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<concept id="GUID-D9EA2B10-B789-5CC1-9BD0-23B4FA7E0821" xml:lang="en"><title>Properties</title><shortdesc>This topic explains the properties of publish and subscribe.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>A property has the two attributes: identity and type.</p>

<section id="GUID-A5CBB14F-D37F-41C5-8D00-37C3D07708F6"><title>Identity</title> <p>A property is identified by a 64-bit integer

made up of two 32-bit parts: the category and the key.</p> <p>A property belongs

to a category, and a category is identified by a UID.</p> <p>A key is a 32-bit

value that identifies a specific property within a category. The meaning applied

to the key depends on the kind of enumeration scheme set up for the category.

At its simplest, a key can be an index value. It can also be another UID,

if the category is designed to be generally extensible.</p> </section>

<section id="GUID-0351DF1A-8DEE-49D6-AD7D-6575E9DF5B56"><title>Type</title> <p>A property can be:</p> <ul>

<li id="GUID-7C7CE552-0608-502F-8A2D-87071C4157AE"><p>a single 32-bit value</p> </li>

<li id="GUID-E96FE3B3-C66F-5CE9-9C0F-A04BBB20C7BE"><p>a contiguous set of

bytes, referred to as a byte-array, whose length can vary from 0 to 512 bytes</p> </li>

<li id="GUID-4F8B6257-4410-5F40-A211-B14239CEC64F"><p>Unicode text.</p> </li>

</ul> <p>From the viewpoint of the implementation, Unicode text is treated

as a byte-array; the detail is hidden by the API.</p> <p>Once defined, a property

value can change, but the property type cannot. Byte-array type properties

can also change length provided the length does not exceed the value <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-57E64054-610A-31D1-AD7F-E2F9F9FC1DCB"><apiname>RProperty::KMaxPropertySize</apiname></xref>.

The   limit on size of property ensures some limit on RAM usage.</p><p>  

    limit on size of property ensures some limit on RAM usage.</p><p>The API

allows byte-array and Unicode text type properties to be  pre-allocated when

they are defined. This means that the time taken to set the   values is bounded.

However, if the length of these property types subsequently increases, then

memory allocation may take place, and no guarantees can be made  on the time

taken to set them.</p><p>Note that the<xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-F8DE443B-B208-353C-A98E-AA52C4FE6530"><apiname>RProperty::ELargeByteArray</apiname></xref> property

type can never provide a real-time guarantee.</p><p>Properties and their values

are defined, set and retrieved using  <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita"><apiname>RProperty</apiname></xref> .  </p></section>