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.

Variables can be used freely throughout a handler; simply choose a name, assign it to a function, and use it in an expression. SystemTap automatically identifies whether a variable should be identified as a string or integer, based on the function it is assigned to. For instance, if you use set the variable foo to gettimeofday_s() (as in foo = gettimeofday_s()), then foo can be used as an integer argument (%d) in printf(). 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 first using global outside of any probe. Consider the following example: global count_jiffies, count_ms probe 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 () } attempts to compute 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). The ++ notation in (i.e. 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.

In some cases, the output of a SystemTap script may be too big. To address this, you need to further refine the script's logic in order to delimit the output into something more relevant or useful to your probe. You can do this by using conditionals in handlers. SystemTap accepts the following types of conditional statements: If/Else Statements Format: if (condition) {statement} else {statement} global countread, countnonread probe kernel.function("vfs_read"),kernel.function("vfs_write") { if (probefunc()=="vfs_read") { countread ++ } else { countnonread ++ } } probe timer.s(5) { exit() } probe end { printf("VFS reads total %d\n VFS writes total %d\n", countread, countnonread) } is a script that counts how many virtual file system reads (vfs_read) and writes (vfs_write) the system performs within a 5-second span. When run, the script increments the value of the variable countread by 1 if the name of the function it probed matches vfs_read (as noted by the condition if (probefunc()=="vfs_read")); otherwise, it increments countnonread (else {countnonread ++}). While Loops Format: while (condition) {statement} For Loops Format: for (argument1; argument2; argument3) {statement} need simple, simple examples for FOR and WHILE Aside from == ("is equal to"), you can also use the following operators in your conditional statements: >= Greater than or equal to <= Less than or equal to != Is not equal to

You can also allow a SystemTap script to accept simple command-line arguments and declare them in the script without using target(). One way to do this is to use the variable notation $ or @. probe kernel.function(@1) { } probe kernel.function(@1).return { } is similar to , except that it allows you to pass the kernel function to be probed as a command-line argument (as in stap commandlineargs.stp kernel function). You can also specify the script to accept multiple command-line arguments, noting them as @1, @2, and so on, in the order they are entered by the user. Both variable notations $ and @ also represent a specific variable type. Use $ if you are expecting the user to enter an integer as a command-line argument, and @ if you are expecting a string.