Define in and define out

Recently, when looking at the code , I found that there are many variables or functions in front of IN or OUT“ modification ”, This is not a type , Find the definition , Discovery is defined like this ：#define IN
At first glance, I don't know what I mean , Different from common macro definitions , Its second value is empty . There is a saying that may explain , Record here .

This is a programming specification
such as #define IN
#define OUT
Define a function int fun(IN char *param,OUT *char result)
Indicates that the first parameter is the incoming parameter , The second parameter is the callback result , because IN OUT Itself is empty , So it doesn't affect the compilation , Just for code specification , Let people more clearly define .