# Programming Ruby ### The Pragmatic Programmer's Guide
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module Errno

### Index:

Ruby exception objects are subclasses of `Exception`. However, operating systems typically report errors using plain integers. Module `Errno` is created dynamically to map these operating system errors to Ruby classes, with each error number generating its own subclass of `SystemCallError`. As the subclass is created in module `Errno`, its name will start `Errno::`.

The names of the `Errno` `::` classes depend on the environment in which Ruby runs. On a typical Unix or Windows platform, there are `Errno` classes such as `Errno::EACCES`, `Errno::EAGAIN`, `Errno::EINTR`, and so on.

The integer operating system error number corresponding to a particular error is available as the class constant `Errno::` error `::Errno`.

`Errno::EACCES::Errno` | » | `13`
`Errno::EAGAIN::Errno` | » | `11`
`Errno::EINTR::Errno` | » | `4`

The full list of operating system errors on your particular platform are available as the constants of `Errno`.

`Errno.constants` | » | `E2BIG, EACCES, EADDRINUSE, EADDRNOTAVAIL, EADV, EAFNOSUPPORT, EAGAIN, ...`

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Extracted from the book "Programming Ruby - The Pragmatic Programmer's Guide"
Copyright © 2001 by Addison Wesley Longman, Inc. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/)).

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