path: root/libical/doc/UsingLibical.txt

Using Libical

Eric Busboom (eric@softwarestudio.org)

May 2000

\tableofcontents{}

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 RFC2445 and RFC2446. Eventually, it will also implement
iRIP and CAP. 

This documentation assumes that you are familiar with the iCalendar
standards RFC2445 and RFC2446. these specifications are online on
the CALSCH webpage at:

http://www.imc.org/ietf-calendar/

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 Example Code

A lot of the documentation for this library is in the form of example
code. These examples are in the ``examples'' directory of the distribution.
Also look in ``src/test'' for more annotated examples. 

2 Building the Library

Libical uses autoconf to generate makefiles, although it uses none
of the autoconf flags to influence the compilation. It should built
with no adjustments on Linux, FreeBSD and Solaris. 

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 ([fig] \includegraphics{icaluml.eps} ) When
a component is sent across a network, if it is un-encrypted, it will
look something like:

BEGIN:VEVENT 

DTSTAMP:19980309T231000Z 

UID:guid-1.host1.com 

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

ATTENDEE;RSVP=TRUE;ROLE=REQ-PARTICIPANT;CUTYPE=GROUP: 

  MAILTO:employee-A@host.com 

DESCRIPTION:Project XYZ Review Meeting 

CATEGORIES:MEETING 

CLASS:PUBLIC 

CREATED:19980309T130000Z 

SUMMARY:XYZ Project Review 

DTSTART;TZID=US-Eastern:19980312T083000 

DTEND;TZID=US-Eastern:19980312T093000 

LOCATION:1CP Conference Room 4350 

END:VEVENT 

3.1 Core iCal classes

3.1.1 Components 

3.1.2 Properties 

3.1.3 Values 

3.1.4 Parameters 

3.2 Other elements of libical

In addition to the core iCal classes, libical has many other types,
structures, classes that aid in creating and using iCal components. 

3.2.1 Enumerations

3.2.2 Types

3.2.3 The Parser

3.2.4 Restrictions

3.2.5 Error objects

3.2.6 Memory Management

3.2.7 Storage classes

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. For instanace, the
timezone properties associated with daylight savings time 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,this grouping is represented by the XDAYLIGHT component.
Standard iCAL components all start with the letter ``V,'' while pseudo
components start with''X.''

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 

There are three ways to create components in Libical: creating individual
objects and assembling them, building entire objects in massive vaargs
calls, and parsing a text file containing iCalendar data. 

6.1.1 Constructor Interfaces 

Using constructor interfaces, you create each of the objects seperately
and them assemble them in to components: 

icalcomponent *event;

icalproperty *prop;

icalparameter *param;

struct icaltimetype atime;

event = icalcomponent_new(ICAL_VEVENT_COMPONENT);

prop = icalproperty_new_dtstamp(atime) ;

icalcomponent_add_property(event, prop);

prop = icalproperty_new_uid(strdup("guid-1.host1.com")) );

icalcomponent_add_property(event,prop);

prop=icalproperty_new_organizer(strdup("mrbig@host.com")); 

param = icalparameter_new_role(ICAL_ROLE_CHAIR) 

icalproperty_add_parameter(prop, param);

icalcomponent_add_property(event,prop);

While we are on this example, you should notice that libical uses a
semi-object-oriented style of interface. Most things you work with
are objects, that are instantiated with a constructor that has ``new''
in the name. Also note that, other than the object reference, most
structure data is passed in to libical routines by value. Strings,
of course, are passed in by reference, but libical will take ownership
of the memory, so you had beter strdup() the data unless you want
a core dump when the memory is freed for the second time. Libical
has some complex but very regular memory handling rules. These are
detailed in section \ref{sec:memory}.

If any of the constructors fail, they will return 0. If you try to
insert 0 into a property or component, or use a zero-valued object
reference, libical will either silently ignore the error or will abort
with an error message. This behavior is controlled by a compile time
flag (ICAL_ERRORS_ARE_FATAL), and will abort by default. 

