C中的副作用是什么? [英] What is side effect in C?

问题描述

维基百科说:

在计算机科学中，如果操作、函数或表达式在其本地环境之外修改某些状态变量值，则称其具有副作用，也就是说具有除了向操作的调用者返回一个值(主要效果)之外，还有一个可观察的效果.

In computer science, an operation, function or expression is said to have a side effect if it modifies some state variable value(s) outside its local environment, that is to say has an observable effect besides returning a value (the main effect) to the invoker of the operation.

但是我们如何访问一个变量在其本地环境之外，有没有人能通俗地解释一下这种情况、副作用、主效应和序列点?

But how can we access a variable outside its local environment, can anyone explain this situation, side effect, main effect and sequence point comprehensibly?

推荐答案

一个函数是(应该)一个黑盒子，其中返回值，或者通过引用传递的变量的值，应该是唯一的东西可能会因输入参数而异.

A function is (should be) a black box, in which the return value, or the value of a variable passed by reference, should be the only thing that may change depending upon the input parameters.

函数在这些情况之外产生的任何其他可观察到的变化都是副作用.最著名的例子可能是 printf() 函数，它除了返回写入的字符数外，还改变标准输出的内容，这意味着改变一些与管道相关的内存缓冲区，一个文件或屏幕，例如，不属于函数的本地环境.

Any other observable change that the function produces outside these cases, is a side-effect. The most well-known example may be the printf() function which, besides returning the number of written characters, changes the contents of the standard output, which means altering some memory buffer associated with a pipe, a file, or the screen, for instance, and which doesn't belong to the local environment of the function.

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