我听说 i++ 不是线程安全的，++i 线程安全吗？

我听说 i++ 不是线程安全的语句，因为在汇编中它减少为将原始值存储为临时值，递增它，然后替换它，这可能会被上下文切换中断。

但是，我想知道 ++i 。据我所知，这将减少为一条汇编指令，例如“add r1, r1, 1”，并且由于它只是一条指令，因此它不会被上下文切换中断。

谁能澄清一下吗？我假设正在使用 x86 平台。

You've heard wrong. It may well be that "i++" is thread-safe for a specific compiler and specific processor architecture but it's not mandated in the standards at all. In fact, since multi-threading isn't part of the ISO C or C++ standards ^(a), you can't consider anything to be thread-safe based on what you think it will compile down to.

这是非常可行的++i可以编译为任意序列，例如：

load r0,[i]  ; load memory into reg 0
incr r0      ; increment reg 0
stor [i],r0  ; store reg 0 back to memory

在我的（假想的）没有内存增量指令的 CPU 上，这不是线程安全的。或者它可能很聪明并将其编译为：

lock         ; disable task switching (interrupts)
load r0,[i]  ; load memory into reg 0
incr r0      ; increment reg 0
stor [i],r0  ; store reg 0 back to memory
unlock       ; enable task switching (interrupts)

where lock残疾人和unlock启用中断。但是，即使如此，在具有多个共享内存的 CPU 的体系结构中，这也可能不是线程安全的（lock只能禁用一个 CPU 的中断）。

语言本身（或它的库，如果它没有内置到语言中）将提供线程安全的构造，您应该使用它们，而不是依赖于您对将生成什么机器代码的理解（或可能误解）。

Things like Java synchronized and pthread_mutex_lock() (available to C/C++ under some operating systems) are what you need to look into ^(a).

^(a) This question was asked before the C11 and C++11 standards were completed. Those iterations have now introduced threading support into the language specifications, including atomic data types (though they, and threads in general, are optional, at least in C).