module Errno

When an operating system encounters an error, it typically reports the error as an integer error code:

$ ls nosuch.txt
ls: cannot access 'nosuch.txt': No such file or directory
$ echo $? # Code for last error.
2

When the Ruby interpreter interacts with the operating system and receives such an error code (e.g., 2), it maps the code to a particular Ruby exception class (e.g., Errno::ENOENT):

File.open('nosuch.txt')
# => No such file or directory @ rb_sysopen - nosuch.txt (Errno::ENOENT)

Each such class is:

Thus:

Errno::ENOENT.superclass # => SystemCallError
Errno::ENOENT::Errno     # => 2

The names of nested classes are returned by method Errno.constants:

Errno.constants.size         # => 158
Errno.constants.sort.take(5) # => [:E2BIG, :EACCES, :EADDRINUSE, :EADDRNOTAVAIL, :EADV]

As seen above, the error code associated with each class is available as the value of a constant; the value for a particular class may vary among operating systems. If the class is not needed for the particular operating system, the value is zero:

Errno::ENOENT::Errno      # => 2
Errno::ENOTCAPABLE::Errno # => 0