⁠3.3.3. Conditional Statements

In some cases, the output of a SystemTap script may be too large. 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.
Do this by using conditionals in handlers. SystemTap accepts the following types of conditional statements:
If/Else Statements
Format:
if (condition) statement1else statement2
The statement1 is executed if thecondition expression isnon-zero. The statement2 isexecuted if the conditionexpression is zero. The else clause(else statement2) is optional. Bothstatement1 andstatement2 can be statementblocks.
⁠

Example 3.11. ifelse.stp

global countread, countnonreadprobe 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)}
Example 3.11, “ifelse.stp” 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
So long as condition is non-zerothe block of statements instatement are executed. Thestatement is often a statementblock and it must change a value socondition will eventually be zero.
For Loops
Format:
for (initialization; conditional; increment) statement
The for loop is shorthand for a while loop. Thefollowing is the equivalent while loop:
initializationwhile (conditional) { statement increment}
Conditional Operators
Aside from == ("is equal to"), following operators can also be used in conditional statements:
>=
Greater than or equal to
<=
Less than or equal to
!=
Is not equal to