6.1.2 vaargs Constructors 

There is another way to create complex components, which is arguable
more elegant, if you are not horrified by varargs. The varargs constructor
interface all you to create intricate components in a single block
of text. 

    calendar = 

        icalcomponent_vanew(

            ICAL_VCALENDAR_COMPONENT,

            icalproperty_new_version(strdup("2.0")),

            icalproperty_new_prodid(strdup("-//RDU Software//NONSGML HandCal//EN")),

            icalcomponent_vanew(

                ICAL_VEVENT_COMPONENT,

                icalproperty_new_dtstamp(atime),

                icalproperty_new_uid(strdup("guid-1.host1.com")),

                icalproperty_vanew_organizer(

                    strdup("mrbig@host.com"),

                    icalparameter_new_role(ICAL_ROLE_CHAIR),

                    0

                    ),

                icalproperty_vanew_attendee(

                    strdup("employee-A@host.com"),

                    icalparameter_new_role(ICAL_ROLE_REQPARTICIPANT),

                    icalparameter_new_rsvp(1),

                    icalparameter_new_cutype(ICAL_CUTYPE_GROUP),

                    0

                    ),

                icalproperty_new_location(strdup("1CP Conference Room 4350")),

                0

                ),

            0

            );

This form is similar to the regular constructor, except that they have
``vanew'' instead of ``new'' in the name. The arguments are similar
too, except that the component contstructor can have a list of properties,
and the property constructor can have a list or parameters. Be sure
to terminate every list with a '0', or your code will crash, if you
are lucky. 

6.1.3 Parsing Text Files 

The final way to create components will probably be the most common;
you can create components from RFC2445 compliant text. If you have
the string in memory, use

icalcomponent* icalparser_parse_string(char* str);

This may seem wasteful if you want to pull a large component off of
the network; you may prefer to parse the component line by line. This
is possible too by using:

icalparser* icalparser_new(); 

void icalparser_free(icalparser* parser);

icalparser_get_line(parser,read_stream);

icalparser_add_line(parser,line);

icalparser_set_gen_data(parser,stream)

These routines will construct a parser object to which you can add
lines of input and retrieve any components that the parser creates
from the input. For an example:

char* read_stream(char *s, size_t size, void *d) 

{ 

  char *c = fgets(s,size, (FILE*)d);

  return c;

}

main() {

  char* line; 

  icalcomponent *c; 

  icalparser *parser = icalparser_new();

  FILE* stream = fopen(argv[1],"r");

  icalparser_set_gen_data(parser,stream);

  do{ 

    line = icalparser_get_line(parser,read_stream); 

    c = icalparser_add_line(parser,line);

    if (c != 0){ 

     printf("%s",icalcomponent_as_ical_string(c));     

     icalparser_claim(parser); 

     printf("\n---------------\n"); 

     icalcomponent_free(c); 

   }

  } while ( line != 0);

}

The parser object parameterizes the routine used to get input lines
with icalparser_set_gen_data() and icalparser_get_line(). In this
example, the routine read_stream() will fetch the next line from a
stream, with the stream passed in as the void* parameter d. The parser
calls read_stream() from icalparser_get_line(), but it also needs
to know what stream to use. This is set by the call to icalparser_set_gen_data(). 

Using the same mechanism, other implmentations could read from memory
buffers, sockets or other interfaces. 

Since the example code is a very common way to use the parser, there
is a convienience routine;

icalcomponent* icalparser_parse(icalparser *parser, 

               char* (*line_gen_func)(char *s, size_t size, void* d))

To use this routine, you still must construct the parser object and
pass in a reference to a line reading routine. If the parser can create
a single component from the input, it will return a pointer to the
newly constructed component. If the parser can construct multiple
cmponents from the input, it will return a reference to an XROOT component
( of type ICAL_XROOT_COMPONENT.) This XROOT component will hold all
of the components constructed from the input as children. See section
6.2.2 for how to iterate through the child components. 

6.2 Accessing Components 

Given a reference to a component, you probably will want to access
the properties, parameters and values inside. Libical interface let
you find sub-component, add and remove sub-components, and do the
same three operations on properties. 

6.2.1 Finding Components 

To find a sub-component of a component, use: 

icalcomponent* icalcomponent_get_first_component(

                                  icalcomponent* component, 

                                  icalcomponent_kind kind);

This routine will return a reference to the first component of the
type 'kind.' The key kind values, listed in icalenums.h are: 

ICAL_ANY_COMPONENT

ICAL_VEVENT_COMPONENT

ICAL_VTODO_COMPONENT

ICAL_VJOURNAL_COMPONENT

ICAL_VCALENDAR_COMPONENT 

ICAL_VFREEBUSY_COMPONENT

ICAL_VALARM_COMPONENT

These are only the most common components; there are many more listed
in icalenums.h.

