Dick's Portability Layer

One of the most common problems that people face when porting applications from Windows to Linux using Mono are paths.

The Problem

Windows developers are used to a case-insensitive file system, which means that they might create a file called "mydata" in one place, and try to access it somewhere else as "MyData" or as "MYDATA". This of course breaks on most Unix setups because Windows is case insensitive[1].

Another problem is that developers on Windows are known to hardcode the directory separator character in their source code ("\") instead of using Path.DirectorySeparator and using Path.Combine for combining this paths. This is a problem because "\" is a valid file name components on Unix. This means that if an application hardcodes for example "Logs\access_log", in Unix this will not store the contents in the "Logs" directory as the file "access_log". Instead, it will store the results in a file called "Logs\access_log".

Only a few applications cope with drive letters, but they might still pose a problem as the colon is a valid filename in Unix, which means that "A:\file" is a valid filename in the current directory.

Although .NET provides the tools to write code that is portable, in practice, they do not use these features (the exception is Path.Combine, which some people use, as it is genuinely useful on its own).

The Usual Solution

When moving applications from Windows to Linux, it is always necessary to run the application, run its test suite, and validate that the application works as intended. With the path problems described above, the process above included a number of iterations to fix the assumptions made by programmers about the file system.

This process could be time consuming, because identifying where the mistakes were made could take some time, the program might fail with FileNotFound exceptions (when referencing files that were not there), data would show up empty (listing contents of a directory that had nothing, as all the data went elsewhere) but it was doable.

This process works as long as you have the source code to all the components that you are porting, but if you were using a third-party library that you had no source code for, you would not be able to fix the problems.

The New Solution

This week, Dick Porter introduced a portability layer into Mono that will address those problems without requiring changes to your code. This will remove a large component of the porting cycle as a whole class of obnoxious problems are gone.

The new portability framework is enabled by setting the environment variable MONO_IOMAP (which we will likely rename to something shorter) to one of the following values:

• case: makes all file system access case insensitive.
• drive: strips drive name from pathnames.
• all: enables both case and drive.

In addition, if any of those options are enabled, the directory separator mapping is also turned on. So this basically means that you have to type this, or include this in your script that launches your application:

	$ export MONO_IOMAP=all
	$ mono myapp.exe
	

For ASP.NET applications hosted with mod_mono, you can add the following directive to your Apache configuration file:

	MonoSetEnv MONO_IOMAP=all
	

This new feature will appear in Mono 1.1.18.

The downside is that Mono will have to do some extra work when coping with your file system, to search for case insensitive file names. So if your application is still a portable application, you will be much better off without this switch.

[1] Some Linux file systems are case insensitive, and some folks have used a combination of hacks, including doing loopback CIFS mounts to get case sensitivity issues out of the way; OS X does not have this particular problem, but it still has the others.