path: root/doc/UsingLibical.md

# Using Libical

> Author: Eric Busboom <eric@civicknowledge.com>
>
> Date: January 2001

## 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 so users can
store their calendar data and arrange meetings with other users.

Libical implements [RFC5545][], [RFC5546][], [RFC7529][]; the
iCalendar extensions in [RFC6638][]; and some of [RFC6047][].

This documentation assumes that you are familiar with the iCalendar
standards RFC5545 and RFC5546. These specifications are available
at the [IETF Tools][] website:

[RFC5545]: https://tools.ietf.org/html/rfc5545
[RFC5546]: https://tools.ietf.org/html/rfc5546
[RFC7529]: https://tools.ietf.org/html/rfc7529
[RFC6638]: https://tools.ietf.org/html/rfc6638
[RFC6047]: https://tools.ietf.org/html/rfc6047
[IETF Tools]: https://tools.ietf.org/

### 1.1 The libical project

This code is under active development. If you would like to contribute
to the project, visit <https://libical.github.io/libical/>

### 1.2 License

The code and datafiles in this distribution are licensed under the
Mozilla Public License version 2.0. See <https://www.mozilla.org/MPL>
for a copy of the license. Alternately, you may use libical under
the terms of the GNU Lesser General Public License, version 2.1.
See <https://www.gnu.org/licenses/lgpl-2.1.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 additional annotated examples.

## 2 Building the Library

Libical uses autoconf to generate makefiles. It should build with no
adjustments on Linux, FreeBSD and Solaris under `gcc`. Some versions
have been successfully built on MacOS, Solaris, UnixWare, And
Tru64 UNIX without `gcc`, but you may run into problems with a particular
later version.

For a more complete guide to building the library, see the `README` file
in the distribution.

## 3 Structure

The iCalendar data model is based on four types of objects: *components*,
*properties*, *values* and *parameters*.

Properties are the fundamental unit of information in iCalendar, 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

```ical
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. Components are
delimited by `BEGIN` and `END` tags.

When a component is sent across a network, if it is un-encrypted, it
will look something like:

```ical
BEGIN:VCALENDAR
METHOD:REQUEST
PRODID: -//hacksw/handcal//NONSGML v1.0//EN
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
END:VCALENDAR
```

Note that components can be nested; this example has both a VCALENDAR
and a VEVENT component, one nested inside the other.

### 3.1 Core iCal classes

Libical is an object-based, data-oriented library. Nearly all of the
routines in the library are associated with an opaque data types and
perform some operation on that data type. Although the library does
not actually have classes, we will use those terms since the behavior
of these associations of data and routines is very similar to a class.

#### 3.1.1 Properties

Properties are represented with the `icalproperty` class and its many
"derived" classes with one "derived" class per property type in [RFC5545][].
Again, there is no actual inheritance relations, but there are clusters
of routines that make this term useful. A property is a container
for a single value and a set of parameters.

#### 3.1.2 Components

In libical, components are represented with the `icalcomponent` class.
`icalcomponent` is a container for a set of other components and properties.

#### 3.1.3 Values

Values are represented in a similar way to properties; a base class
and many "derived " classes. A value is essentially a abstract handle
on a single fundamental type, a structure or a union.

#### 3.1.4 Parameters

Parameters are represented in a similar way to properties, except that
they contain only one value.

### 3.2 Other elements of libical

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

#### 3.2.1 Enumerations and types

Libical is strongly typed, so every component, property, parameter,
and value type has an enumeration, and some have an associated structure
or union.

#### 3.2.2 The parser

The libical parser offers a variety of ways to convert [RFC5545][] text
into a libical internal component structure. The parser can parse
blocks of text as a string, or it can parse line-by-line.

#### 3.2.3 Error objects

Libical has a substantial error reporting system for both programming
errors and component usage errors.

#### 3.2.4 Memory Management

Since many of libical's interfaces return strings, the library has its
own memory management system to eliminate the need to free every string
returned from the library.  See [Memory Management](#memory).

#### 3.2.5 Storage classes

The library also offers several classes to store components to files,
memory or databases.

## 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 key 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 instance, 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 [RFC5545][] (see [section 3.6.5][RFC5545 3.6.5])
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 classes
of `VALARM`. [RFC5546][] 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`.

[RFC5545 3.6.5]: <https://tools.ietf.org/html/rfc5545#section-3.6.5>

### 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 cumbersome 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 RFC5545 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,