As you might guess, if there is more than one subcomponent of the type
you have chosen, this routine will return only the first. to get at
the others, you need to iterate through the component. 

6.2.2 Interating Through Components

Iteration requires a second routine to get the next subcomponent after
the first:

icalcomponent* icalcomponent_get_next_component(

               icalcomponent* component, icalcomponent_kind kind);

With the 'first' and 'next' routines, you can create a for loop to
iterate through all of a components subcomponents

  for(c = icalcomponent_get_first_component(comp,ICAL_ANY_COMPONENT); 

         c != 0; 

         c = icalcomponent_get_next_component(comp,ICAL_ANY_COMPONENT))

{ 

      do_something(c);

}

This code bit wil iterate through all of the subcomponents in 'comp'
but you can select a specific type of component by changing ICAL_ANY_COMPONENT
to another component type.

6.2.3 Removing Components 

Libical component have internal iterators, so you can only have one
iteration over a component at a time. 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(parent_comp,ICAL_ANY_COMPONENT); 

       c != 0; 

       c = next

{     

    next = icalcomponent_get_next_component(parent_comp,ICAL_ANY_COMPONENT);

    icalcomponent_remove_component(parent_comp,c); 

}

6.2.4 Working with properties and parameters

Finding, iterating and removing properties works the same as it does
for components, using the property-specific or parameter-specific
interfaces: 

icalproperty* icalcomponent_get_first_property(

     icalcomponent* component, 

     icalproperty_kind kind); 

icalproperty* icalcomponent_get_next_property(

     icalcomponent* component, 

     icalproperty_kind kind);

void icalcomponent_add_property(

     icalcomponent* component, 

     icalproperty* property);

void icalcomponent_remove_property(

     icalcomponent* component, 

     icalproperty* property);

icalparameter* icalproperty_get_first_parameter(

     icalproperty* prop, 

     icalparameter_kind kind); 

icalparameter* icalproperty_get_next_parameter(

     icalproperty* prop, 

     icalparameter_kind kind);

void icalproperty_add_parameter(

     icalproperty* prop,

     icalparameter* parameter);

void icalproperty_remove_parameter(

     icalproperty* prop, 

     icalparameter_kind kind);

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

RFC 2446 defines rules for what properties must exist in a component
to be used for transfering scheduling data. Most of these rules relate
to the existence of properties relative to the METHOD property, which
declares what operation a remote reciever should use to process a
component. For instance, if the METHOD is REQUEST and the component
is a VEVENT, the sender is probably asking the reciever to join in
a meeting. I this case, RFC2446 says that the component must specify
a start time (DTSTART) and list the reciever as an attendee (ATTENDEE). 

Libical can check these restrictions with the routine:

int icalrestriction_check(icalcomponent* comp);

This routine returns 0 if the component does not pass RFC2446 restrictions,
or if the component is malformed. The component you pass in must be
a VCALENDAR, with one or more children, like the examples in RFC2446. 

When this routine runs, it will insert new properties into the component
to indicate any errors it finds. See section 6.5.3, X-LIC-ERROR for
more information about these error properties. 

6.2.11 Converting Components to Text

To create an RFC2445 compliant text representtion of an object, use
one of the *_as_ical_string() routines:

char* icalcomponent_as_ical_string (icalcomponent* component)

char* icalproperty_as_ical_string (icalproperty* property)

char* icalparameter_as_ical_string (icalparameter* parameter)

char* icalvalue_as_ical_string (icalvalue* value)

In most cases, you will only use icalcomponent_as_ical_string (), since
it will cascade and convert all of the parameters, properties and
values that are attached to the root component.

Icalproperty_as_ical_string() will terminate each line with the RFC2445
specified line terminator ``\r\n'' However, if you compile with the
symbol ICAL_UNIX_NEWLINE defined, it will terminate lines with ``\n''

Remember that the string returned by these routines is owned by the
library, and will eventually be re-written. You should copy it if
you want to preserve it. 

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. ( & currently, not at all.
) 

6.4 \label{sec:memory}Memory Management 

Libical relies heavily on dynamic allocation for both the core objects
and for the strings used to hold values. Some of this memory the library
caller owns and must free, and some of the memory is managed by the
library. Here is a summary of the memory rules. 

