SystemTap also supports the use of associative arrays. While an ordinary variable represents a single value, associative arrays can represent a collection of values. Simply put, an associative array is a collection of unique keys; each key in the array has a value associated with it.
Since associative arrays are normally processed in multiple probes (as we will demonstrate later), they should be declared as
global variables in the SystemTap script. The syntax for accessing an element in an associative array is similar to that of
awk, and is as follows:
is any arbitrary name the array uses. The
is used to refer to a specific unique key in the array. To illustrate, let us try to build an array named
foo that specifies the ages of three people (i.e. the unique keys):
harry. To assign them the ages (i.e. associated values) of 23, 24, and 25 respectively, we'd use the following array statements:
Example 3.11. Basic Array Statements
foo["tom"] = 23
foo["dick"] = 24
foo["harry"] = 25
You can specify up to nine index expressons in an array statement, each one delimited by a comma (
). This is useful if you wish to have a key that contains multiple pieces of information. The following line from disktop.stp
uses 5 elements for the key: process ID, executable name, user ID, parent process ID, and string "W". It associates the value of
with that key.
device[pid(),execname(),uid(),ppid(),"W"] = devname
All associate arrays must be declared as
global, regardless of whether the associate array is used in one or multiple probes.