```ical
CATEGORIES: work, home
```

becomes in libical's internal representation

```ical
CATEGORIES: work
CATEGORIES: home
```

Oddly, [RFC5545][] 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 Using libical

### 5.1 Creating Components

There are three ways to create components in Libical:

1. creating individual objects and assembling them,
2. building entire objects in massive vargs calls,
3. parsing a text file containing iCalendar data.

#### 5.1.1 Constructor Interfaces

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

```c
icalcomponent *event;
icalproperty *prop;
icalparameter *param;
struct icaltimetype atime;

// create new VEVENT component
event = icalcomponent_new(ICAL_VEVENT_COMPONENT);

// add DTSTAMP property to the event
prop = icalproperty_new_dtstamp(atime);
icalcomponent_add_property(event, prop);

// add UID property to the event
prop = icalproperty_new_uid("guid-1.example.com");
icalcomponent_add_property(event, prop);

// add ORGANIZER (with ROLE=CHAIR) to the event
prop = icalproperty_new_organizer("mrbig@example.com");
param = icalparameter_new_role(ICAL_ROLE_CHAIR);
icalproperty_add_parameter(prop, param);
icalcomponent_add_property(event, prop);
```

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. Libical has some complex but very regular memory
handling rules. These are detailed in section [Memory Management](#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.

#### 5.1.2 varargs Constructors

There is another way to create complex components, which is arguably
more elegant, if you are not horrified by varargs. The varargs constructor
interface allows you to create intricate components in a single block
of code. Here is the previous examples in the vaargs style.

```c
icalcomponent *calendar;
struct icaltimetype atime;

calendar =
    icalcomponent_vanew(
        ICAL_VCALENDAR_COMPONENT,
        icalproperty_new_version("2.0"),
        icalproperty_new_prodid(
             "-//RDU Software//NONSGML HandCal//EN"),
        icalcomponent_vanew(
            ICAL_VEVENT_COMPONENT,
            icalproperty_new_dtstamp(atime),
            icalproperty_new_uid("guid-1.host1.com"),
            icalproperty_vanew_organizer(
                "mrbig@host.com",
                icalparameter_new_role(ICAL_ROLE_CHAIR),
                NULL),
            icalproperty_vanew_attendee(
                "employee-A@host.com",
                icalparameter_new_role(
                    ICAL_ROLE_REQPARTICIPANT),
                icalparameter_new_rsvp(1),
                icalparameter_new_cutype(ICAL_CUTYPE_GROUP),
                NULL),
            icalproperty_new_location(
               "1CP Conference Room 4350"),
            NULL),
        NULL);
```

This form is similar to the constructor form, except that the constructors
have `vanew` instead of `new` in the name. The arguments are similar
too, except that the component constructor can have a list of properties,
and the property constructor can have a list of parameters.

*Be sure to terminate every list with a `NULL` (or a *`(void 0)`*, or your code
will crash, if you are lucky*. The reason you can't use 0 itself is that
depending on what platform you are on, `sizeof(int) ≠ sizeof(void *)`.

#### 5.1.3 Parsing Text Files

The final way to create components will probably be the most common;
you can create components from [RFC5545][] compliant text. If you have
the string in memory, use

```c
icalcomponent* icalparser_parse_string(char* str);
```

If the string contains only one component, the parser will return the
component in libical form. If the string contains multiple components,
the multiple components will be returned as the children of an
`ICAL_XROOT_COMPONENT` component.

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

```c
icalparser* icalparser_new();

void icalparser_free(
    icalparser* parser);

icalparser_get_line(
    icalparser *parser,
    char* (*read_stream)(char *s, size_t size,  void *d));

icalparser_add_line(
    icalparser *parser,
    char *line);

icalparser_set_gen_data(
    icalparser *parser,
    void *data);
```

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. These routines work by specifying an adaptor routine
to get string data from a source. For example:

```c
char* read_stream(char *s, size_t size, void *d)
{
    return fgets(s, size, (FILE*)d);
}

int main(int argc, char *argv[])
{
    char *line;
    icalcomponent *component;
    icalparser *parser = icalparser_new();

