handlers SystemTap handler constructs SystemTap handlers SystemTap handler constructs handlers SystemTap handler constructs syntax and format SystemTap handlers SystemTap handler constructs syntax and format syntax and format SystemTap handler constructs handlers 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.

handlers SystemTap handler constructs variables SystemTap statements SystemTap handler constructs variables variables SystemTap handler constructs handlers format and syntax SystemTap handler constructs handlers Variables can be used freely throughout a handler; simply choose a name, 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 an number and can be printed in a printf() with the integer format specifier (%d). handlers SystemTap handler constructs global SystemTap statements SystemTap handler constructs global global SystemTap handler constructs handlers 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: 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 () } CONFIG_HZ, computing for 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). 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.

handlers conditional statements SystemTap statements conditional statements 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 handlers conditional statements if/else SystemTap statements conditional statements if/else if/else conditional statements handlers Format: if (condition) statement1 else statement2 The statement1 is executed if the condition expression is non-zero. The statement2 is executed if the condition expression is zero. The else clause (else statement2)is optional. Both statement1 and statement2 can be statement blocks. 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 handlers conditional statements while loops SystemTap statements conditional statements while loops while loops conditional statements handlers Format: while (condition) statement So long as condition is non-zero the block of statements in statement are executed. The statement is often a statement block and it must change a value so condition will eventually be zero. For Loops handlers conditional statements for loops SystemTap statements conditional statements for loops for loops conditional statements handlers Format: for (initialization; conditional; increment) statement The for loop is simply shorthand for a while loop. The following is the equivalent while loop: initialization while (conditional) { statement increment } need simple, simple examples for FOR and WHILE handlers conditional statements conditional operators SystemTap statements conditional statements conditional operators conditional operators conditional statements handlers 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

handlers SystemTap handler constructs command-line arguments SystemTap statements SystemTap handler constructs command-line arguments command-line arguments SystemTap handler constructs handlers You can also allow a SystemTap script to accept simple command-line arguments using a $ or @ immediately followed by the number of the argument on the command line. Use $ if you are expecting the user to enter an integer as a command-line argument, and @ if you are expecting a string. 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. handlers SystemTap handler constructs variable notations SystemTap statements SystemTap handler constructs variable notations variable notations SystemTap handler constructs handlers