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)
  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.

Example 3.11. ifelse.stp

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)
}

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-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:

for (initialization; conditional; increment) statement

The for loop is shorthand for a while loop. The following is the equivalent while loop:

initialization
while (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