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FreeBSD uses the `DDB_CTF` kernel option to enable support for loading `CTF` data from kernel modules and the kernel itself. `CTF` is the Solaris(TM) Compact C Type Format which encapsulates a reduced form of debugging information similar to `DWARF` and the venerable stabs. `CTF` data is added to binaries by the `ctfconvert` and `ctfmerge` build tools. The `ctfconvert` utility parses `DWARF``ELF` debug sections created by the compiler and `ctfmerge` merges `CTF``ELF` sections from objects into either executables or shared libraries.
Some different providers exist for FreeBSD than for Solaris(TM). Most notable is the `dtmalloc` provider, which allows tracing `malloc()` by type in the FreeBSD kernel. Some of the providers found in Solaris(TM), such as `cpc` and `mib`, are not present in FreeBSD. These may appear in future versions of FreeBSD. Moreover, some of the providers available in both operating systems are not compatible, in the sense that their probes have different argument types. Thus, `D` scripts written on Solaris(TM) may or may not work unmodified on FreeBSD, and vice versa.
Due to security differences, only `root` may use DTrace on FreeBSD. Solaris(TM) has a few low level security checks which do not yet exist in FreeBSD. As such, the [.filename]#/dev/dtrace/dtrace# is strictly limited to `root`.
DTrace falls under the Common Development and Distribution License (`CDDL`) license. To view this license on FreeBSD, see [.filename]#/usr/src/cddl/contrib/opensolaris/OPENSOLARIS.LICENSE# or view it online at http://opensource.org/licenses/CDDL-1.0[http://opensource.org/licenses/CDDL-1.0]. While a FreeBSD kernel with DTrace support is `BSD` licensed, the `CDDL` is used when the modules are distributed in binary form or the binaries are loaded.
Enabling DTrace Support
In FreeBSD 9.2 and 10.0, DTrace support is built into the [.filename]#GENERIC# kernel. Users of earlier versions of FreeBSD or who prefer to statically compile in DTrace support should add the following lines to a custom kernel configuration file and recompile the kernel using the instructions in crossref:kernelconfig[kernelconfig,Configuring the FreeBSD Kernel]:
options DDB_CTF
makeoptions DEBUG=-g
makeoptions WITH_CTF=1
Users of the AMD64 architecture should also add this line:
This option provides support for `FBT`. While DTrace will work without this option, there will be limited support for function boundary tracing.
Once the FreeBSD system has rebooted into the new kernel, or the DTrace kernel modules have been loaded using `kldload dtraceall`, the system will need support for the Korn shell as the DTrace Toolkit has several utilities written in `ksh`. Make sure that the package:shells/ksh93[] package or port is installed. It is also possible to run these tools under package:shells/pdksh[] or package:shells/mksh[].
Finally, install the current DTrace Toolkit, a collection of ready-made scripts for collecting system information. There are scripts to check open files, memory, `CPU` usage, and a lot more. FreeBSD 10 installs a few of these scripts into [.filename]#/usr/share/dtrace#. On other FreeBSD versions, or to install the full DTrace Toolkit, use the package:sysutils/dtrace-toolkit[] package or port.
The scripts found in [.filename]#/usr/share/dtrace# have been specifically ported to FreeBSD. Not all of the scripts found in the DTrace Toolkit will work as-is on FreeBSD and some scripts may require some effort in order for them to work on FreeBSD.
The DTrace Toolkit includes many scripts in the special language of DTrace. This language is called the D language and it is very similar to C++. An in depth discussion of the language is beyond the scope of this document. It is covered extensively in the http://www.dtrace.org/guide[Illumos Dynamic Tracing Guide].
Using DTrace
DTrace scripts consist of a list of one or more _probes_, or instrumentation points, where each probe is associated with an action. Whenever the condition for a probe is met, the associated action is executed. For example, an action may occur when a file is opened, a process is started, or a line of code is executed. The action might be to log some information or to modify context variables. The reading and writing of context variables allows probes to share information and to cooperatively analyze the correlation of different events.
To view all probes, the administrator can execute the following command:
# dtrace -l | more
Each probe has an `ID`, a `PROVIDER` (dtrace or fbt), a `MODULE`, and a `FUNCTION NAME`. Refer to man:dtrace[1] for more information about this command.
The examples in this section provide an overview of how to use two of the fully supported scripts from the DTrace Toolkit: the [.filename]#hotkernel# and [.filename]#procsystime# scripts.