需要帮助理解这种在C环code [英] Need help understanding this for loop code in C

问题描述

考虑以下code在C：

Consider the following code in C:

void main()
{
    int a=0;

    for(printf("\nA"); a; printf("\nB"));

    printf("\nC");
    printf("\nD");     
}

当我使用TURB C ++ 3.0版本和gcc-4.3.4，我得到以下为两种情况下输出编译：

When I compile it using Turb C++ version 3.0 and gcc-4.3.4, I get the following as the output in BOTH the cases :

A
C
D

不过，如果我编译以下code：

However, if I compile the following code:

void main()
{
    for(printf("\nA"); 0; printf("\nB"));

    printf("\nC");
    printf("\nD");
}

由GCC-4.3.4的输出是一样的，在previous案件，但涡轮C ++ 3.0生成以下的输出：

The output by gcc-4.3.4 is the same as in the previous case but turbo c++ 3.0 produces the following output :

A
B
C
D

首先，我不知道这里发生了什么！另外，怎么来用gcc编译器的输出是相同的两个在codeS，但TURBOC ++ 3.0编译器的情况下，输出不同呢？是否有人可以提供一些线索？

First of all, I have no idea what's happening here! Plus, how come the output by the gcc compiler is the same for both the codes but in the case of turboc++ 3.0 compiler, the output is different? Can someone please shed some light?

编辑：

其实有人问一家IT公司的采访这个问题时，他没有给出答案，面试官作了这样的解释。但我觉得这是愚蠢的。你怎么能要求别人用错误，仿佛它是由语言设施提供？对于它被称为工厂和技术，我们是否传递0作为第二恩pression文字或一个变量，其值为0，结局应该是一样的。

Actually someone was asked this question in an interview for an IT company and when he failed to give the answer, the interviewer gave this explanation. But I find this to be stupid. How can you ask someone to use a "bug" as if it's a "facility" provided by the language? For it to be called a "facility" and "technique", whether we pass 0 as a literal in the second expression or a variable whose value is 0, the outcome should have been the same.

我错了总结面试官是非常愚蠢的问这样的问题，它显示了他的无能？

Am I wrong concluding that the interviewer was very dumb to ask a question like that and that it shows his incompetence?

推荐答案

在TCC输出第二个例子是错误的。

The TCC output for the 2nd example is wrong.

从C99标准：

语句

的（ 第1 的 前pression-2 的 的前pression-3 的） 语句的

行为如下：这位前pression的前pression-2 的是控制
  这是循环体的每次执行之前评估前pression。
  这位前pression的前pression-3 的评价为无效前pression后
  循环体的每次执行。 [...] 的

behaves as follows: The expression expression-2 is the controlling expression that is evaluated before each execution of the loop body. The expression expression-3 is evaluated as a void expression after each execution of the loop body. [...]

显然，没有迭代这里，所以前pression -3- 的不应该被执行。

Obviously, there are no iterations here, so expression-3 should never be executed.

类似地，在C90标准（或至少在一个草案，我发现的），它说：

Similarly, in the C90 standard (or at least in a draft that I found), it says:

除了continue语句的循环体的行为时，
  声明

Except for the behavior of a continue statement in the loop body, the statement

     for (  expression-1 ;  expression-2 ;  expression-3 )  statement

和语句序列

      expression-1 ;
     while ( expression-2) {
               statement
              expression-3 ;
     }

  
  

是等效的。

are equivalent.

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