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:

• A nested class in this module, Errno.

• A subclass of class SystemCallError.

• Associated with an error code.

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