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

array operations assigning associated values assigning associated values array operations values, assignment of array operations 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: assigning associated values array operations associating timestamps to process names array operations assigning associated values associating timestamps to process names operations assigning associated values associating timestamps to process names assigning associated values associating timestamps to process names array operations associating timestamps to process names assigning associated values array operations timestamps, association thereof to process names assigning associated values array operations foo[tid()] = gettimeofday_s() Whenever an event invokes the statement in , SystemTap returns the appropriate tid() value (i.e. 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().

reading values from arrays array operations array operations reading values from arrays operations 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: reading values from arrays array operations using arrays in simple computations array operations reading values from arrays using arrays in simple computations operations reading values from arrays using arrays in simple computations using arrays in simple computations reading values from arrays array operations algebraic formulas using arrays reading values from arrays array operations array operations reading values from arrays computing for timestamp deltas operations reading values from arrays computing for timestamp deltas computing for timestamp deltas reading values from arrays array operations reading values from arrays computing for timestamp deltas array operations timestamp deltas, computing for reading values from arrays array operations 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. reading values from arrays array operations empty unique keys array operations reading values from arrays empty unique keys operations reading values from arrays empty unique keys empty unique keys reading values from arrays array operations 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.

array operations incrementing associated values incrementing associated values array operations operations 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: incrementing associated values array operations tallying virtual file system reads (VFS reads) array operations incrementing associated values tallying virtual file system reads (VFS reads) operations incrementing associated values tallying virtual file system reads (VFS reads) tallying virtual file system reads (VFS reads) incrementing associated values array operations VFS reads, tallying of incrementing associated values array operations probe vfs.read { reads[execname()] ++ } In , the first time that the probe returns the process name gnome-terminal (i.e. 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.

multiple elements in an array array operations array operations multiple elements in an array operations multiple elements in an array array operations processing multiple elements in an array processing multiple elements in an array array operations operations 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? array operations processing multiple elements in an array foreach operations processing multiple elements in an array foreach processing multiple elements in an array foreach array operations foreach processing multiple elements in an array array operations array operations processing multiple elements in an array iterations, processing elements in an array as operations processing multiple elements in an array iterations, processing elements in an array as iterations, processing elements in an array as processing multiple elements in an array array operations The best way to process all key pairs in an array (as an iteration) is to use the foreach statement. Consider the following example: array operations processing multiple elements in an array cumulative virtual file system reads, tallying operations processing multiple elements in an array cumulative virtual file system reads, tallying cumulative virtual file system reads, tallying processing multiple elements in an array array operations processing multiple elements in an array cumulative virtual file system reads, tallying array operations virtual file system reads (cumulative), tallying processing multiple elements in an array array operations 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. array operations processing multiple elements in an array limiting the output of foreach operations processing multiple elements in an array limiting the output of foreach processing multiple elements in an array limiting the output of foreach array operations limiting the output of foreach processing multiple elements in an array array operations array operations processing multiple elements in an array ordering the output of foreach operations processing multiple elements in an array ordering the output of foreach processing multiple elements in an array ordering the output of foreach array operations ordering the output of foreach processing multiple elements in an array array operations 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 (i.e. print the first 10, starting with the highest value).

array operations deleting arrays and array elements operations deleting arrays and array elements array operations clearing arrays/array elements operations clearing arrays/array elements clearing arrays/array elements array operations 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. array operations clearing arrays/array elements delete operator operations clearing arrays/array elements delete operator clearing arrays/array elements array operations delete operator delete operator clearing arrays/array elements array operations 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: array operations clearing arrays/array elements virtual file system reads (non-cumulative), tallying operations clearing arrays/array elements virtual file system reads (non-cumulative), tallying clearing arrays/array elements array operations virtual file system reads (non-cumulative), tallying virtual file system reads (non-cumulative), tallying clearing arrays/array elements array operations 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. array operations clearing arrays/array elements multiple array operations within the same probe operations clearing arrays/array elements multiple array operations within the same probe clearing arrays/array elements array operations multiple array operations within the same probe multiple array operations within the same probe clearing arrays/array elements array operations 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.

array operations conditional statements, using arrays in operations conditional statements, using arrays in conditional statements, using arrays in array operations if/else statements, using arrays in array operations 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: 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. array operations conditional statements, using arrays in testing for array membership operations conditional statements, using arrays in testing for array membership conditional statements, using arrays in array operations testing for array membership testing for array membership conditional statements, using arrays in array operations membership (in array), testing for conditional statements, using arrays in array operations 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: 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.

statistical aggregates array operations aggregates (statistical) array operations array operations computing for statistical aggregates operations computing for statistical aggregates computing for statistical aggregates array operations Statistical aggregates are used to collect statistics on numerical values where it is important to accumulate new data quickly and in large volume (i.e. storing only aggregated stream statistics). Statistical aggregates can be used in global variables or as elements in an array. array operations computing for statistical aggregates adding values to statistical aggregates operations computing for statistical aggregates adding values to statistical aggregates computing for statistical aggregates array operations adding values to statistical aggregates adding values to statistical aggregates computing for statistical aggregates array operations To add value to a statistical aggregate, use the operator <<< value. need more examples of supported rvalues, e.g. length, count, and what each one does. global reads probe vfs.read { reads[execname()] <<< count } array operations computing for statistical aggregates count (operator) operations computing for statistical aggregates count (operator) computing for statistical aggregates array operations count (operator) count operator computing for statistical aggregates array (operator) In , the operator <<< count stores the amount returned by count to 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. In the context of , count returns the amount of data written by the returned execname() to the virtual file system. array operations computing for statistical aggregates extracting data collected by statistical aggregates operations computing for statistical aggregates extracting data collected by statistical aggregates computing for statistical aggregates array operations extracting data collected by statistical aggregates extracting data collected by statistical aggregates computing for statistical aggregates array operations integer extractors computing for statistical aggregates array operations 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 array operations computing for statistical aggregates @count (integer extractor) operations computing for statistical aggregates @count (integer extractor) computing for statistical aggregates array operations @count (integer extractor) @count (integer extractor) computing for statistical aggregates array operations Returns the number of all values stored into the variable/array index expression. Given the sample probe in , the expression @count(writes[execname()]) will return how many values are stored in each unique key in array writes. sum array operations computing for statistical aggregates @sum (integer extractor) operations computing for statistical aggregates @sum (integer extractor) computing for statistical aggregates array operations @sum (integer extractor) @sum (integer extractor) computing for statistical aggregates array operations Returns the sum of all values stored into the variable/array index expression. Again, given sample probe in , the expression @sum(writes[execname()]) will return the total of all values stored in each unique key in array writes. min array operations computing for statistical aggregates @min (integer extractor) operations computing for statistical aggregates @min (integer extractor) computing for statistical aggregates array operations @min (integer extractor) @min (integer extractor) computing for statistical aggregates array operations Returns the smallest among all the values stored in the variable/array index expression. max array operations computing for statistical aggregates @max (integer extractor) operations computing for statistical aggregates @max (integer extractor) computing for statistical aggregates array operations @max (integer extractor) @max (integer extractor) computing for statistical aggregates array operations Returns the largest among all the values stored in the variable/array index expression. avg array operations computing for statistical aggregates @avg (integer extractor) operations computing for statistical aggregates @avg (integer extractor) computing for statistical aggregates array operations @avg (integer extractor) @avg (integer extractor) computing for statistical aggregates array operations Returns the average of all values stored in the variable/array index expression. 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: 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 (i.e. var1 and var2) contained in the first instance of the array reads from the first probe.