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

Each class in Errno can be created with optional messages:

Errno::EPIPE.new                  # => #<Errno::EPIPE: Broken pipe>
Errno::EPIPE.new("foo")           # => #<Errno::EPIPE: Broken pipe - foo>
Errno::EPIPE.new("foo", "here")   # => #<Errno::EPIPE: Broken pipe @ here - foo>

See SystemCallError.new.

System call error module used by webrick for cross platform compatibility.

EPROTO: protocol error
ECONNRESET: remote host reset the connection request
ECONNABORTED: Client sent TCP reset (RST) before server has accepted the connection requested by client.

System call error module used by webrick for cross platform compatibility.

EPROTO:: protocol error ECONNRESET:: remote host reset the connection request ECONNABORTED:: Client sent TCP reset (RST) before server has accepted the connection requested by client.