Add Errno#errno_message getter

[wooster0]

This adds Errno#errno_message getter which returns the last raised errno message.

Usecase:
I want to print the error message from errno in my application just like this: No such file or directory instead of this: Unhandled exception: some_call: No such file or directory (Errno).

[ysbaddaden]

Why not.

[RX14]

Perhaps this should be a method instead of a class method

[asterite]

Once we make the error hierarchy independent of the OS, will this still be here?

I don't quite understand the use case.

[wooster0]

@RX14 In the end there isn't really a difference and the other methods are also already class methods.
But if we make self.message an instance method, the others should be instance methods too I think.

@asterite Yes I think it will still be here. The use case is just that when I for example read a file and an error happens, that I can print the reason why the file couldn't be read.
I could right now for example do:

begin
  File.read("a")
rescue ex : Errno
  puts ex
end

and it would print Error opening file 'a' with mode 'r': No such file or directory. But I just want to print the errno message No such file or directory in my program.

[asterite]

@r00ster91 then let's save @message in an instance variable and expose that as a getter.

[asterite]

Hm, never mind, the name collides with Exception's message. Maybe @errno_message, though...

[wooster0]

@asterite Hmm so we always save the last raised errno message in @errno_message. But what if I for example want to get some other errno message with a number instead of just the last one that has been raised? I think we should rather use a method.

[asterite]

But what if I for example want to get some other errno message with a number instead of just the last one that has been raised?

What would be an example for this? Errno gets re-written each time there's an error, so it only makes sense to reference the last one.

With that you could do:

begin
  File.read("a")
rescue ex : Errno
  puts ex.errno_message
end

But more specifically, why don't you do:

begin
  File.read("a")
rescue ex
  puts "Can't read file a"
end

I don't think Errno as an exception will survive in the future, we'll probably have a different exception hierarchy and exposing errno message will probably don't exist.

[straight-shoota]

But what if I for example want to get some other errno message with a number

You could always just call LibC.error directly.

[wooster0]

But more specifically, why don't you do:

begin
 File.read("a")
rescue ex
 puts "Can't read file a"
end

I prefer to give the user specific information why the program couldn't read the file. Because just this is IMO not very helpful to solve the problem.

But okay I will change it to a getter then. I agree. In the end when really need it, I could just call LibC.strerror myself.