Using Libical

Eric Busboom (eric@softwarestudio.org)

January 2000

1 Introduction 

Libical is an Open Source implementation of the iCalendar protocols
and protocol data units. The iCalendar specification describes how
calendar clients can communicate with calendar servers for users can
store their calendar data and arrange meetings with other users. 

Libical implements the following specifications and protocols

+----------+-------+
|iCal Core | 2445  |
+----------+-------+
+----------+-------+
|  iTIP    | 2446  |
+----------+-------+
+----------+-------+
|  iMIP    | 2447  |
+----------+-------+
+----------+-------+
|  iRIP    | draft |
+----------+-------+
+----------+-------+
|   CAP    | draft |
+----------+-------+


(The current version, 0.14, does not implement iRip or CAP. ) 

This documentation assumes that you are familiar with the iCalendar
standards RFC2445 and RFC2446.

1.1 The libical project

This code is under active development. If you would like to contribute
to the project, you can contact me, Eric Busboom, at eric@softwarestudio.org.
The project has a webpage at

http://softwarestudio.org/libical/index.html 

and a mailing list that you can join by sending the following mail:

------------ 

To: minimalist@softwarestudio.org 

Subject: subscribe libical 

------------

1.2 License

The code and datafiles in this distribution are licensed under the
Mozilla Public License. See http://www.mozilla.org/NPL/MPL-1.0.html
for a copy of the license. Alternately, you may use libical under
the terms of the GNU Library General Public License. See http://www.fsf.org/copyleft/lesser.html
for a copy of the LGPL.

This dual license ensures that the library can be incorporated into
both proprietary code and GPL'd programs, and will benefit from improvements
made by programmers in both realms. I will only accept changes into
my version of the library if they are similarly dual-licensed.

1.3 Purpose & Goals 

1.4 Document version

$Id$

2 Building the Library

3 Structure 

The iCal calendar model is based on four types of objects: components,
properties, values and parameters. 

Properties are the fundamental unit of information in iCal, and they
work a bit like a hash entry, with a constant key and a variable value.
Properties may also have modifiers, called parameters. In the iCal
content line

ORGANIZER;ROLE=CHAIR:MAILTO:mrbig@host.com

The property name is ``ORGANIZER,'' the value of the property is ``mrbig@host.com''
and the ``ROLE'' parameter specifies that Mr Big is the chair of the
meetings associated with this property. 

Components are groups of properties that represent the core objects
of a calendar system, such as events or timezones. 

The central goal of libical is to parse iTIP data into an internal
representation of Components, Properties, Parameters an Values, and
to allow the user to manipulate the data in various ways

3.1 Components 

3.2 Properties 

3.3 Values 

3.4 Parameters 

3.5 Enumerations

3.6 Types

3.7 The Parser

3.8 Restrictions

3.9 Memory Management

4 Differences From RFCs 

Libical has been designed to follow the standards as closely as possible,
so that the key objects in the standards are also keey objects in
the library. However, there are a few areas where the specifications
are (arguably) irregular, and following them exactly would result
in an unfriendly interface. These deviations make libical easier to
use by maintaining a self-similar interface. 

4.1 Pseudo Components 

Libical defines components for groups of properties that look and act
like components, but are not defined as components in the specification.
XDAYLIGHT and XSTANDARD are notable examples. These pseudo components
group properties within the VTIMEZONE components. XDAYLIGHT starts
with ``BEGIN:DAYLIGHT'' and ends with ``END:DAYLIGHT, just like other
components, but is not defined as a component in RFC2445. ( See RFC2445,
page 61 ) In Libical, it is a component. 

There are also pseudo components that are conceptually derived classess
of VALARM. RFC2446 defines what properties may be included in each
component, and for VALARM, the set of properties it may have depends
on the value of the ACTION property. 

For instance, if a VALARM component has an ACTION property with the
value of ``AUDIO,'' the component must also have an ``ATTACH'' property.
However, if the ACTION value is ``DISPLAY,'' the component must have
a DESCRIPTION property. 

To handle these various, complex restrictions, libical has pseudo components
for each type of alarm: XAUDIOALARM, XDISPLAYALARM, XEMAILALARM and
XPROCEDUREALARM. 

