指数;    为（superIndex = 0; superIndex＆下; SUPER_SIZE-SUB_SIZE; superIndex + = 1）
    {
      对于（分类指数= 0;＆分类指数LT; SUB_SIZE;子索引+ = 1）
        如果（superArray [superIndex +子索引]！=子阵[子索引]）
          转到break_then_continue;      // code，它执行，如果子阵是一个子阵列      break_then_continue：
    }
 

解决方案

在标准对其进行了明确表示，标签属于一个说法，因此一个简单的分号（; ）后您的标签可以绕过你在运行的问题，因为这算作一个声明。

有甚至是使用的一个例子是空的 ¹ 在声明中的 6.8.3 / 6

  

例3 ：一种空语句还可以用于携带一个标签只是
  复合语句的结束}之前

 而（循环1）{
  / * ... * /  而（循环2）{
    / * ... * /    如果（want_out）
      转到end_loop1;    / * ... * /
  }  / * ... * /  end_loop1：;
}
 

¹ 的在标准这被称为空语句。的

---

  

6.8.1标记语句

 语法
  1标记的语句：
      鉴定网络连接器：声明
      案例不断-EX pression：声明
      默认：声明
 

的注意语句不在上述引文可选的。的

---

• open-std.org：n1124.pdf

I am writing some C code and in my code I have two nested loops. On a particular condition I want to break out of the inner loop and continue the outer loop. I tried to achieve this using a label at the end on the outer loop's code and, on the condition, goto that label. However gcc gives an error that I cannot have a label at the end of a compound statement. Why not?

Note 1: This is not a switch statement and that question has been answered elsewhere.

Note 2: This is not a question about style and whether I should or should not be using goto statements or conditional variables instead.

EDIT: People have asked for an example and I can give a slightly facile example of checking if an array is a subarray of another array

    int superArray[SUPER_SIZE] = {...}, subArray[SUB_SIZE] = {...};
    int superIndex, subIndex;

    for (superIndex=0; superIndex<SUPER_SIZE-SUB_SIZE; superIndex+=1)
    {
      for (subIndex=0; subIndex<SUB_SIZE; subIndex+=1)
        if (superArray[superIndex+subIndex] != subArray[subIndex])
          goto break_then_continue;

      // code that executes if subArray is a sub array

      break_then_continue:
    }

解决方案

In the standard it's explicitly said that labels belong to a statement, therefore a simple semicolon (;) after your label can circumvent the problem you are running in to, since that counts as a statement.

There is even an example of the use of an "empty"¹ statement in 6.8.3/6.

EXAMPLE 3 A null statement may also be used to carry a label just before the closing } of a compound statement

while (loop1) {
  /* ... */

  while (loop2) {
    /* ... */

    if (want_out)
      goto end_loop1;

    /* ... */
  }

  /* ... */

  end_loop1: ;
}

¹ In the standard this is referred to as a null statement.

---

6.8.1 Labeled statements

Syntax
  1 labeled-statement:
      identifier : statement
      case constant-expression : statement
      default : statement

Notice that statement isn't optional in the above quotation.

---

• open-std.org: n1124.pdf

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