# Appendix: Application Programming Interface

A simple API is available to allow for embedded usage of MHonArc within other Perl programs.

- Initialization
- Processing Input
- Callbacks
  - $mhonarc::CBDbPreLoad
  - $mhonarc::CBDbPreSave
  - $mhonarc::CBDbSave
  - $mhonarc::CBMessageBodyRead
  - $mhonarc::CBMessageHeadRead
  - $mhonarc::CBRawMessageBodyRead
  - $mhonarc::CBRcVarExpand
- Notes

## Initialization

Before calling any MHonarc routines, you must initialize the MHonArc library. The following code snippet shows you how to initialize MHonArc:

    # Require MHonArc library
    require 'mhamain.pl';
    # Initialize MHonArc
    mhonarc::initialize();

NOTE: | The `mhonarc::initialize()` routine should only be called once within your program.

NOTE: | If `mhamain.pl` is not in perl's library search path, you will need to add the directory path to perl's search path before calling `require`.

## Processing Input

To instruct MHonArc to process input, use the following routine:

    # Tell MHonArc to start processing
    mhonarc::process_input();

When `mhonarc::process_input()` is called with no arguments, it parses `@ARGV` for command-line arguments. If you pass a list of arguments into `mhonarc::process_input()` then that list will be processed for the command-line arguments. For example:

    mhonarc::process_input(
        '-quiet',
        '-outdir', $archive_path,
        '-rcfile', $rcfile,
        $mailbox_filename
    );

The return value of `mhonarc::process_input()` will be the CPU time, in seconds, MHonArc used. Example usage:

    $cpu_time = mhonarc::process_input();

To determine what the status of the processing was, you can query the `$mhonarc::CODE` variable. The value of this variable reflects what the exit status of MHonArc would be if invoked from the shell. I.e. If `$mhonarc::CODE` is equal to 0, then no errors occured during processing. A non-zero value indicates some error occured. Example usage:

    mhonarc::process_input(
        '-quiet',
        '-outdir', $archive_path,
        '-rcfile', $rcfile,
        $mailbox_filename
    );
    if ($mhonarc::CODE) {
        # error code here
    }

NOTE: | If `$mhonarc::CODE` is equal to `75`, this indicates that MHonArc was unable to obtain a lock on the archive. This exit code is recognized by MTAs like `sendmail` to requeue a message and try to deliver it again later. This is useful when MHonArc is invoked by a `sendmail` alias.

It is okay to call `mhonarc::process_input()` multiple times within a single program. This is useful if your program wants to process multiple archives.

## Callbacks

Support is available for registering callbacks to be invoked when MHonArc is processing input. To register a callback, all you need to do is set the appriopriate MHonArc variable to a routine reference (hard or symbolic). For example, to set the callback when a message header is read, you can do something like the following:

    $mhonarc::CBMessageHeadRead = \&my_callback_routine;

NOTE: | The `mhasiteinit.pl` site initialization library can be used to register callbacks. The advantages for using `mhasiteinit.pl` is that it is executed each time MHonArc is executed, and you do not have to create custom front-ends to MHonArc if all you want to do is register callbacks. See Installation and the example `mhasiteinit.pl` provided in the `examples/` directory of the MHonArc distribution for more information about `mhasiteinit.pl`.

What follows is the type of callbacks supported by MHonArc:

### $mhonarc::CBDbPreLoad

Invoked just before the database file is loaded.

Synopsis:

    $do_load = &$mhonarc::CBDbPreLoad($pathname);

Arguments:

`$pathname`

  Pathname to database file that will be loaded.

Return Value:

If a true value, MHonArc will load the database denoted by `$pathname`. If a false value, MHonArc will skip loading the database file.

Notes:

- This callback, along with `$mhonarc::CBDbPreSave` (in theory), can be used to provide a customized loading and saving of MHonArc archive information.

### $mhonarc::CBDbPreSave

Invoked before data is saved to the database file.

Synopsis:

    $do_save = &$mhonarc::CBDbPreSave($pathname, $tmp_pathname);

Arguments:

`$pathname`

  Pathname to database file that will be written to.

