'goto * foo'foo不是指针。这是什么？ [英] 'goto *foo' where foo is not a pointer. What is this?

问题描述

我正在玩标签作为值，并以此代码结束。

  int foo = 0; 
 goto * foo; 
  

我的C / C ++经验告诉我 * foo 意味着 dereference foo ，并且这不会被编译，因为 foo 不是一个指针。但它确实编译。这实际上是做什么的？

gcc（Ubuntu 4.9.2-0ubuntu1〜12.04）4.9.2 ，if

解决方案

这是gcc中已知的一个bug。 gcc的文档扩展名允许表单的声明

  goto * ptr; 
  

其中 ptr 可以是任何类型的表达式 void *的。作为该扩展的一部分，对标签名称应用一个&& 产生标签的地址，类型为 void *


  int foo 
 = 0; 
 goto * foo; 
  

foo 显然是 int ，而不是类型 void * 。一个 int 值可以被转换为 void * ，但只能用明确的转换（除了特殊情况空白指针常量，这里不适用）。
$ b $

表达式 * foo 本身被正确诊断为错误。这个：



  goto * 42; 
  

无错地编译（生成的机器代码似乎是跳转到地址 42 ，如果我正确读取汇编代码）。



快速实验表明gcc为



  goto * 42; 
  

如同它为



  goto *（void *）42; 
  

后者是文档扩展名的正确使用方式，对于某些用户来说，原因，你想跳转到地址42。

我已经提交了错误报告 - 该错误报告很快以此错误复制报告，于2007年提交。

I was playing around with labels as values and ended up with this code.

int foo = 0;
goto *foo;

My C/C++ experience tells me *foo means dereference foo and that this won't compile because foo isn't a pointer. But it does compile. What does this actually do?

gcc (Ubuntu 4.9.2-0ubuntu1~12.04) 4.9.2, if important.

解决方案

This is a known bug in gcc.

gcc has a documented extension that permits a statement of the form

goto *ptr;

where ptr can be any expression of type void*. As part of this extension, applying a unary && to a label name yields the address of the label, of type void*.

In your example:

int foo = 0;
goto *foo;

foo clearly is of type int, not of type void*. An int value can be converted to void*, but only with an explicit cast (except in the special case of a null pointer constant, which does not apply here).

The expression *foo by itself is correctly diagnosed as an error. And this:

goto *42;

compiles without error (the generated machine code appears to be a jump to address 42, if I'm reading the assembly code correctly).

A quick experiment indicates that gcc generates the same assembly code for

goto *42;

as it does for

goto *(void*)42;

The latter is a correct use of the documented extension, and it's what you should probably if, for some reason, you want to jump to address 42.

I've submitted a bug report -- which was quickly closed as a duplicate of this bug report, submitted in 2007.

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