4.2 Combined Values 

Many values can take more than one type. TRIGGER, for instance, can
have a value type of with DURATION or of DATE-TIME. These multiple
types make it difficult to create routines to return the value associated
with a property. 

It is natural to have interfaces that would return the value of a property,
but it is cumbersone for a single routine to return multiple types.
So, in libical, properties that can have multiple types are given
a single type that is the union of their RFC2445 types. For instance,
in libical, the value of the TRIGGER property resolves to struct icaltriggertype.
This type is a union of a DURATION and a DATE-TIME. 

4.3 Multi-Valued Properties

Some properties, such as CATEGORIES have only one value type, but each
CATEGORIES property can have multiple value instances. This also results
in a cumbersome interface -- CATEGORIES accessors would have to return
a list while all other accessors returned a single value. In libical,
all properties have a single value, and multi-valued properties are
broken down into multiple single valued properties during parsing.
That is, an input line like, 

CATEGORIES: work, home

becomes in libical's internal representation

CATEGORIES: work

CATEGORIES: home

Oddly, RFC2445 allows some multi-valued properties ( like FREEBUSY
) to exist as both a multi-values property and as multiple single
value properties, while others ( like CATEGORIES ) can only exist
as single multi-valued properties. This makes the internal representation
for CATEGORIES illegal. However when you convert a component to a
string, the library will collect all of the CATEGORIES properties
into one. 

5 Implementation Limitations 

6 Using libical

6.1 Creating Components 

6.1.1 Constructor Interfaces 

6.1.2 vaargs Constructors 

6.1.3 Parsing Text Files 

6.2 Accessing Components 

6.2.1 Finding Components 

6.2.2 Removing Components 

Removing an element from a list while iterating through the list can
cause problems, since you will probably be removing the element that
the internal iterator points to. This will result in the iteration
loop terminating immediately after removing the element. To avoid
the problem, you will need to step the iterator ahead of the element
you are going to remove, like this:

for(c = icalcomponent_get_first_component(s); 

    c != 0; 

    c = next)

{ 

    next = icalcomponent_get_next_component(s);

    icalcomponent_remove_component(s,c); 

}

6.2.3 Finding Properties 

6.2.4 Removing Properties 

6.2.5 Getting Values 

6.2.6 Setting Values 

6.2.7 Getting Parameters

6.2.8 Setting Parameters 

6.2.9 Removing Parameters 

6.2.10 Checking Component Validity

6.3 Storing Objects

The libical distribution inclues a seperate library, libicalss, that
allows you to store iCal component data to disk in a variety of ways.
This library is documented seperately. 

6.4 Memory Management 

Here are the memory rules for the library:

1) If the function name has "new" in it, the caller gets control
  of the memory. ( such as icalcomponent_new(), or icalproperty_new_clone()
  ) 

2) If you got the memory from a routine with new in it, you must
  call the corresponding *_free routine to free the memory. ( Use
  icalcomponent_free() to free objects created with icalcomponent_new()) 

3) If the function name has "add" in it, the caller is transfering
  control of the memory to the routine. ( icalproperty_add_parameter() )

4) If the function name has "remove" in it, the caller passes in
  a pointer to an object and after the call returns, the caller owns
  the object. So, before you call icalcomponent_remove_property(comp,foo),
  you do not own "foo" and after the call returns, you do. 

5) If the routine returns a string, libical owns the memory and will
  put it on a ring buffer to reclaim later. You'd better strdup it
  if you want to keep it, and you don't have to delete it. 

6.5 Error Handling

6.5.1 Return values

6.5.2 icalerrno

6.5.3 Component errors

6.6 Naming Standard

Structures that you access with the ``struct'' keyword, such as ``struct
icaltimetype'' are things that you are allowed to see inside and poke
at. 

Structures that you access though a typedef, such as ``icalcomponent''
are things where all of the data is hidden. 

Component names that start with ``V'' are part of RFC 2445 or another
iCal standard. Component names that start with ``X'' are also part
of the spec, but they are not actually components in the spec. However,
they look and act like components, so they are components in libical.
Names that start with ``XLIC'' or ``X-LIC'' are not part of any iCal
spec. They are used internally by libical. 

7 Hacks and Bugs