解引用NULL指针和未初始化指针之间的差异 [英] Difference between dereferencing NULL pointer and uninitialised pointer

问题描述

代码块（C ++）

 ＃include< bits / stdc ++。 
 using namespace std; 
 int main（）{
 int * p; 
 cout<< * p; 
} 
  

产生垃圾值，而

 ＃include< bits / stdc ++。h> 
 using namespace std; 
 int main（）{
 int * p = NULL; 
 cout<< * p; 
} 
  

会导致运行时错误。
我期望两个运行时错误（ideone产生两个运行时错误），因为这两个都是错误的指针。

解决方案

您对运行时错误的期望有缺陷。

意味着潜在的症状范围：

• 没有发生任何事情


• 发生某些情况


• 发生运行时错误


• 您的源代码会自动编辑为使用正确的标准标头，而不是误导地黑客实施位


• 
  
• 您的猫被杀害


• 您的猫未被杀害

<

• 您的猫会自杀


• 您的猫打开一个黑洞


• 一只猫在黑洞中开启

等等。

这也适用于解除引用NULL，但是现代商品硬件倾向于特别处理NULL解引用，通常保证分段错误有助于诊断。显然，CPU不能为任意指针值，因为它们可以是有效的，只要它知道！

On code blocks(C++)

#include<bits/stdc++.h>
using namespace std;
int main(){
    int *p;
    cout<<*p;
}

produces garbage value, whereas

#include<bits/stdc++.h>
using namespace std;
int main(){
    int *p=NULL;
    cout<<*p;
}

results in runtime error. I expected runtime error for both (ideone produces runtime error for both) because both are bad pointers. How can one explain this?

解决方案

Your expectation of a runtime error is flawed.

Dereferencing an uninitialised/invalid pointer with arbitrary value can do anything at all.

That means the potential symptoms range from:

• nothing happens
• something happens
• a runtime error happens
• your source code is spontaneously edited to use proper standard headers instead of misguidedly hacking in implementation "bits"
• your attitude in the comments section is magically improved
• your cat is murdered
• your cat is not murdered
• your cat is murdered and not murdered
• your cat murders itself
• a black hole opens inside your cat
• a cat opens up inside a black hole

and so on.

This is true for dereferencing NULL, too, but modern commodity hardware tends to treat NULL dereferences specially, usually guaranteeing a segmentation fault to aid in diagnostics. Obviously, a CPU cannot do that for arbitrary pointer values, because they may be valid as far as it knows!

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