3.3.2. Conditional Statements
3.3.2. 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 Format: 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. Example 3.9. ifelse.stp Example 3.9, "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: 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 Format: The for loop is simply shorthand for a while loop. The following is the equivalent while loop: Conditional Operators 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
if ( condition ) statement1 else statement2 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) } while ( condition ) statement for ( initialization ; conditional ; increment ) statement initialization while ( conditional ) { statement increment }