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.
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token：ab。关于macro的处理，参考下列解释：
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"。
4种delaying 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. |