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+
+
+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