我应该担心在指针铸造对齐？ [英] Should I worry about the alignment during pointer casting?

问题描述

在我的项目，我们有一张code是这样的：

  //原始数据由4个整数
unsigned char型数据[16];
INT I1，I2，I3，I4;
I1 = *（（为int *）的数据）;
I2 = *（（为int *）（数据+ 4））;
I3 = *（（为int *）（数据+ 8））;
I4 = *（（为int *）（数据+ 12））;
 

我跟我的技术主管，这code可能无法移植，因为它试图投无符号字符* 到 INT * 通常具有更严格对齐要求。但是，技术主管说，没关系，大多数编译器仍然铸造后相同的指针值，我可以只写code这样的。

要坦率地说，我真的不相信。经过研究，我发现一些人对使用指针件类似上面，例如，<一的href="https://www.securecoding.cert.org/confluence/display/cplusplus/EXP36-CPP.+Do+not+convert+pointers+into+more+strictly+aligned+pointer+types">here这里 。

因此​​，这里是我的问题：

1. 铸造一个真正的项目之后，是不是真的安全取消引用指针？
2. 有C风格的铸件之间有什么区别和 reinter pret_cast ？
3. 有C和C ++？
有什么区别

解决方案

  

1。铸造一个真正的项目之后，是不是真的安全取消引用指针？

如果指针恰好没有正确对齐它真的会导致问题。我亲眼所见和固定造成的铸造的char * 来一个更严格对齐类型总线错误。即使你没有得到一个明显的错误，你可以有不太明显的问题，如性能下降。严格按照标准，以避免UB是，即使你没有立即看到任何问题，是个好主意。 （和code是打破规则是严格走样规则第3.10 / 10）

一个更好的替代方法是使用的std ::的memcpy（）或的std :: memmove与如果缓冲区重叠（或更好，但 bit_cast＆LT;＆GT;（） ）

  unsigned char型数据[16];
INT I1，I2，I3，I4;
的std ::的memcpy（安培; I1，数据的sizeof（INT））;
的std ::的memcpy（安培; I2，数据+ 4的sizeof（INT））;
的std ::的memcpy（安培;酷睿i3，数据+ 8的sizeof（INT））;
的std ::的memcpy（安培; I4，数据+ 12，的sizeof（INT））;
 

一些编译器的工作比别人更努力，以确保字符数组排列更严格的比必要的，因为程序员常常犯了这个错误，但。

 的#include＆LT; cstdint＆GT;
＃包括＆LT;所属类别＆GT;
＃包括＆LT;的iostream＆GT;

模板＆LT; typename的T＆GT;无效check_aligned（无效* P）{
    性病::法院＆LT;＆LT; P＆LT;＆LT; 是＆其中;＆其中;
      （0 ==（reinter pret_cast＆LT;的std ::使用intptr_t＆GT（P）％alignof（T））：NOT）＆LT;＆LT;
      对齐的类型＆LT;＆LT; typeid的（T）。名称（）＆LT;＆LT; '\ N';
}

无效foo1（）{
    所以char a;
    炭B〔的sizeof（INT）];
    check_aligned＆LT; INT＆GT;（二）; //未对齐铛
}

的struct {
    所以char a;
    炭B〔的sizeof（INT）];
};

无效foo2的（）{
    šš;
    check_aligned＆LT; INT＆GT;（S.B）; //未对齐的叮当声和MSVC
}

šš;

无效foo3（）{
    check_aligned＆LT; INT＆GT;（S.B）; //未对齐铛，MSVC和海湾合作委员会
}

诠释的main（）{
    foo1（）;
    foo2的（）;
    foo3（）;
}
 

http://ideone.com/FFWCjf

  

2。有没有C风格的铸造和reinter pret_cast？

有什么区别

这要看情况。 C风格的类型转换操作取决于所涉及的类型的不同的事情。指针类型之间的C风格的铸件会导致同样的事情作为一个rein​​ter pret_cast;见§5.4的显式类型转换（转换符号）的节和第5.2.9-11。

  

3。有C和C ++？

有什么区别

不应该有，只要你正在处理的是合法的C型。

In my project we have a piece of code like this:

// raw data consists of 4 ints
unsigned char data[16];
int i1, i2, i3, i4;
i1 = *((int*)data);
i2 = *((int*)(data + 4));
i3 = *((int*)(data + 8));
i4 = *((int*)(data + 12));

I talked to my tech lead that this code may not be portable since it's trying to cast a unsigned char* to a int* which usually has a more strict alignment requirement. But tech lead says that's all right, most compilers remains the same pointer value after casting, and I can just write the code like this.

To be frank, I'm not really convinced. After researching, I find some people against use of pointer castings like above, e.g., here and here.

So here are my questions:

1. Is it REALLY safe to dereference the pointer after casting in a real project?
2. Is there any difference between C-style casting and reinterpret_cast?
3. Is there any difference between C and C++?

解决方案

1. Is it REALLY safe to dereference the pointer after casting in a real project?

If the pointer happens to not be aligned properly it really can cause problems. I've personally seen and fixed bus errors caused by casting a char* to a more strictly aligned type. Even if you don't get an obvious error you can have less obvious issues like slower performance. Strictly following the standard to avoid UB is a good idea even if you don't immediately see any problems. (And the rule the code is breaking is the strict aliasing rule, § 3.10/10)

A better alternative is to use std::memcpy() or std::memmove if the buffers overlap (or better yet bit_cast<>())

unsigned char data[16];
int i1, i2, i3, i4;
std::memcpy(&i1, data     , sizeof(int));
std::memcpy(&i2, data +  4, sizeof(int));
std::memcpy(&i3, data +  8, sizeof(int));
std::memcpy(&i4, data + 12, sizeof(int));

Some compilers work harder than others to make sure char arrays are aligned more strictly than necessary because programmers so often get this wrong though.

#include <cstdint>
#include <typeinfo>
#include <iostream>

template<typename T> void check_aligned(void *p) {
    std::cout << p << " is " <<
      (0==(reinterpret_cast<std::intptr_t>(p) % alignof(T))?"":"NOT ") <<
      "aligned for the type " << typeid(T).name() << '\n';
}

void foo1() {
    char a;
    char b[sizeof (int)];
    check_aligned<int>(b); // unaligned in clang
}

struct S {
    char a;
    char b[sizeof(int)];
};

void foo2() {
    S s;
    check_aligned<int>(s.b); // unaligned in clang and msvc
}

S s;

void foo3() {
    check_aligned<int>(s.b); // unaligned in clang, msvc, and gcc
}

int main() {
    foo1();
    foo2();
    foo3();
}

http://ideone.com/FFWCjf

2. Is there any difference between C-style casting and reinterpret_cast?

It depends. C-style casts do different things depending on the types involved. C-style casting between pointer types will result in the same thing as a reinterpret_cast; See § 5.4 Explicit type conversion (cast notation) and § 5.2.9-11.

3. Is there any difference between C and C++?

There shouldn't be as long as you're dealing with types that are legal in C.

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