DTrace automatically loads some dtrace kernel modules when the dtrace command references the probes that are associated with a dtrace kernel module. You can use this convenient method to load dtrace modules, rather than manually loading them.

To find out which modules are automatically loaded in this manner, use the following command:

# cat /etc/dtrace-modules
sdt
systrace
profile
fasttrap

Additional modules can be added to this list after it is determined that they are fully tested.

To determine whether a particular module has been loaded in the Linux kernel, use the lsmod command. For example, you would run the following command to determine whether the sdt module is loaded:

# lsmod | grep sdt

If the module is not loaded, the command most likely will result in no output. If the module is loaded, the output is similar to the following:

sdt                    20480   0
dtrace                 151552  4 sdt,fasttrap,systrace,profile

To use DTrace providers, their supported kernel modules must be loaded each time the kernel is booted.

If the dtrace kernel module is not already loaded, when the dtrace command is run, the dtrace module and all of the modules that are listed in /etc/dtrace-modules are all loaded automatically. However, if the dtrace kernel module is already loaded, the automatic kernel module loading mechanism is not triggered.

You can load modules manually by using the modprobe command. For example, to use the fbt kernel module if it is not in the default list, you would run the following command:

# modprobe fbt

The modprobe action also loads the dtrace kernel module as a dependency so that a subsequent dtrace command does not drive automatic loading of other dtrace modules until the dtrace kernel module is no longer loaded. The drace kernel module will no longer be loaded upon another boot of the system or after the manual removal of the dtrace kernel modules.

The suggested practice is to use the dtrace -l command to trigger automatic module loading and thereby also confirm basic dtrace functionality. Then, use the modprobe command to load additional modules that are not in the default list, such as fbt, as needed.

/* hello.d -- A simple D program that uses the BEGIN probe */

BEGIN
{
  /* This is a C-style comment */
  trace("hello, world");
  exit(0);
}

A D program consists of a series of clauses, where each clause describes one or more probes to enable, and an optional set of actions to perform when the probe fires. The actions are listed as a series of statements enclosed in braces {} following the probe name. Each statement ends with a semicolon (;).

In this example, the function trace directs DTrace to record the specified argument, the string ”hello, world”, when the BEGIN probe fires, and then print it out. The function exit() tells DTrace to cease tracing and exit the dtrace command.

The full name of the BEGIN probe is dtrace:::BEGIN. dtrace provides three probes: dtrace:::BEGIN, dtrace:::END, and dtrace:::ERROR. Because these probe names are unique to the dtrace provider, their names can be shortened to BEGIN, END, and ERROR.

When you have saved your program, you can run it by using the dtrace command with the -s option, which specifies the name of the file that contains the D program:

# dtrace -s hello.d
dtrace: script 'hello.d' matched 1 probe
CPU     ID                    FUNCTION:NAME
  0      1                           :BEGIN   hello, world

DTrace interprets and runs the script. You will notice that in addition to the string "hello,world", the default behavior of DTrace is to display information about the CPU on which the script was running when a probe fired, the ID of the probe, the name of the function that contains the probe, and the name of the probe itself. The function name is displayed as blank for BEGIN, as DTrace provides this probe.

You can suppress the probe information in a number of different ways, for example, by specifying the -q option:

# dtrace -q -s hello.d
hello, world

Copy the hello.d program to the file goodbye.d. Edit this file so that it traces the string "goodbye, world" and uses the END probe instead of BEGIN. When you run this new script, you need to type Ctrl-C to cause the probe to fire and exit dtrace.

(Estimated completion time: 5 minutes)

The following is an example of a simple D program that demonstrates the use of the END probe:

/* goodbye.d -- Simple D program that demonstrates the END probe */

END
{
  trace("goodbye, world");
}

# dtrace -s goodbye.d
dtrace: script 'goodbye.d' matched 1 probe
^C
CPU     ID                    FUNCTION:NAME
  3      2                             :END   goodbye, world

# dtrace -q -s ./goodbye.d
                     ^C
goodbye, world