    // open file (first command-line argument)
    FILE* stream = fopen(argv[1], "r");

    // associate the FILE with the parser so that read_stream
    // will have access to it
    icalparser_set_gen_data(parser, stream);

    do {
        // read the file, line-by-line, and parse the data
        line = icalparser_get_line(parser, read_stream);
        component = icalparser_add_line(parser, line);

        // if icalparser has finished parsing a component,
        // it will return it
        if (component != 0) {
            // print the parsed component
            printf("%s", icalcomponent_as_ical_string(component));
            icalparser_clean(parser);

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

            icalcomponent_free(component);
        }
    } while (line != 0);

    return 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()`.
By using a different routine for `read_stream()` or passing in different
data with `icalparser_set_gen_data()`, you can connect to any data source.

Using the same mechanism, other implementations 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 convenience routine;

```c
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
components 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.

```c
char* read_stream(char *s, size_t size, void *d)
{
    return fgets(s, size, (FILE*)d);
}

int main(int argc, char *argv[])
{
    char* line;
    icalcomponent *component;
    icalparser *parser = icalparser_new();

    // open file (first command-line argument)
    FILE* stream = fopen(argv[1], "r");

    // associate the FILE with the parser so that read_stream
    // will have access to it
    icalparser_set_gen_data(parser, stream);

    // parse the opened file
    component = icalparser_parse(parser, read_stream);

    if (component != 0) {
        // print the parsed component
        printf("%s", icalcomponent_as_ical_string(component));
        icalcomponent_free(component);
    }

    icalparser_free(parser);

    return 0;
}
```

### 5.2 Accessing Components

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

#### 5.2.1 Finding Components

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

```c
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.

#### 5.2.2 Iterating Through Components

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

```c
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

```c
icalcomponent *c;

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 will 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.

#### 5.2.3 Using Component Iterators

The iteration model in the previous section requires the component
to keep the state of the iteration. So, you could not use this model
to perform a sorting operations, since you'd need two iterators and
there is only space for one. If you ever call `icalcomponent_get_first_component()`
when an iteration is in progress, the pointer will be reset to the
beginning.

To solve this problem, there are also external iterators for components.
The routines associated with these external iterators are:

```c
icalcompiter icalcomponent_begin_component(
    icalcomponent* component,
    icalcomponent_kind kind);

icalcompiter icalcomponent_end_component(
    icalcomponent* component,
    icalcomponent_kind kind);

icalcomponent* icalcompiter_next(
    icalcompiter* i);

icalcomponent* icalcompiter_prior(
    icalcompiter* i);

icalcomponent* icalcompiter_deref(
    icalcompiter* i);
```

The `*_begin_*()` and `*_end_*()` routines return a new iterator that points
to the beginning and end of the list of subcomponent for the given
component, and the kind argument works like the kind argument for
internal iterators.

After creating an iterators, use `*_next()` and `*_prior()` to step forward
and backward through the list and get the component that the iterator
points to, and use `_deref()` to return the component that the iterator
points to without moving the iterator. All routines will return 0
when they move to point off the end of the list.

Here is an example of a loop using these routines:

```c
for(i = icalcomponent_begin_component(impl->cluster, ICAL_ANY_COMPONENT);
    icalcompiter_deref(&i)!= 0;
    icalcompiter_next(&i))
{
    icalcomponent *this = icalcompiter_deref(&i);
}
```

#### 5.2.4 Removing Components

Removing an element from a list while iterating through the list with
the internal iterators can cause problems, since you will probably
be removing the element that the internal iterator points to. The
`_remove()` routine will keep the iterator valid by moving it to the
next component, but in a normal loop, this will result in two advances
per iteration, and you will remove only every other component.  To
avoid the problem, you will need to step the iterator ahead of the
element you are going to remove, like this:

```c
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);
}
```

Another way to remove components is to rely on the side effect of
`icalcomponent_remove_component()`:
if component iterator in the parent component is pointing to the child
that will be removed, it will move the iterator to the component after
the child. The following code will exploit this behavior:

```c
icalcomponent_get_first_component(parent_comp,ICAL_VEVENT_COMPONENT);

while((c=icalcomponent_get_current_component(c)) != 0){
   if(icalcomponent_isa(c) == ICAL_VEVENT_COMPONENT){
      icalcomponent_remove_component(parent_comp,inner);
   } else {
      icalcomponent_get_next_component(parent_comp,ICAL_VEVENT_COMPONENT);
   }
}
```

#### 5.2.5 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:

```c
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);
```

For parameters:

```c
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_by_kind(
     icalproperty* prop,
     icalparameter_kind kind);
```

Note that since there should be only one parameter of each type in
a property, you will rarely need to use `icalparameter_get_next_parameter()`.

