Welcome to Dynamic Tracing in the Solaris Operating System! If you have ever wanted to understand the behavior of your system, DTrace is the tool for you. DTrace is a comprehensive dynamic tracing facility that is built into Solaris that can be used by administrators and developers on live production systems to examine the behavior of both user programs and of the operating system itself. DTrace enables you to explore your system to understand how it works, track down performance problems across many layers of software, or locate the cause of aberrant behavior. As you'll see, DTrace lets you create your own custom programs to dynamically instrument the system and provide immediate, concise answers to arbitrary questions you can formulate using the DTrace D programming language. The first section of this chapter provides a quick introduction to DTrace and shows you how to write your very first D program. The rest of the chapter introduces the complete set of rules for programming in D as well as tips and techniques for performing in-depth analysis of your system. You can share your DTrace experiences and scripts with the rest of the DTrace community on the web at http://www.sun.com/bigadmin/content/dtrace/. All of the example scripts presented in this guide can be found on your Solaris system in the directory /usr/demo/dtrace.