path: root/docs/reference/gdk/tmpl/properties.sgml

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Properties and Atoms

<!-- ##### SECTION Short_Description ##### -->
Functions to manipulate properties on windows

<!-- ##### SECTION Long_Description ##### -->
<para>
Each window under X can have any number of associated
<firstterm>properties</firstterm> attached to it.
Properties are arbitrary chunks of data identified by
<firstterm>atom</firstterm>s. (An <firstterm>atom</firstterm>
is a numeric index into a string table on the X server. They are used
to transfer strings efficiently between clients without
having to transfer the entire string.) A property
has an associated type, which is also identified
using an atom.
</para>
<para>
A property has an associated <firstterm>format</firstterm>,
an integer describing how many bits are in each unit
of data inside the property. It must be 8, 16, or 32.
When data is transferred between the server and client,
if they are of different endianesses it will be byteswapped
as necessary according to the format of the property.
Note that on the client side, properties of format 32
will be stored with one unit per <emphasis>long</emphasis>,
even if a long integer has more than 32 bits on the platform.
(This decision was apparently made for Xlib to maintain
compatibility with programs that assumed longs were 32
bits, at the expense of programs that knew better.)
</para>
<para>
The functions in this section are used to add, remove
and change properties on windows, to convert atoms
to and from strings and to manipulate some types of
data commonly stored in X window properties.
</para>

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<para>

</para>

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<!-- ##### SECTION Image ##### -->


<!-- ##### STRUCT GdkAtom ##### -->
<para>
An opaque type representing a string as an index into a table
of strings on the X server.
</para>


<!-- ##### MACRO GDK_ATOM_TO_POINTER ##### -->
<para>
Converts a #GdkAtom into a pointer type. 
</para>

@atom: a #GdkAtom.


<!-- ##### MACRO GDK_POINTER_TO_ATOM ##### -->
<para>
Extracts a #GdkAtom from a pointer. The #GdkAtom must have been
stored in the pointer with GDK_ATOM_TO_POINTER().
</para>

@ptr: a pointer containing a #GdkAtom.


<!-- ##### MACRO GDK_NONE ##### -->
<para>
A null value for #GdkAtom, used in a similar way as <literal>None</literal>
in the Xlib API.
</para>



<!-- ##### FUNCTION gdk_text_property_to_text_list ##### -->
<para>
Converts a text string from the encoding as it is stored in
a property into an array of strings in the encoding of
the current local. (The elements of the array represent
the nul-separated elements of the original text string.)
</para>

@encoding: an atom representing the encoding. The most common
           values for this are <literal>STRING</literal>,
           or <literal>COMPOUND_TEXT</literal>. This is
           value used as the type for the property.
@format: the format of the property.
@text: the text data.
@length: the length of the property, in items.
@list: location to store a terminated array of strings
       in the encoding of the current locale. This
       array should be freed using gdk_free_text_list().
@Returns: the number of strings stored in @list, or 0,
          if the conversion failed.


<!-- ##### FUNCTION gdk_text_property_to_text_list_for_display ##### -->
<para>

</para>

@display: 
@encoding: 
@format: 
@text: 
@length: 
@list: 
@Returns: 


<!-- ##### FUNCTION gdk_free_text_list ##### -->
<para>
Frees the array of strings created by
gdk_text_property_to_text_list().
</para>

@list: the value stored in the @list parameter by
       a call to gdk_text_property_to_text_list().


<!-- ##### FUNCTION gdk_text_property_to_utf8_list ##### -->
<para>

</para>

@encoding: 
@format: 
@text: 
@length: 
@list: 
@Returns: 


<!-- ##### FUNCTION gdk_text_property_to_utf8_list_for_display ##### -->
<para>

</para>

@display: 
@encoding: 
@format: 
@text: 
@length: 
@list: 
@Returns: 


<!-- ##### FUNCTION gdk_string_to_compound_text ##### -->
<para>
Converts a string from the encoding of the current locale 
into a form suitable for storing in a window property.
</para>

@str: a nul-terminated string.
@encoding: location to store the encoding atom (to be used as the type for the property).
@format: location to store the format for the property.
@ctext: location to store newly allocated data for the property.
@length: location to store the length of @ctext in items.
@Returns: 0 upon sucess, non-zero upon failure.


<!-- ##### FUNCTION gdk_string_to_compound_text_for_display ##### -->
<para>

</para>

@display: 
@str: 
@encoding: 
@format: 
@ctext: 
@length: 
@Returns: 


<!-- ##### FUNCTION gdk_free_compound_text ##### -->
<para>
Frees the data returned from gdk_string_to_compound_text().
</para>

@ctext: The pointer stored in @ctext from a call to gdk_string_to_compound_text().