`$tmp_pathname`

  Pathname temporary file that data will be written to before replacing `$pathname`. Data is written to a temporary file first to prevent any I/O, or other, errors leaving a corrupt database. If the data is written successfully, MHonArc renames `$tmp_pathname` to `$pathname`.

Return Value:

If a true value, MHonArc will save the data to `$pathname`. If a false value, MHonArc will skip writing the database file.

Notes:

- Generally, this function should always return a true value. This can easily be done by having the following statement at the end of the callback: `return 1;`

  A possible scenario where a false value may be returned if for cases where a customized format is used to save the data and the `$mhonarc::CBDbPreLoad` function is defined to provide a customized way to load database data.

### $mhonarc::CBDbSave

Invoked when data has been written to database file.

Synopsis:

    &$mhonarc::CBDbSave($db_fh);

Arguments:

`$db_fh`

  Open filehandle to database file. This filehandle can be used to write custom information to the database file.

  Note: Any data written to `$db_fh` must be legal Perl code.

Return Value:

N/A

### $mhonarc::CBMessageBodyRead

The callback function after a mail message body has been read a converted.

Synopsis:

    &$mhonarc::CBMessageBodyRead(
    		$fields_hash_ref, $html_text_ref, $files_array_ref);

Arguments:

`$fields_hash_ref`

  Reference to hash containing parsed message header. The structure of this hash is the same as described for the `$mhonarc::CBMessageHeadRead` callback.

`$html_text_ref`

  Reference to string contain the HTML markup for the body. Modifications to the referenced data will be reflected in the message page generated. Therefore, care should be observed when doing any modification.

  If MHonArc was unable to convert the body of the message, the following expression will evaluate to true:

      $$html_text_ref eq ""

  If this is the case, you could set the value of $$html_text_ref to something else to customize the warning text MHonArc uses in the message page written.

`$files_array_ref`

  Reference to array of derived files when the body was converted. Each file is typically relative to `$mhonarc::OUTDIR`, unless it is a full pathname. the `mhonarc::OSis_absolute_path($filename)` can be used to determine if a file is an absolute pathname or not. Note, it is possible that a file could designate a directory; this indicates that the directory, and all files in the directory, are derived.

  Modifications to the array will affect the list of derived files MHonArc stores for the message. You can add files to the array if your routine creates files, but you can also delete items if your routine removes files; CAUTION: the HTML markup typically contains links to derived files so removing files could cause broken links unless `$html_text_ref` is modified to reflect the file deletions.

Return Value:

N/A

Notes:

- To distinquish between SINGLE operation mode and archive operation mode, you can check the `$mhonarc::SINGLE` variable. For example:

      if ($mhonarc::SINGLE) {
          # single message-based processing here
      } else {
          # archive-based processing here
      }

- The `$mhonarc::CBMessageBodyRead` routine can be used to trigger automatic virus scanning of attachments.

### $mhonarc::CBMessageHeadRead

The callback function after a mail message header is read and before any other processing is done. Note, the function is called after any exclusion checks (CHECKNOARCHIVE and MSGEXCFILTER) are performed by MHonArc.

Synopsis:

    $boolean = &$mhonarc::CBMessageHeadRead(
    			    $fields_hash_ref, $raw_header_txt);

Arguments:

`$fields_hash_ref`

  Reference to hash containing parsed message header. Keys are the lowercase field names and the values are references to array contain the values for each field. If a field is only declared once in the header, the array will only contain one item. For example, to access the raw subject text, do the following: `$fields_hash_ref->{'subject'}[0];`

  The hash also contains special keys represented the values MHonArc has extracted when parsing the message header. The values of these keys are regular scalars and NOT array references. The following summarizes the keys made available:

  `x-mha-index`
    The assigned index given to the message by MHonArc.
  `x-mha-message-id`
    The message-id MHonArc extracted. Note, if the message did not specified a message ID, MHonArc auto-generates one.
  `x-mha-from`
    Who MHonArc thinkgs the message is from. This value is controled by the FROMFIELDS resource.
  `x-mha-subject`
    The message subject that will be used by MHonArc. The value may be different from the raw subject text of the message since SUBJECTSTRIPCODE code will have been applied. If no subject is defined, then the value is the empty string.
  `x-mha-content-type`
    The content-type of the message MHonArc will use for the message.

