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<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-E651C7A7-D6EB-533E-A97A-360D089DE7A5" xml:lang="en"><title>Inter-thread
data transfer</title><shortdesc>Describes how data is transferred between threads.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>The client-server message protocol supports the passing of a 32-bit request
code and four 32-bit parameters from client to server and returning a 32-bit
result from the server to the client. The parameters may be interpreted as
plain integers or pointers; pointer types may be untyped (<codeph>TAny*</codeph>)
or descriptor types (<codeph>TDesC8*</codeph> and <codeph>TDesC16*</codeph>)
that the server can use to access the client’s address space. The request
code, parameters, and the parameter types are packaged into a <xref href="GUID-4AD02F14-1142-372F-9D11-224595932034.dita"><apiname>TIpcArgs</apiname></xref> object.
The parameter types are stored in the kernel side message object so that the
kernel can check that subsequent operations requested by the server using
the message are: </p>
<ul>
<li id="GUID-1F925301-81D0-5F84-ABEE-BB4DB576DBC1"><p>correct; for example,
checking that the source and target descriptors are either both 8-bit or both
16-bit descriptors </p> </li>
<li id="GUID-EE7CCC5F-0013-55EA-B473-763785F5CD90"><p>permitted by the client;
for example, checking that when the server tries to write to a client descriptor,
that the client descriptor is <xref href="GUID-49D4E917-57EA-39AE-8941-144AA8AC2584.dita"><apiname>TDes</apiname></xref> -derived (modifiable),
rather than <xref href="GUID-52D07F46-2162-380C-A775-C3BB335C42F5.dita"><apiname>TDesC</apiname></xref> -derived (non-modifiable) </p> </li>
</ul>
<p><note>NOTE:</note> although you can pass untyped pointers, it is not possible
to directly access memory in another thread’s address space using an arbitrary
pointer as this is inherently insecure. The ability to pass a pointer between
a client and server is therefore only of value when the client and server
are within the same process. In this case, the use of a pointer is obviously
not limited to pointing to a descriptor, but may also be used to point to
an arbitrary data structure containing information to be shared between the
client and server. </p>
<p>It is important to note that a server may not run until some arbitrary
time after a client issues a request. Any descriptor containing data to be
passed to the server must be guaranteed to exist until the request completes.
For this reason, any such descriptor must <i>not</i> live on the program stack.
Typically, such a descriptor would be a component of an object which is allocated
on the heap. </p>
<p>The following diagram illustrates the general idea. In this case, there
are three parameters, one of which is an integer, and the other two being
pointers to descriptors. </p>
<fig id="GUID-C3369B01-2A0D-5AB2-973B-386FDBBB6B86">
<image href="GUID-2105B5F0-2D00-5ECA-8859-A8A432423327_d0e237328_href.png" placement="inline"/>
<p>Inter-thread data transfer</p>
</fig>
<section id="GUID-EE636E39-A171-484D-AAF4-7996D04C8EEC"><title>See also</title> <p> <xref href="GUID-79BAF19D-F003-5468-9C01-6E918B06C36D.dita">Descriptor
concepts</xref>. </p> </section>
</conbody></concept>