问题描述
我一直在阅读有关无锁编程的文章在MSDN中.它说:
I have been reading an article about Lockless Programming in MSDN. It says :
并给出了一些示例:
// This write is not atomic because it is not natively aligned.
DWORD* pData = (DWORD*)(pChar + 1);
*pData = 0;
// This is not atomic because it is three separate operations.
++g_globalCounter;
// This write is atomic.
g_alignedGlobal = 0;
// This read is atomic.
DWORD local = g_alignedGlobal;
我读了很多答案和评论，说在C ++中没有任何东西可以保证是原子性的，甚至在标准语言中也没有提到，因此现在我有点困惑.我误解了这篇文章吗?还是文章作者谈论的是非标准的，特定于MSVC ++编译器的内容?
I read lots of answers and comments saying, nothing is guaranteed to be atomic in C++ and it is not even mentioned in standarts, in SO and now I am a bit confused. Am I misinterpreting the article? Or does the article writer talk about things that are non-standart and specific to MSVC++ compiler?
所以根据这篇文章，以下分配必须是原子的，对吧?
So according to the article the below assignments must be atomic, right?
struct Data
{
char ID;
char pad1[3];
short Number;
char pad2[2];
char Name[5];
char pad3[3];
int Number2;
double Value;
} DataVal;
DataVal.ID = 0;
DataVal.Number = 1000;
DataVal.Number2 = 0xFFFFFF;
DataVal.Value = 1.2;
如果为true，用std::string Name;替换Name[5]和pad3[3]是否对内存对齐有任何影响? Number2和Value变量的分配是否仍是原子的?
If it is true, does replacing Name[5] and pad3[3] with std::string Name; make any difference in memory-alignment ? Will the assignments to Number2 and Value variables be still atomic?
有人可以解释吗?
推荐答案
此建议特定于体系结构.对于x86& x86_64(在低级编程中).您还应该检查编译器不要对代码重新排序.您可以为此使用编译器内存屏障".
This recommendation is architecture-specific. It is true for x86 & x86_64 (in a low-level programming). You should also check that compiler don't reorder your code. You can use "compiler memory barrier" for that.
x86的低级原子读写在英特尔参考手册英特尔®64和IA-32体系结构软件开发人员手册"第3A卷(http://www.intel.com/Assets/PDF/manual/253668.pdf )，第8.1.1节
Low-level atomic read and writes for x86 is described in Intel Reference manuals "The Intel® 64 and IA-32 Architectures Software Developer’s Manual" Volume 3A ( http://www.intel.com/Assets/PDF/manual/253668.pdf) , section 8.1.1
Intel486处理器(以及以后的较新处理器)保证了以下各项 基本的内存操作将始终以原子方式进行:
The Intel486 processor (and newer processors since) guarantees that the following basic memory operations will always be carried out atomically:
- 读取或写入字节
- 读取或写入在16位边界上对齐的单词
- 读取或写入在32位边界上对齐的双字
- 读取或写入在64位边界上对齐的四字
- 16位访问适合32位数据总线的未缓存内存位置
- 未对齐的16位，32位和64位对适合缓存的缓存的访问线
- Unaligned 16-, 32-, and 64-bit accesses to cached memory that fit within a cacheline
该文档还对像Core2这样的较新处理器进行了原子上的更多描述. 并非所有未对齐的操作都是原子操作.
This document also have more description of atomically for newer processors like Core2. Not all unaligned operations will be atomic.
其他intel手册推荐此白皮书:
Other intel manual recommends this white paper:
这篇关于C ++中的原子性:神话还是现实的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！