#### 5.2.6 Working with values

Values are typically part of a property, although they can exist on
their own. You can manipulate them either as part of the property
or independently.

The most common way to work with values to is to manipulate them from
the properties that contain them. This involves fewer routine calls
and intermediate variables than working with them independently, and
it is type-safe.

For each property, there are a `_get_()` and a `_set_()` routine that
accesses the internal value. For instanace, for the `UID` property, the
routines are:

```c
void icalproperty_set_uid(
    icalproperty* prop,
    const char* v);

const char* icalproperty_get_uid(
    icalproperty* prop);
```

For multi-valued properties, like `ATTACH`, the value type is usually
a struct or union that holds both possible types.

If you want to work with the underlying value object, you can get and
set it with:

```c
icalvalue* icalproperty_get_value(
    icalproperty* prop);

void icalproperty_set_value(
    icalproperty* prop,
    icalvalue* value);
```

`icalproperty_get_value()` will return a reference that you can manipulate
with other icalvalue routines. Most of the time, you will have to
know what the type of the value is. For instance, if you know that
the value is a `DATETIME` type, you can manipulate it with:

```c
struct icaltimetype icalvalue_get_datetime(
    icalvalue* value);

void icalvalue_set_datetime(
    icalvalue* value,
    struct icaltimetype v);
```

When working with an extension property or value (and `X-PROPERTY` or
a property that has the parameter `VALUE=x-name`), the value type is
always a string. To get and set the value, use:

```x
void icalproperty_set_x(
    icalproperty* prop,
    char* v);

char* icalproperty_get_x(
    icalproperty* prop);
```

All X properties have the type of `ICAL_X_PROPERTY`, so you will need
these routines to get and set the name of the property:

```c
char* icalproperty_get_x_name(
    icalproperty* prop)

void icalproperty_set_x_name(
    icalproperty* prop,
    char* name);
```

#### 5.2.7 Checking Component Validity

[RFC5546][] defines rules for what properties must exist in a component
to be used for transferring scheduling data. Most of these rules relate
to the existence of properties relative to the `METHOD` property, which
declares what operation a remote receiver 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 receiver to join in
a meeting. In this case, RFC5546 says that the component must specify
a start time (`DTSTART`) and list the receiver as an attendee
(`ATTENDEE`).

Libical can check these restrictions with the routine:

```c
int icalrestriction_check(icalcomponent* comp);
```

This routine returns 0 if the component does not pass RFC5546 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 RFC5546.

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.

5.2.8 Converting Components to Text

To create an RFC5545 compliant text representation of an object, use
one of the `*_as_ical_string()` routines:

```c
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.

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.

### 5.3 Time

#### 5.3.1 Time structure

Libical defines its own time structure for storing all dates and times.
It would have been nice to re-use the C library's struct `tm`, but that
structure does not differentiate between dates and times, and between
local time and UTC. The libical structure is:

```c
struct icaltimetype {
  int year;
  int month;
  int day;
  int hour;
  int minute;
  int second;
  int is_utc; /* 1-> time is in UTC timezone */
  int is_date; /* 1 -> interpret this as date. */
};
```

The `year`, `month`, `day`, `hour`, `minute` and `second` fields
hold the broken-out
time values. The `is_utc` field distinguishes between times in UTC and
a local time zone. The `is_date` field indicates if the time should
be interpreted only as a date. If it is a date, the hour, minute and
second fields are assumed to be zero, regardless of their actual values.

#### 5.3.2 Creating time structures

There are several ways to create a new icaltimetype structure:

```c
struct icaltimetype icaltime_from_string(
    const char* str);

struct icaltimetype icaltime_from_timet_with_zone(
    time_t v,
    int is_date,
    icaltimezone* zone);
```

`icaltime_from_string()` takes any RFC5545 compliant time string:

```c
struct icaltimetype tt = icaltime_from_string("19970101T103000");
```

`icaltime_from_timet_with_zone()` takes a `time_t` value, representing seconds past
the POSIX epoch, a flag to indicate if the time is a date, and a time zone.
Dates have an identical structure to a time, but the time portion (hours,
minutes and seconds) is always 00:00:00. Dates act differently in
sorting and comparison, and they have a different string representation
in [RFC5545][].

#### 5.3.3 Time manipulating routines

The `null` time value is used to indicate that the data in the structure
is not a valid time.

