How to set errno value?
===

I have two calls to two different methods : 

    void func1() 
    {
      // do something 
      if (fail) 
      {
        // then set errno to EEXIST
    
      }
    
    }


And the second method : 

    void func2() 
    {
      // do something 
      if (fail) 
      {
        // then set errno to ENOENT
    
      }
    
    }


1. When I set the `errno` to some value , what does it do ? just error checking ? 

2. How can I set `errno` in the above methods `func1` and `func2` to `EEXIST` and `ENOENT` 

Thanks 