RHEL 6 SystemTap Guide - 3.5.6. Using Arrays in Conditional Statements

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3.5.6. Using Arrays in Conditional Statements

You can also use associative arrays in if statements. This is useful if you want to execute a subroutine once a value in the array matches a certain condition. Consider the following example:

Example 3.17. vfsreads-print-if-1kb.stp

global reads
probe vfs.read
{
  reads[execname()] ++
}

probe timer.s(3)
{
  printf("=======\n")
  foreach (count in reads-)
    if (reads[count] >= 1024)
      printf("%s : %dkB \n", count, reads[count]/1024)
    else
      printf("%s : %dB \n", count, reads[count])
}

Every three seconds, Example 3.17, “vfsreads-print-if-1kb.stp” prints out a list of all processes, along with how many times each process performed a VFS read. If the associated value of a process name is equal or greater than 1024, the if statement in the script converts and prints it out in kB.

Testing for Membership

You can also test whether a specific unique key is a member of an array. Further, membership in an array can be used in if statements, as in:

if([index_expression] in array_name) statement

To illustrate this, consider the following example:

Example 3.18. vfsreads-stop-on-stapio2.stp

global reads

probe vfs.read
{
  reads[execname()] ++
}

probe timer.s(3)
{
  printf("=======\n")
  foreach (count in reads+) 
    printf("%s : %d \n", count, reads[count])
  if(["stapio"] in reads) {
    printf("stapio read detected, exiting\n")
    exit()
  }
}

The if(["stapio"] in reads) statement instructs the script to print stapio read detected, exiting once the unique key stapio is added to the array reads.