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Date: 九月 5, 2016

什么是“Pure Function”

下列文字摘自The Definition of “Pure Function”

A pure function is a function that depends only on its declared inputs and its internal algorithm to produce its output. It does not read any other values from “the outside world” — the world outside of the function’s scope — and it does not modify any values in the outside world.

1-Pure-Function-Equation

C语言中的“#”和“##”

参考stackoverflow上的这篇帖子

#include <stdio.h>
#define f(a,b) a##b
#define g(a)   #a
#define h(a) g(a)

int main()
{
        printf("%s\n",h(f(1,2)));
        printf("%s\n",g(f(1,2)));
        return 0;
}

运行结果如下:

12
f(1,2)

#a会创建一个字符串:"a"a##b会创建一个新的tokenab。关于macro的处理,参加下列解释:

An occurrence of a parameter in a function-like macro, unless it is the operand of # or ##, is expanded before substituting it and rescanning the whole for further expansion. Because g’s parameter is the operand of #, the argument is not expanded but instead immediately stringified (“f(1,2)”). Because h’s parameter is not the operand of # nor ##, the argument is first expanded (12), then substituted (g(12)), then rescanning and further expansion occurs (“12”).

总结一下,对于function-like macro的参数,如果是#或者##的操作数,则会马上扩展为相应的字符串;否则会先扩展,再替换,循环此操作,直到完成。因此对于g(f(1,2)),会马上替换成"f(1,2)"这个字符串。而对于h(f(1,2)),由于其参数不是#..或者..#..,所以会先把f(1,2)扩展成12,然后替换变成g(12),最后变成"12"

FreeBSD kernel 笔记(13)——delaying execution

4delaying execution方法(选自:FreeBSD Device Drivers):

Sleeping Sleeping is done when you must wait for something to occur before you can proceed.
Event Handlers Event handlers let you register one or more functions to be executed when an event occurs.
Callouts Callouts let you perform asynchronous code execution. Callouts are used to execute your functions at a specific time.
Taskqueues Taskqueues also let you perform asynchronous code execution. Taskqueues are used for deferred work.

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