# Array Operations in SystemTap

This section enumerates some of the most commonly used array operations in SystemTap.

## Assigning an Associated Value

Use `=` to set an associated value to indexed unique pairs, as in:

    array_name[index_expression] = value

shows a very basic example of how to set an explicit associated value to a unique key. You can also use a handler function as both your `index_expression` and `value`. For example, you can use arrays to set a timestamp as the associated value to a process name (which you wish to use as your unique key), as in:

Associating Timestamps to Process Names

    foo[tid()] = gettimeofday_s()

Whenever an event invokes the statement in , SystemTap returns the appropriate `tid()` value (that is, the ID of a thread, which is then used as the unique key). At the same time, SystemTap also uses the function `gettimeofday_s()` to set the corresponding timestamp as the associated value to the unique key defined by the function `tid()`. This creates an array composed of key pairs containing thread IDs and timestamps.

In this same example, if `tid()` returns a value that is already defined in the array `foo`, the operator will discard the original associated value to it, and replace it with the current timestamp from `gettimeofday_s()`.

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## Reading Values From Arrays

You can also read values from an array the same way you would read the value of a variable. To do so, include the `array_name[index_expression]` statement as an element in a mathematical expression. For example:

Using Array Values in Simple Computations

    delta = gettimeofday_s() - foo[tid()]

This example assumes that the array `foo` was built using the construct in (from ). This sets a timestamp that will serve as a reference point, to be used in computing for `delta`.

The construct in computes a value for the variable `delta` by subtracting the associated value of the key `tid()` from the current `gettimeofday_s()`. The construct does this by reading the value of `tid()` from the array. This particular construct is useful for determining the time between two events, such as the start and completion of a read operation.

Note

If the `index_expression` cannot find the unique key, it returns a value of 0 (for numerical operations, such as ) or a null/empty string value (for string operations) by default.

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## Incrementing Associated Values

Use `++` to increment the associated value of a unique key in an array, as in:

    array_name[index_expression] ++

Again, you can also use a handler function for your `index_expression`. For example, if you wanted to tally how many times a specific process performed a read to the virtual file system (using the event `vfs.read`), you can use the following probe:

vfsreads.stp

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

In , the first time that the probe returns the process name `gnome-terminal` (that is, the first time `gnome-terminal` performs a VFS read), that process name is set as the unique key `gnome-terminal` with an associated value of 1. The next time that the probe returns the process name `gnome-terminal`, SystemTap increments the associated value of `gnome-terminal` by 1. SystemTap performs this operation for all process names as the probe returns them.

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## Processing Multiple Elements in an Array

Once you've collected enough information in an array, you will need to retrieve and process all elements in that array to make it useful. Consider : the script collects information about how many VFS reads each process performs, but does not specify what to do with it. The obvious means for making useful is to print the key pairs in the array `reads`, but how?

The best way to process all key pairs in an array (as an iteration) is to use the `foreach` statement. Consider the following example:

cumulative-vfsreads.stp

    global reads
    probe vfs.read
    {
      reads[execname()] ++
    }
    probe timer.s(3)
    {
      foreach (count in reads)
        printf("%s : %d \n", count, reads[count])
    }

In the second probe of , the `foreach` statement uses the variable `count` to reference each iteration of a unique key in the array `reads`. The `reads[count]` array statement in the same probe retrieves the associated value of each unique key.

Given what we know about the first probe in , the script prints VFS-read statistics every 3 seconds, displaying names of processes that performed a VFS-read along with a corresponding VFS-read count.

Now, remember that the `foreach` statement in prints all iterations of process names in the array, and in no particular order. You can instruct the script to process the iterations in a particular order by using `+` (ascending) or `-` (descending). In addition, you can also limit the number of iterations the script needs to process with the `limit value` option.

For example, consider the following replacement probe:

    probe timer.s(3)
    {
      foreach (count in reads- limit 10)
        printf("%s : %d \n", count, reads[count])
    }

This `foreach` statement instructs the script to process the elements in the array `reads` in descending order (of associated value). The `limit 10` option instructs the `foreach` to only process the first ten iterations (that is, print the first 10, starting with the highest value).

