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Oracle Solaris 11.1 Dynamic Tracing Guide     Oracle Solaris 11.1 Information Library
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Document Information


1.  About DTrace

2.  D Programming Language

D Program Structure

Probe Clauses and Declarations

Probe Descriptions



Order of Execution

Use of the C Preprocessor

Compilation and Instrumentation

Variables and Arithmetic Expressions


Output Formatting


External Symbols and Types

Types, Operators, and Expressions

Identifier Names and Keywords

Data Types and Sizes


Arithmetic Operators

Relational Operators

Logical Operators

Bitwise Operators

Assignment Operators

Increment and Decrement Operators

Conditional Expressions

Type Conversions



Scalar Variables

Associative Arrays

Thread-Local Variables

Clause-Local Variables

Built-in Variables

External Variables

Pointers and Arrays

Pointers and Addresses

Pointer Safety

Array Declarations and Storage

Pointer and Array Relationship

Pointer Arithmetic

Generic Pointers

Multi-Dimensional Arrays

Pointers to DTrace Objects

Pointers and Address Spaces


String Representation

String Constants

String Assignment

String Conversion

String Comparison

Structs and Unions


Pointers to Structs


Member Sizes and Offsets


Type and Constant Definitions




Type Namespaces

3.  Aggregations

4.  Actions and Subroutines

5.  Buffers and Buffering

6.  Output Formatting

7.  Speculative Tracing

8.  dtrace(1M) Utility

9.  Scripting

10.  Options and Tunables

11.  Providers

12.  User Process Tracing

13.  Statically Defined Tracing for User Applications

14.  Security

15.  Anonymous Tracing

16.  Postmortem Tracing

17.  Performance Considerations

18.  Stability

19.  Translators

20.  Versioning



One major difference between D and other programming languages such as C, C++, and the Java programming language is the absence of control-flow constructs such as if-statements and loops. D program clauses are written as single straight-line statement lists that trace an optional, fixed amount of data. D does provide the ability to conditionally trace data and modify control flow using logical expressions called predicates that can be used to prefix program clauses. A predicate expression is evaluated at probe firing time prior to executing any of the statements associated with the corresponding clause. If the predicate evaluates to true, represented by any non-zero value, the statement list is executed. If the predicate is false, represented by a zero value, none of the statements are executed and the probe firing is ignored.

Type the following source code for the next example and save it in a file named countdown.d:

        i = 10;

/i > 0/

/i == 0/

This D program implements a 10-second countdown timer using predicates. When executed, countdown.d counts down from 10 and then prints a message and exits:

# dtrace -s countdown.d
dtrace: script 'countdown.d' matched 3 probes
CPU     ID                    FUNCTION:NAME
        0  25499                       :tick-1sec 10
        0  25499                       :tick-1sec 9
        0  25499                       :tick-1sec 8
        0  25499                       :tick-1sec 7
        0  25499                       :tick-1sec 6
        0  25499                       :tick-1sec 5
        0  25499                       :tick-1sec 4
        0  25499                       :tick-1sec 3
        0  25499                       :tick-1sec 2
        0  25499                       :tick-1sec 1
        0  25499                       :tick-1sec blastoff!

This example uses the BEGIN probe to initialize an integer i to 10 to begin the countdown. Next, as in the previous example, the program uses the tick-1sec probe to implement a timer that fires once per second. Notice that in countdown.d, the tick-1sec probe description is used in two different clauses, each with a different predicate and action list. The predicate is a logical expression surrounded by enclosing slashes / / that appears after the probe name and before the braces { } that surround the clause statement list.

The first predicate tests whether i is greater than zero, indicating that the timer is still running:

/i > 0/

The relational operator > means greater than and returns the integer value zero for false and one for true. All of the C relational operators are supported in D; the complete list is found in Types, Operators, and Expressions. If i is not yet zero, the script traces i and then decrements it by one using the - operator.

The second predicate uses the == operator to return true when i is exactly equal to zero, indicating that the countdown is complete:

/i == 0/

Similar to the first example, hello.d, countdown.d uses a sequence of characters enclosed in double quotes, called a string constant, to print a final message when the countdown is complete. The exit function is then used to exit dtrace and return to the shell prompt.

If you look back at the structure of countdown.d, you will see that by creating two clauses with the same probe description but different predicates and actions, we effectively created the logical flow:

i = 10
once per second,
        if i is greater than zero
        otherwise if i is equal to zero

When you wish to write complex programs using predicates, try to first visualize your algorithm in this manner, and then transform each path of your conditional constructs into a separate clause and predicate.

Now let's combine predicates with a new provider, the syscall provider, and create our first real D tracing program. The syscall provider permits you to enable probes on entry to or return from any Oracle Solaris system call. The next example uses DTrace to observe every time your shell performs a read(2) or write(2) system call. First, open two terminal windows, one to use for DTrace and the other containing the shell process you're going to watch. In the second window, type the following command to obtain the process ID of this shell:

# echo $$

Now go back to your first terminal window and type the following D program and save it in a file named rw.d. As you type in the program, replace the integer constant 12345 with the process ID of the shell that was printed in response to your echo command.

/pid == 12345/

Notice that the body of rw.d's probe clause is left empty because the program is only intended to trace notification of probe firings and not to trace any additional data. Once you're done typing in rw.d, use dtrace to start your experiment and then go to your second shell window and type a few commands, pressing return after each command. As you type, you should see dtrace report probe firings in your first window, similar to the following example:

# dtrace -s rw.d
dtrace: script 'rw.d' matched 2 probes
CPU     ID                    FUNCTION:NAME
        0     34                      write:entry
        0     32                      read:entry
        0     34                      write:entry
        0     32                      read:entry
        0     34                      write:entry
        0     32                      read:entry
        0     34                      write:entry
        0     32                      read:entry

You are now watching your shell perform read(2) and write(2) system calls to read a character from your terminal window and echo back the result! This example includes many of the concepts described so far and a few new ones as well. First, to instrument read(2) and write(2) in the same manner, the script uses a single probe clause with multiple probe descriptions by separating the descriptions with commas like this:


For readability, each probe description appears on its own line. This arrangement is not strictly required, but it makes for a more readable script. Next the script defines a predicate that matches only those system calls that are executed by your shell process:

/pid == 12345/

The predicate uses the predefined DTrace variable pid, which always evaluates to the process ID associated with the thread that fired the corresponding probe. DTrace provides many built-in variable definitions for useful things like the process ID. Here is a list of a few DTrace variables you can use to write your first D programs:

Variable Name
Data Type
Current errno value for system calls
Name of the current process's executable file
Process ID of the current process
Thread ID of the current thread
Current probe description's provider field
Current probe description's module field
Current probe description's function field
Current probe description's name field

Now that you've written a real instrumentation program, try experimenting with it on different processes running on your system by changing the process ID and the system call probes that are instrumented. Then, you can make one more simple change and turn rw.d into a very simple version of a system call tracing tool like truss(1). An empty probe description field acts as a wildcard, matching any probe, so change your program to the following new source code to trace any system call executed by your shell:

/pid == 12345/

Try typing a few commands in the shell such as cd, ls, and date and see what your DTrace program reports.