Get source and compile
Even if you don't plan to develop, just the act of downloading and compiling can uncover problems. If the version you download doesn't build on your platform, file a bug.
Getting the source
Download Source Archives (Zipfile/Tarball)
OBSOLETE! Linden Lab provides current and past downloads in an archived format, see the source downloads page. Be sure to get all three: source, artwork, and libraries.
Linden Lab maintains a version control repository containing various (experimental) branches. See the version control repository page for the correct repository url.
Subsequently, if you are using a command line, you could check out the source code with
svn checkout repository-url target-directory
For example to checkout snowglobe 1.4:
cd snowglobe-1.x-svn svn checkout https://svn.secondlife.com/svn/linden/projects/2009/snowglobe/trunk linden
Note that using 'https://' as opposed to 'http://' is only really necessary if you are a committer and need repository write access.
Per platform instructions are available in these topics:
- Common compilation problems if you run into errors while building.
- Improving compilation time for information on how to compile faster
- LLMozLib2 for building LLMozLib2.
Hints on running
Channels and Versions
Channels are just groupings of versions. Linden Lab uses channels to track and supply updates for First Look viewers separately from Release and Release Candidate viewers.
If you get a message while trying to start your compiled viewer that there is a required update, don't panic. All of your work is not lost. You (usually) don't have to apply your changes to a new source release to test your code. That's where channels come in.
On login, the combination of channel plus version is checked against a list. Within each channel, some versions are allowed and some are blocked. Viewers are blocked that are unsafe (security issues), incompatible (the protocol has been changed in some fundamental way), not supported (so old Linden Lab can't afford to provide support resources), or (most relevant here) are in a test channel (Release Candidate, First Look) and Linden Lab only wants test data from the most recent.
See Setting a Channel and Version for an Open Source Viewer for an explanation of how to do it in the source code.
See Channel and Version Requirements for a detailed explanation and official policy on how Linden Lab deals with channels and versions in Second Life.
The Third Party Viewer Policy requires that all viewers use a unique channel name. Please see here for more information.