3.4. Associative Arrays

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:

array_name[index_expression]

Here, the array_name is any arbitrary name the array uses. The index_expression 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 (that is the unique keys): tom, dick, and harry. To assign them the ages (that is associated values) of 23, 24, and 25 respectively, we can use the following array statements:

foo["tom"] = 23
foo["dick"] = 24
foo["harry"] = 25

You can specify up to nine index expressions 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 devname with that key.

device[pid(),execname(),uid(),ppid(),"W"] = devname