```c
struct icaltimetype icaltime_null_time(void);

int icaltime_is_null_time(struct icaltimetype t);
```

It is sensible for the broken-out time fields to contain values that
are not permitted in an ISO compliant time string. For instance, the
seconds field can hold values greater than 59, and the hours field
can hold values larger than 24. The excessive values will be rolled
over into the next larger field when the structure is normalized.

```c
struct icaltimetype icaltime_normalize(struct icaltimetype t);
```

Normalizing allows you to do arithmetic operations on time values.

```c
struct icaltimetype tt = icaltime_from_string("19970101T103000");

tt.days +=3
tt.second += 70;

tt = icaltime_normalize(tt);
```

There are several routines to get the day of the week or month, etc,
from a time structure.

```c
short icaltime_day_of_year(
    struct icaltimetype t);

struct icaltimetype icaltime_from_day_of_year(
    short doy,
    short year);

short icaltime_day_of_week(
    struct icaltimetype t);

short icaltime_start_doy_week(
    struct icaltimetype t,
    int fdow);

short icaltime_week_number(
    short day_of_month,
    short month,
    short year);

short icaltime_days_in_month(
    short month,
    short year);
```

Two routines convert time structures to and from the number of seconds
since the POSIX epoch. The `is_date` field indicates whether or not
the hour, minute and second fields should be used in the conversion.

```c
struct icaltimetype icaltime_from_timet_with_zone(
    time_t v,
    int is_date,
    icaltimezone* zone);

time_t icaltime_as_timet(
    struct icaltimetype);
```

The compare routine works exactly like `strcmp()`, but on time structures.

```c
int icaltime_compare(
    struct icaltimetype a,
    struct icaltimetype b);
```

The following routines convert between UTC and a named timezone. The
tzid field must be a timezone name from the Olsen database, such as
`America/Los_Angeles`.

The `utc_offset` routine returns the offset of the named time zone from
UTC, in seconds.

The `tt` parameter in the following routines indicates the date on which
the conversion should be made. The parameter is necessary because
timezones have many different rules for when daylight savings time
is used, and these rules can change over time. So, for a single timezone
one year may have daylight savings time on March 15, but for other
years March 15 may be standard time, and some years may have standard
time all year.

```c
int icaltime_utc_offset(
    struct icaltimetype tt,
    char* tzid);

int icaltime_local_utc_offset();

struct icaltimetype icaltime_as_utc(
    struct icaltimetype tt,
    char* tzid);

struct icaltimetype icaltime_as_zone(
    struct icaltimetype tt,
    char* tzid);

struct icaltimetype icaltime_as_local(
    struct icaltimetype tt);
```

### 5.4 Storing Objects

The libical distribution includes a separate library, libicalss, that
allows you to store iCal component data to disk in a variety of ways.

The file storage routines are organized in an inheritance hierarchy
that is rooted in icalset, with the derived class icalfileset and
icaldirset. Icalfileset stores components to a file, while icaldirset
stores components to multiple files, one per month based on DTSTAMP.
Other storages classes, for storage to a heap or a mysql database
for example, could be added in the future.

All of the icalset derived classes have the same interface:

```c
icaldirset* icaldirset_new(
    const char* path);

void icaldirset_free(
    icaldirset* store);

const char* icaldirset_path(
    icaldirset* store);

void icaldirset_mark(
    icaldirset* store);

icalerrorenum icaldirset_commit(
    icaldirset* store);

icalerrorenum icaldirset_add_component(
    icaldirset* store,
    icalcomponent* comp);

icalerrorenum icaldirset_remove_component(
    icaldirset* store,
    icalcomponent* comp);

int icaldirset_count_components(
    icaldirset* store,
    icalcomponent_kind kind);

icalerrorenum icaldirset_select(
    icaldirset* store,
    icalcomponent* gauge);

