D programs consist of a set of clauses that describe probes to enable and predicate actions to bind to these probes. D programs can also contain declarations of variables and definitions of new types.
A D program source file consists of one or more probe clauses that describe the instrumentation that must be enabled by DTrace. Each probe clause has the following general form:
probe descriptions
/ predicate /
{
action statements
}
You can omit the predicate and list of action statements. Any directives found outside probe clauses are referred to as declarations. Declarations must be made outside of probe clauses. No declarations inside of the enclosing { } are permitted and declarations may not be interspersed between the elements of the probe clause shown in the preceding example. You can use whitespace to separate any D program elements and to indent action statements.
You can use declarations to declare D variables and external C symbols. For more information, see Variables in DTrace. You can use declarations to define types in D. For more information, see Type and Constant Definitions in DTrace. Special D compiler directives called pragmas may also appear anywhere in a D program, including outside of probe clauses. D pragmas are specified on lines beginning with a # character. For example, D pragmas are used to set run-time DTrace options. For more information, see DTrace Options and Tunables.
Every D program clause begins with a list of one or more probe descriptions, each taking the following form:
provider:module:function:name
If one or more fields of the probe description are omitted, the specified fields are interpreted from right to left by the D compiler. For example, the probe description foo:bar would match a probe with function foo() and name bar regardless of the value of the probe's provider and module fields. Therefore, a probe description is really more accurately viewed as a pattern that can be used to match one or more probes based on their names.
You must write D probe descriptions specifying all four field delimiters so that you can specify the desired provider on the left side. If you do not specify the provider, you might obtain unexpected results if multiple providers publish probes with the same name. Similarly, future versions of DTrace might include new providers whose probes unintentionally match your partially specified probe descriptions. You can specify a provider but match any of its probes by leaving any of the module, function, and name fields blank. For example, the description syscall::: can be used to match every probe published by the DTrace syscall provider.
Probe descriptions also support a pattern matching syntax similar to the shell globbing pattern matching syntax. For more information, see the sh(1) man page. Before matching a probe to a description, DTrace scans each description field for the characters *, ?, and [. If one of these characters appears in a probe description field and is not preceded by a \, the field is regarded as a pattern. The description pattern must match the entire corresponding field of a given probe. The complete probe description must match on every field in order to successfully match and enable a probe. A probe description field that is not a pattern must exactly match the corresponding field of the probe. A description field that is empty matches any probe.
The following table lists the special characters that are recognized in probe name patterns.
|
Pattern match characters can be used in any or all of the four fields of the probe descriptions. You can also use patterns to list matching probes by using the patterns on the command line with the dtrace -l command. For example, the command dtrace -l -f kmem_* lists all DTrace probes in functions whose names begin with the prefix kmem_.
If you want to specify the same predicate and actions for more than one probe description or description pattern, you can place the descriptions in a comma-separated list. For example, the following D program would trace a timestamp each time probes associated with entry to system calls containing the words "lwp" or "sock" fire:
syscall::*lwp*:entry, syscall::*sock*:entry
{
trace(timestamp);
}
A probe description may also specify a probe using its integer probe ID. For example:
12345
{
trace(timestamp);
}
You can use the preceding clause to enable probe ID 12345, as reported by the dtrace -l -i 12345 command. You must always write D programs using human-readable probe descriptions. Integer probe IDs are not guaranteed to remain consistent as DTrace provider kernel modules are loaded and unloaded or following a reboot.
Predicates are expressions enclosed in slashes / / that are evaluated at probe firing time to determine whether the associated actions should be executed. Predicates are the primary conditional construct used for building more complex control flow in a D program. You can omit the predicate section of the probe clause entirely for any probe, in which case the actions are always executed when the probe fires.
Predicate expressions can use any of the previously described D operators and may refer to any D data objects such as variables and constants. The predicate expression must evaluate to a value of integer or pointer type so that it can be considered as true or false. As with all D expressions, a zero value is interpreted as false and any non-zero value is interpreted as true.
Probe actions are described by a list of statements separated by semicolons (;) and enclosed in braces { }. If you only want to note that a particular probe fired on a particular CPU without tracing any data or performing any additional actions, you can specify an empty set of braces with no statements inside.
Each clause is represented by its predicate, if any, and the clause's actions. When an enabled probe fires, its actions will execute if the predicate evaluates to true or if no predicate is given. Program order determines the order in which actions are executed. Two or more clauses that enable the same probe will also execute in program order.
The C programming language used for defining system interfaces in Oracle Solaris includes a preprocessor that performs a set of initial steps in C program compilation. The C preprocessor is commonly used to define macro substitutions where one token in a C program is replaced with another predefined set of tokens, or to include copies of system header files. You can use the C preprocessor in conjunction with D programs by specifying the dtrace -C option. This option causes dtrace to first execute the cpp preprocessor on your program source file and then pass the results to the D compiler.
The D compiler automatically loads the set of C type descriptions associated with the operating system implementation, but you can use the preprocessor to include type definitions such as types used in your own C programs. You can also use the preprocessor to perform tasks such as creating macros that expand to chunks of D code and other program elements. If you use the preprocessor with D program, you may only include files that contain valid D declarations. Typical C header files include only external declarations of types and symbols, which will be correctly interpreted by the D compiler. The D compiler cannot parse C header files that include additional program elements like C function source code and produces an appropriate error message.