<!-- ##### FUNCTION gdk_utf8_to_string_target ##### -->
<para>

</para>

@str: 
@Returns: 


<!-- ##### FUNCTION gdk_utf8_to_compound_text ##### -->
<para>

</para>

@str: 
@encoding: 
@format: 
@ctext: 
@length: 
@Returns: 


<!-- ##### FUNCTION gdk_utf8_to_compound_text_for_display ##### -->
<para>

</para>

@display: 
@str: 
@encoding: 
@format: 
@ctext: 
@length: 
@Returns: 


<!-- ##### FUNCTION gdk_atom_intern ##### -->
<para>
Finds or creates an atom corresponding to a given string.
</para>

@atom_name: a string.
@only_if_exists: if %TRUE, GDK is allowed to not create a new atom, but
                 just return %GDK_NONE if the requested atom doesn't already
                 exists. Currently, the flag is ignored, since checking the 
                 existance of an atom is as expensive as creating it.
@Returns: the atom corresponding to @atom_name.


<!-- ##### FUNCTION gdk_atom_intern_static_string ##### -->
<para>

</para>

@atom_name: 
@Returns: 


<!-- ##### FUNCTION gdk_atom_name ##### -->
<para>
Determines the string corresponding to an atom.
</para>

@atom: a #GdkAtom.
@Returns: a newly-allocated string containing the string
          corresponding to @atom. When you are done
          with the return value, you should free it 
          using g_free().


<!-- ##### FUNCTION gdk_property_get ##### -->
<para>
Retrieves a portion of the contents of a property. If the
property does not exist, then the function returns %FALSE,
and %GDK_NONE will be stored in @actual_property_type.
</para>
<note>
<para>
The XGetWindowProperty() function that gdk_property_get()
uses has a very confusing and complicated set of semantics.  
Unfortunately, gdk_property_get() makes the situation
worse instead of better (the semantics should be considered
undefined), and also prints warnings to stderr in cases where it
should return a useful error to the program. You are advised to use 
XGetWindowProperty() directly until a replacement function for 
gdk_property_get()
is provided. 
</para>
</note>

@window: a #GdkWindow.
@property: the property to retrieve.
@type: the desired property type, or %GDK_NONE, if any type of data
       is acceptable. If this does not match the actual
       type, then @actual_format and @actual_length will
       be filled in, a warning will be printed to stderr
       and no data will be returned.
@offset: the offset into the property at which to begin
         retrieving data, in 4 byte units.
@length: the length of the data to retrieve in bytes.  Data is
         considered to be retrieved in 4 byte chunks, so @length 
         will be rounded up to the next highest 4 byte boundary 
         (so be careful not to pass a value that might overflow 
          when rounded up).
@pdelete: if %TRUE, delete the property after retrieving the
          data.
@actual_property_type: location to store the actual type of 
                       the property.
@actual_format: location to store the actual return format of the
                data; either 8, 16 or 32 bits.
@actual_length: location to store the length of the retrieved data, in
                bytes.  Data returned in the 32 bit format is stored
                in a long variable, so the actual number of 32 bit
                elements should be be calculated via
                @actual_length/sizeof(glong) to ensure portability to
                64 bit systems.
@data: location to store a pointer to the data. The retrieved
       data should be freed with g_free() when you are finished
       using it.
@Returns: %TRUE if data was successfully received and stored
          in @data, otherwise %FALSE.


<!-- ##### FUNCTION gdk_property_change ##### -->
<para>
Changes the contents of a property on a window.
</para>

@window: a #GdkWindow.
@property: the property to change.
@type: the new type for the property. If @mode is
       %GDK_PROP_MODE_PREPEND or %GDK_PROP_MODE_APPEND, then this 
       must match the existing type or an error will occur.
@format: the new format for the property. If @mode is
         %GDK_PROP_MODE_PREPEND or %GDK_PROP_MODE_APPEND, then this 
         must match the existing format or an error will occur.
@mode: a value describing how the new data is to be combined
       with the current data.
@data: the data
       (a <literal>guchar *</literal>
        <literal>gushort *</literal>, or 
        <literal>gulong *</literal>, depending on @format), cast to a 
        <literal>guchar *</literal>.
@nelements: the number of elements of size determined by the format,
            contained in @data.


<!-- ##### ENUM GdkPropMode ##### -->
<para>
Describes how existing data is combined with new data when
using gdk_property_change().
</para>

@GDK_PROP_MODE_REPLACE: the new data replaces the existing data.
@GDK_PROP_MODE_PREPEND: the new data is prepended to the existing data.
@GDK_PROP_MODE_APPEND: the new data is appended to the existing data.

<!-- ##### FUNCTION gdk_property_delete ##### -->
<para>
Deletes a property from a window.
</para>

@window: a #GdkWindow.
@property: the property to delete.