  For example, to access the subject text that MHonArc will use, do the following: `$fields_hash_ref->{'x-mha-subject'};`

`$raw_header_txt`

  The raw header data of the message. This data may be useful if pattern matches are desired against header data.

Return Value:

The return value is used by MHonArc to determine if the message should be excluded from any further processing. If the return value evaluates to true, then MHonArc will continue processing of the message. If the return value evaluates to false, the message will be excluded.

Notes:

- To distinquish between SINGLE operation mode and archive operation mode, you can check the `$mhonarc::SINGLE` variable. For example:

      if ($mhonarc::SINGLE) {
          # single message-based processing here
      } else {
          # archive-based processing here
      }

- MHonArc resources exist that allow message exclusion capabilities: CHECKNOARCHIVE, EXPIREAGE, EXPIREDATE, and MSGEXCFILTER. If possible, use these resources to perform message exclusion filtering.

### $mhonarc::CBRawMessageBodyRead

Invoked with the raw message body data is read from input. I.e. The message body has not been converted.

Synopsis:

    &$mhonarc::CBRawMessageBodyRead($fields_hash_ref, $body_data_ref);

Arguments:

`$fields_hash_ref`

  Reference to hash containing parsed message header. The structure of this hash is the same as described for the `$mhonarc::CBMessageHeadRead` callback.

`$body_data_ref`

  Reference to string contain the raw data of the message body. Modifications to the referenced data can be performed to change what data MHonArc will process.

Return Value:

N/A

### $mhonarc::CBRcVarExpand

Invoked when a resource variable is to be expanded. With this callback, you can override and/or augment MHonArc's built-in resource variable expansion support.

Synopsis:

    ($result, $do_expand_again, $can_clip) =
        &$mhonarc::CBRcVarExpand($mha_index, $var_name, $arg_string);

Arguments:

`$mha_index`

  The MHonArc index key of the current message.

`$var_name`

  The variable name being expanded. For example, given the resource variable reference, "`$VARIABLE$`", `$var_name` would be equal to "`VARIABLE`".

`$arg_string`

  The argument string for the resource variable reference. For example, given the resource variable reference, "`$VARIABLE(arg1)$`", `$arg_string` would be equal to "`arg1`".

  Note: MHonArc generally uses the character '`;`' to separate multiple arguments to a resource variable. However, this is only convention, and if defining your own resource variable support via this callback, you can use whatever convention you like.

Return Value:

The return value is a list of values interpreted as follows:

`$result`

  The result (or replace text) for the variable. If the result is equal to `undef`, MHonArc's built-in expansion code will be invoked to expand `$var_name`. If `$result` is defined, then MHonArc's built-in expansion will be skipped.

`$do_expand_again`

  If a true value, MHonArc will parse `$result` and expand any resource variables contained within. Note: `$mhonarc::CBRcVarExpand` will be called for each resource variable found.

`$can_clip`

  If a true value, clipping is allowed to be performed. Clipping is done if max length specification is specified in the resource variable reference.

Notes:

- Use the DEFINEVAR resource whenever possible for defining custom resource variables. The `$mhonarc::CBRcVarExpand` should only be used for cases where the sematics of DEFINEVAR are insufficient.

- Colon variable modifiers, if specified in the resource variable reference, are applied by MHonArc after `$mhonarc::CBRcVarExpand` is invoked.

## Notes

- This API documention is not complete. To get a better idea of what you may be able to do, have a look at the source code for the commands provided in the MHonArc distribution: `mhonarc`, `mha-dbedit`, `mha-dbrecover`, and `mha-decode`. You may also want to look at the source of `mhamain.pl`.

- Only a single archive can be processed at any given time.