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

Libical has several error handling mechanisms for the varioustypes
of programming, semantic and syntactic errors you may encounter.

6.5.1 Return values

Many library routines signal errors through their return values. All
routines that return a pointer, such as icalcomponent_new(), will
return 0 ( zero ) on a fatal error. Some routines will return a value
of enum icalerrorenum. 

6.5.2 icalerrno

Most routines will set the global error value icalerrno on errors.
This variable is an enumeration; permissable values can be found in
libical/icalerror.h. If the routine returns an enum icalerrorenum,
then the return value will be the same as icalerrno. You can use icalerror_strerror()
to get a string that describes the error

6.5.3 X-LIC-ERROR and X-LIC-INVALID-COMPONENT

The library handles semantic and syntactic errors in components by
inserting errors properties into the components. If the parser cannot
parse incoming text ( a syntactic error ) or if the icalrestriction_check()
routine indicates that the component does not meet the requirments
of RFC2446 ( a semantic error) the library will insert properties
of the type X-LIC-ERROR to describe the error. Here is an example
of the error property: 

X-LIC-ERROR;X-LIC-ERRORTYPE=INVALID_ITIP :Failed iTIP restrictions
for property DTSTART. Expected 1 instances of the property and got
0 

This error resulted from a call to icalrestriction_check(), which discovered
that the component does not have a DTSTART property, as required by
RFC2445. 

There are a few routines to manipulate error properties:

+------------------------------------+--------------------------------+
|Routine                             | Purpose                        |
+------------------------------------+--------------------------------+
+------------------------------------+--------------------------------+
|void icalrestriction_check()        | Check   a   component  against |
|                                    | RFC2446 and insert error prop­ |
|                                    | erties to indicate non compli­ |
|                                    | ance                           |
|int icalcomponent_count_errors()    | Return  the  number  of  error |
|                                    | properties  in a component     |
|void icalcomponent_strip_errors()   | Remove all error properties in |
|                                    | as component                   |
+------------------------------------+--------------------------------+
|void icalcomponent_convert_errors() | Convert some error  properties |
|                                    | into  REQUESTS-STATUS to indi­ |
|                                    | cate the inability to  process |
|                                    | the  component  as an iTIP re­ |
|                                    | quest.                         |
+------------------------------------+--------------------------------+
|                                    |                                |
+------------------------------------+--------------------------------+


The types of errors are listed in icalerror.h. They are:

ICAL_XLICERRORTYPE_COMPONENTPARSEERROR

ICAL_XLICERRORTYPE_PARAMETERVALUEPARSEERROR

ICAL_XLICERRORTYPE_PARAMETERNAMEPARSEERROR 

ICAL_XLICERRORTYPE_PROPERTYPARSEERROR

ICAL_XLICERRORTYPE_VALUEPARSEERROR

ICAL_XLICERRORTYPE_UNKVCALPROP

ICAL_XLICERRORTYPE_INVALIDITIP

The libical parser will generate the error that end in PARSEERROR when
it encounters garbage in the input steam. ICAL_XLICERRORTYPE_INVALIDITIP
is inserted by icalrestriction_check(), and ICAL_XLICERRORTYPE_UNKVCALPROP
is generated by icalvcal_convert() when it encounters a vCal property
that it cannot convert or does not know about. 

Icalcomponent_convert_errors() converts some of the error properties
ina component into REQUEST-STATUS properties that indicate a failure.
As of libical version0.18, this routine only convert *PARSEERROR errors
and it always generates a 3.x ( failure ) code. This makes it more
of a good idea than a really useful bit of code. 

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. 

Enums that identify a component, property, value or parameter end with
``_COMPONENT,'' ``_PROPERTY,'' ``_VALUE,'' or ``_PARAMETER''s

Enums that identify a parameter value have the name of the parameter
as the second word. For instance: ICAL_ROLE_REQPARTICIPANT or ICAL_PARTSTAT_ACCEPTED.

The enums for the parts of a recurarance rule and request statuses
are irregular. 

7 Useful Recipies

Iteration

Copying components. Remember that you must clone or remove an object
before putting in on another list. 

Finding compliance errors

8 Performance

Checking restrictions is computationally expensive.

9 Hacks and Bugs

There are a lot of hacks in the library -- bits of code that I am not
proud of and should propbably be changed. These are marked with the
comment string ``HACK.''