void icaldirset_clear(
    icaldirset* store);

icalcomponent* icaldirset_fetch(
    icaldirset* store,
    const char* uid);

int icaldirset_has_uid(
    icaldirset* store,
    const char* uid);

icalcomponent* icaldirset_fetch_match(
    icaldirset* set,
    icalcomponent *c);

icalerrorenum icaldirset_modify(
    icaldirset* store,
    icalcomponent *oldc,
    icalcomponent *newc);

icalcomponent* icaldirset_get_current_component(
    icaldirset* store);

icalcomponent* icaldirset_get_first_component(
    icaldirset* store);

icalcomponent* icaldirset_get_next_component(
    icaldirset* store);
```

#### 5.4.1 Creating a new set

You can create a new set from either the base class or the direved
class. From the base class use one of:

```c
icalset* icalset_new_file(const char* path);

icalset* icalset_new_dir(const char* path);

icalset* icalset_new_heap(void);

icalset* icalset_new_mysql(const char* path);
```

You can also create a new set based on the derived class, For instance,
with icalfileset:

```c
icalfileset* icalfileset_new(
    const char* path);

icalfileset* icalfileset_new_open(
    const char* path,
    int flags,
    int mode);
```

`icalset_new_file()` is identical to `icalfileset_new()`. Both routines will
open an existing file for reading and writing, or create a new file
if it does not exist. `icalfileset_new_open()` takes the same arguments
as the open() system routine and behaves in the same way.

The icalset and icalfilset objects are somewhat interchangeable -- you
can use an `icalfileset*` as an argument to any of the icalset routines.

The following examples will all use icalfileset routines; using the
other icalset derived classes will be similar.

#### 5.4.2 Adding, Finding and Removing Components

To add components to a set, use:

```c
icalerrorenum icalfileset_add_component(
    icalfileset* cluster,
    icalcomponent* child);
```

The fileset keeps an inmemory copy of the components, and this set
must be written back to the file occasionally. There are two routines
to manage this:

```c
void icalfileset_mark(icalfileset* cluster);

icalerrorenum icalfileset_commit(icalfileset* cluster);
```

`icalfileset_mark()` indicates that the in-memory components have changed.
Calling the `_add_component()` routine will call `_mark()` automatically,
but you may need to call it yourself if you have made a change to
an existing component. The `_commit()` routine writes the data base to
disk, but only if it is marked. The `_commit()` routine is called automatically
when the icalfileset is freed.

To iterate through the components in a set, use:

```c
icalcomponent* icalfileset_get_first_component(icalfileset* cluster);

icalcomponent* icalfileset_get_next_component(icalfileset* cluster);

icalcomponent* icalfileset_get_current_component (icalfileset* cluster);
```

These routines work like the corresponding routines from icalcomponent,
except that their output is filtered through a gauge. A gauge is a
test for the properties within a components; only components that
pass the test are returned. A gauge can be constructed from a MINSQL
string with:

```c
icalgauge* icalgauge_new_from_sql(const char* sql);
```

Then, you can add the gauge to the set with :

```c
icalerrorenum icalfileset_select(
    icalfileset* store,
    icalgauge* gauge);
```

Here is an example that puts all of these routines together:

```c
void test_fileset()
{
    icalfileset *fs;
    icalcomponent *c;
    int i;
    char *path = "test_fileset.ics";

    icalgauge  *g = icalgauge_new_from_sql(
        "SELECT * FROM VEVENT WHERE DTSTART > '20000103T120000Z' AND
DTSTART <= '20000106T120000Z'");

    fs = icalfileset_new(path);

    for (i = 0; i!= 10; i++){
        c = make_component(i); /* Make a new component where DTSTART
has month of i */
        icalfileset_add_component(fs,c);
    }

    icalfileset_commit(fs); /* Write to disk */
    icalfileset_select(fs,g); /* Set the gauge to filter components */

    for (c = icalfileset_get_first_component(fs);
         c != 0;
         c = icalfileset_get_next_component(fs))
    {
        struct icaltimetype t = icalcomponent_get_dtstart(c);
        printf("%s\n",icaltime_as_ctime(t));

