⁠3.3. Basic SystemTap Handler Constructs

SystemTap supports the use of several basic constructs in handlers. The syntax for most of these handler constructs are mostly based on C and awk syntax. This section describes several of the most useful SystemTap handler constructs, which should provide you with enough information to write simple yet useful SystemTap scripts.

⁠3.3.1. Variables

Variables can be used freely throughout a handler; simply choose aname, assign a value from a function or expression to it, and use it in an expression. SystemTap automatically identifies whether a variable should be typed as a string or integer, based on the type of the values assigned to it. For instance, if you use set the variable foo to gettimeofday_s() (as in foo = gettimeofday_s()), then foo is typed as a number and can be printed in a printf() with the integer format specifier (%d).
Note, however, that by default variables are only local to the probe they are used in. This means that variables are initialized, used and disposed at each probe handler invocation. To share a variable between probes, declare the variable name using global outside of the probes. Consider the following example:
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Example 3.8. timer-jiffies.stp

global count_jiffies, count_msprobe timer.jiffies(100) { count_jiffies ++ }probe timer.ms(100) { count_ms ++ }probe timer.ms(12345){ hz=(1000*count_jiffies) / count_ms printf ("jiffies:ms ratio %d:%d => CONFIG_HZ=%d\n", count_jiffies, count_ms, hz) exit ()}
Example 3.8, “timer-jiffies.stp” computes the CONFIG_HZ setting of the kernel using timers that count jiffies and milliseconds, then computing accordingly. The global statement allows the script to use the variables count_jiffies and count_ms (set in their own respective probes) to be shared with probe timer.ms(12345).

Note

The ++ notation in Example 3.8, “timer-jiffies.stp” (that is, count_jiffies ++ and count_ms ++) is used to increment the value of a variable by 1. In the following probe, count_jiffies is incremented by 1 every 100 jiffies:
probe timer.jiffies(100) { count_jiffies ++ }
In this instance, SystemTap understands that count_jiffies is an integer. Because no initial value was assigned to count_jiffies, its initial value is zero by default.