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## Clearing/Deleting Arrays and Array Elements

Sometimes, you may need to clear the associated values in array elements, or reset an entire array for re-use in another probe. in allows you to track how the number of VFS reads per process grows over time, but it does not show you the number of VFS reads each process makes per 3-second period.

To do that, you will need to clear the values accumulated by the array. You can accomplish this using the `delete` operator to delete elements in an array, or an entire array. Consider the following example:

noncumulative-vfsreads.stp

    global reads
    probe vfs.read
    {
      reads[execname()] ++
    }
    probe timer.s(3)
    {
      foreach (count in reads)
        printf("%s : %d \n", count, reads[count])
      delete reads
    }

In , the second probe prints the number of VFS reads each process made within the probed 3-second period only. The `delete reads` statement clears the `reads` array within the probe.

Note

You can have multiple array operations within the same probe. Using the examples from and , you can track the number of VFS reads each process makes per 3-second period and tally the cumulative VFS reads of those same processes. Consider the following example:

    global reads, totalreads

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

    probe timer.s(3)
    {
      printf("=======\n")
      foreach (count in reads-)
        printf("%s : %d \n", count, reads[count])
      delete reads
    }

    probe end
    {
      printf("TOTALS\n")
      foreach (total in totalreads-)
        printf("%s : %d \n", total, totalreads[total])
    }

In this example, the arrays `reads` and `totalreads` track the same information, and are printed out in a similar fashion. The only difference here is that `reads` is cleared every 3-second period, whereas `totalreads` keeps growing.

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## 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:

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, 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:

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    if([index_expression] in array_name) statement

To illustrate this, consider the following example:

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`.

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## Computing for Statistical Aggregates

Statistical aggregates are used to collect statistics on numerical values where it is important to accumulate new data quickly and in large volume (that is, storing only aggregated stream statistics). Statistical aggregates can be used in global variables or as elements in an array.

To add value to a statistical aggregate, use the operator `<<< value`.

need more examples of supported rvalues, for example, length, count, and what each one does.

stat-aggregates.stp

    global reads
    probe vfs.read
    {
      reads[execname()] <<< $count
    }

In , the operator `<<< $count` stores the amount returned by `$count` to the associated value of the corresponding `execname()` in the `reads` array. Remember, these values are stored; they are not added to the associated values of each unique key, nor are they used to replace the current associated values. In a manner of speaking, think of it as having each unique key (`execname()`) having multiple associated values, accumulating with each probe handler run.

Note

In the context of , `count` returns the amount of data read by the returned `execname()` to the virtual file system.

To extract data collected by statistical aggregates, use the syntax format `@extractor(variable/array index expression)`. `extractor` can be any of the following integer extractors:

count

  Returns the number of all values stored into the variable/array index expression. Given the sample probe in , the expression `@count(reads[execname()])` will return how many values are stored in each unique key in array `reads`.

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sum

  Returns the sum of all values stored into the variable/array index expression. Again, given sample probe in , the expression `@sum(reads[execname()])` will return the total of all values stored in each unique key in array `reads`.

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min

  Returns the smallest among all the values stored in the variable/array index expression.

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max

  Returns the largest among all the values stored in the variable/array index expression.

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avg

  Returns the average of all values stored in the variable/array index expression.

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When using statistical aggregates, you can also build array constructs that use multiple index expressions (to a maximum of 5). This is helpful in capturing additional contextual information during a probe. For example:

Multiple Array Indexes

    global reads
    probe vfs.read
    {
      reads[execname(),pid()] <<< 1
    }
    probe timer.s(3)
    {
      foreach([var1,var2] in reads)
        printf("%s (%d) : %d \n", var1, var2, @count(reads[var1,var2]))
    }

In , the first probe tracks how many times each process performs a VFS read. What makes this different from earlier examples is that this array associates a performed read to both a process name and its corresponding process ID.

The second probe in demonstrates how to process and print the information collected by the array `reads`. Note how the `foreach` statement uses the same number of variables (that is, `var1` and `var2`) contained in the first instance of the array `reads` from the first probe.

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