    }

    icalfileset_free(fs);
}
```

#### 5.4.3 Other routines

There are several other routines in the icalset interface, but they
not fully implemented yet.

#### <a id="memory"></a>5.5 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 (such as `icalcomponent_new()`,
   or `icalpropert_new_clone()`), the caller gets control
   of the memory.

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

3. If the function name has "add" in it, the caller is transferring
   control of the memory to the routine, for example the function
   ` 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 and its name does NOT end in `_r`,
   libical owns the memory and will put it on a ring buffer to reclaim
   later. For example, `icalcomponent_as_ical_string()`. You better
   `strdup()` it if you want to keep it, and you don't have to delete it.

6. If the routine returns a string and its name *does* end in `_r`, the
   caller gets control of the memory and is responsible for freeing it.
   For example, `icalcomponent_as_ical_string_r()` does the same thing as
   `icalcomponent_as_ical_string()`, except you now have control of the
   string buffer it returns.

### 5.6 Error Handling

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

#### 5.6.1 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`.

5.6.2 `icalerrno`

Most routines will set the global error value `icalerrno` on errors.
This variable is an enumeration; permissible 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.
The enumerations are:

- `ICAL_BADARG_ERROR`: One of the arguments to a routine was bad.
  Typically for a null pointer.

- `ICAL_NEWFAILED_ERROR`: A `new()` or `malloc()` failed.

- `ICAL_MALFORMEDDATA_ERROR`: An input string was not in the correct format

- `ICAL_PARSE_ERROR`: The parser failed to parse an incoming component

- `ICAL_INTERNAL_ERROR`: Largely equivalent to an assert

- `ICAL_FILE_ERROR`: A file operation failed. Check errno for more detail.

- `ICAL_ALLOCATION_ERROR`: ?

- `ICAL_USAGE_ERROR`: ?

- `ICAL_NO_ERROR`: No error

- `ICAL_MULTIPLEINCLUSION_ERROR`: ?

- `ICAL_TIMEDOUT_ERROR`: For CSTP and acquiring locks

- `ICAL_UNKNOWN_ERROR`: ?

#### 5.6.3 `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 requirements
of RFC5546 (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:

```ical
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
RFC5545.

There are a few routines to manipulate error properties:

The following data is supposed to be in a table. It looks OK in LyX,
but does not format properly in output.

+-------------------------------------+---------------------------------------------------------+
| Routine                             | Purpose                                                 |
+-------------------------------------+---------------------------------------------------------+
| void icalrestriction_check()        | Check a component against RFC5546 and insert            |
+-------------------------------------+---------------------------------------------------------+
|                                     | error properties to indicate non compliance             |
+-------------------------------------+---------------------------------------------------------+
| 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 proprties to indicate the inability to  |
+-------------------------------------+---------------------------------------------------------+
|                                     | process the component as an iTIP request.               |
+-------------------------------------+---------------------------------------------------------+

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
in a component into `REQUEST-STATUS` properties that indicate a failure.
As of libical version 0.18, this routine only converts `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.

#### 5.6.4 `ICAL_ERRORS_ARE_FATAL` and `icalerror_errors_are_fatal`

If `icalerror_get_errors_are_fatal()` returns 1, then any error
condition will cause the program to abort. The abort occurs
in `icalerror_set_errno()`, and is done with an assert(0) if NDEBUG
is undefined, and with `icalerror_crash_here()` if NDEBUG is defined.
Initially, `icalerror_get_errors_are_fatal()` is 1 when `ICAL_ERRORS_ARE_FATAL`
is defined, and 0 otherwise. Since `ICAL_ERRORS_ARE_FATAL` is defined
by default, `icalerror_get_errors_are_fatal()` is also set to 1 by default.

You can change the compiled-in `ICAL_ERRORS_ARE_FATAL` behavior at runtime
by calling `icalerror_set_errors_are_fatal(0)` (i.e, errors are not fatal)
or `icalerror_set_errors_are_fatal(1)` (i.e, errors are fatal).

### 5.7 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 RFC5545 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`"

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 recurrence rule and request statuses
are irregular.

## 6 Hacks and Bugs

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

## 7 Library Reference

### 7.1 Manipulating struct icaltimetype

#### 7.1.1 Struct icaltimetype

```c
struct icaltimetype

{
    int year;
    int month;
    int day;
    int hour;
    int minute;
    int second;
    int is_utc;
    int is_date;
    const char* zone;
};
```