== The irritation of single-instance applications

There is a certain habit in applications these days of only
allowing you to run one copy of themselves; for example, [[Firefox
../unix/WeirdFirefoxRemoteControl]]. I find myself more and more
irritated by this, because it is usually implemented sloppily.

A good implementation must hide the fact that it is not actually
running a second copy of the application. Sloppy implementations fail
to completely hide this fact, generally by missing various differences
between the environment that the original version of the application is
running in and the environment that the second copy is running in.

Firefox on Unix provides a particularly [[blatant example
../unix/WeirdFirefoxRemoteControl]] of this, but it's a hard problem
in general; there are a *lot* of different bits of the environment and
it is in practice very challenging to make sure that you have covered
everything. It may even be impossible; what if you're using a library
that changes its behavior in response to environment variables? If this
seems far-fetched, consider _$PRINTER_ and _$EDITOR_.

Given the difficulties, you should really not do this unless you have
to, for example because running two copies of the application at once
corrupts data somewhere. If you want to offer this as a convenience
feature, please make sure that it can be turned off, ideally with both a
global configuration option and a command line switch.

=== Sidebar: how to do this really badly

Even worse than sloppy single-instance applications are programs that do
not even bother pretending, where running a second copy of the program
does bad things to whatever you were doing in the first copy. For
example, Adobe Acrobat Reader, where you can only ever be reading one
PDF at once; if you try to start a second copy of _acroread_, the second
PDF *replaces* what you were reading in the first one.

(Back in the days when Acroread was my only real choice
for PDFs, this was infuriating. Now that I have [[evince
http://www.gnome.org/projects/evince/]] it is merely extremely
irritating.)