# Tapset Development Guidelines

This chapter describes the upstream guidelines on proper tapset documentation. It also contains information on how to properly document your tapsets, to ensure that they are properly defined in this guide.

## Writing Good Tapsets

The first step to writing good tapsets is to create a simple model of your subject area. For example, a model of the process subsystem might include the following:

## Key Data

- process ID

- parent process ID

- process group ID

## State Transitions

- forked

- exec'd

- running

- stopped

- terminated

### Note

Both lists are examples, and are not meant to represent a complete list.

Use your subsystem expertise to find probe points (function entries and exits) that expose the elements of the model, then define probe aliases for those points. Be aware that some state transitions can occur in more than one place. In those cases, an alias can place a probe in multiple locations.

For example, process execs can occur in either the `do_execve()` or the `compat_do_execve()` functions. The following alias inserts probes at the beginning of those functions:

    probe kprocess.exec = kernel.function("do_execve"),
    kernel.function("compat_do_execve")
    {probe body}

Try to place probes on stable interfaces (i.e., functions that are unlikely to change at the interface level) whenever possible. This will make the tapset less likely to break due to kernel changes. Where kernel version or architecture dependencies are unavoidable, use preprocessor conditionals (see the `stap(1)` man page for details).

Fill in the probe bodies with the key data available at the probe points. Function entry probes can access the entry parameters specified to the function, while exit probes can access the entry parameters and the return value. Convert the data into meaningful forms where appropriate (e.g., bytes to kilobytes, state values to strings, etc).

You may need to use auxiliary functions to access or convert some of the data. Auxiliary functions often use embedded C to do things that cannot be done in the SystemTap language, like access structure fields in some contexts, follow linked lists, etc. You can use auxiliary functions defined in other tapsets or write your own.

In the following example, `copy_process()` returns a pointer to the `task_struct` for the new process. Note that the process ID of the new process is retrieved by calling `task_pid()` and passing it the `task_struct` pointer. In this case, the auxiliary function is an embedded C function defined in `task.stp`.

    probe kprocess.create = kernel.function("copy_process").return
    {
       task = $return
       new_pid = task_pid(task)
    }

It is not advisable to write probes for every function. Most SystemTap users will not need or understand them. Keep your tapsets simple and high-level.

## Elements of a Tapset

The following sections describe the most important aspects of writing a tapset. Most of the content herein is suitable for developers who wish to contribute to SystemTap's upstream library of tapsets.

### Tapset Files

Tapset files are stored in ``src`/tapset/` of the SystemTap GIT directory. Most tapset files are kept at that level. If you have code that only works with a specific architecture or kernel version, you may choose to put your tapset in the appropriate subdirectory.

Installed tapsets are located in `/usr/share/systemtap/tapset/` or `/usr/local/share/systemtap/tapset`.

Personal tapsets can be stored anywhere. However, to ensure that SystemTap can use them, use `-I `tapset_directory`` to specify their location when invoking `stap`.

### Namespace

Probe alias names should take the form ``tapset_name.probe_name``. For example, the probe for sending a signal could be named `signal.send`.

Global symbol names (probes, functions, and variables) should be unique accross all tapsets. This helps avoid namespace collisions in scripts that use multiple tapsets. To ensure this, use tapset-specific prefixes in your global symbols.

Internal symbol names should be prefixed with an underscore (`_`).

### Comments and Documentation

All probes and functions should include comment blocks that describe their purpose, the data they provide, and the context in which they run (e.g. interrupt, process, etc). Use comments in areas where your intent may not be clear from reading the code.

Note that specially-formatted comments are automatically extracted from most tapsets and included in this guide. This helps ensure that tapset contributors can write their tapset and document it in the same place. The specified format for documenting tapsets is as follows:

    /**
     * probe tapset.name - Short summary of what the tapset does.
     * @argument: Explanation of argument.
     * @argument2: Explanation of argument2. Probes can have multiple arguments.
     *
     * Context:
     * A brief explanation of the tapset context.
     * Note that the context should only be 1 paragraph short.
     *
     * Text that will appear under "Description."
     *
     * A new paragraph that will also appear under the heading "Description".
     *
     * Header:
     * A paragraph that will appear under the heading "Header".
     **/

For example:

    /**
     * probe vm.write_shared_copy- Page copy for shared page write.
     * @address: The address of the shared write.
     * @zero: Boolean indicating whether it is a zero page
     *         (can do a clear instead of a copy).
     *
     * Context:
     *  The process attempting the write.
     *
     *  Fires when a write to a shared page requires a page copy.  This is
     *  always preceded by a vm.write_shared.
     **/

To override the automatically-generated `Synopsis` content, use:

     * Synopsis:
     * New Synopsis string
     *

For example:

    /**
     * probe signal.handle - Fires when the signal handler is invoked
     * @sig: The signal number that invoked the signal handler
     *
     * Synopsis:
     * <programlisting>static int handle_signal(unsigned long sig, siginfo_t *info, struct k_sigaction *ka,
     * sigset_t *oldset, struct pt_regs * regs)</programlisting>
     */

It is recommended that you use the `<programlisting>` tag in this instance, since overriding the `Synopsis` content of an entry does not automatically form the necessary tags.

For the purposes of improving the DocBook XML output of your comments, you can also use the following XML tags in your comments:

- `command`

- `emphasis`

- `programlisting`

- `remark` (tagged strings will appear in Publican beta builds of the document)