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<!DOCTYPE concept
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<concept id="GUID-A4799558-AF8C-5E97-9B03-7D1C04FEC243" xml:lang="en"><title>Duplicating
handles to Kernel side objects</title><shortdesc>This document describes when you should duplicate a handle in kernel
side programming.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<section id="GUID-E9B67C16-E1D1-42D4-872D-6731F94DBC87"><title>Duplicating a thread-relative handle in another thread</title> <p>A
handle in one thread (a thread-relative handle) may be duplicated in another
thread within the same process.</p> <p>Typically, thread B may want to access
a Kernel object which already exists and for which thread A, within the same
process, already holds a handle.</p> <p>For example, if thread A has a handle
on a semaphore, it is insufficient to pass a copy of the handle, an <codeph>RSemaphore</codeph>,
to thread B. The handle passed to thread B does not refer to anything known
by thread B. In order for thread B to gain a valid handle on the DLL, the
handle must be duplicated.</p> <p>Note that, in practice, real code may or
may not want to duplicate a semaphore handle, however the principle remains
valid.</p> <p>In thread B, then:</p> <codeblock id="GUID-A6110799-7A92-5EF9-9AE8-CBB532D7542B" xml:space="preserve">...
RSemaphore cs;
cs.SetHandle(h.Handle()); 
                          // ...where  h  is an RSemaphore, the
                          // handle to the DLL, and
                          // passed from thread A code.
                                  
cs.Duplicate(tA);
                          // ...where  tA  is an RThread, the handle to
                          // thread A, and passed from threadA code.
                          // Duplicate also opens the resulting
                          // handle, cs.
...
</codeblock> <p>The dotted line in the following diagram shows the effect
of this:</p> <fig id="GUID-731954F6-D85E-5940-96A4-15AC6E3AA5F5">
<image href="GUID-5BABDDEC-1D81-5847-9ADB-65DF8E404BFA_d0e205879_href.png" placement="inline"/>
</fig> <p>Note that for process-relative handles, there is no need to use <codeph>Duplicate()</codeph>;
the handle passed from thread A to thread B can be used directly.</p> </section>
<section id="GUID-83BE173E-F73D-46A8-8267-A1055E486FE3"><title>Making specific handles from generic handles</title> <p>There
are two types of generic handle, one for threads and one for processes, both
of which are created using the default constructor of the appropriate handle.</p> <codeblock id="GUID-F42A0EEB-3142-527A-9BD4-724B9969CF5D" xml:space="preserve">RThread  thisThread;  // Generic handle meaning "the current thread"
RProcess thisProcess; // Generic handle meaning "the current process"
</codeblock> <p>An important fact about generic handles is that they make
no claim on the object that they refer to, so you don't need to call <codeph>Close()</codeph> when
you get rid of them. This is not true of specific handles, which always add
a reference to the object that they refer to, preventing that object being
fully destroyed until all specific handles have been closed.</p> <p>Use Duplicate()
to make a specific handle from a generic handle; for example:</p> <codeblock id="GUID-D5588FB1-8ACE-526F-A8D7-6E2C5C089823" xml:space="preserve">RThread thisThread;                        // generic handle
TInt err=thisThread.Duplicate(thisThread);    // a specific handle
...
thisThread.Close();                        // don't forget.
</codeblock> </section>
<section id="GUID-83C281F6-1C91-4F1A-9C69-3C5BF3843DE5"><title>Promoting a thread-relative handle to a process-wide handle</title> <p>The
second parameter passed to <codeph>Duplicate()</codeph> specifies whether
the resulting handle is specific to the calling thread, or is valid for all
threads in the same process.</p> <p><codeph>Duplicate()</codeph> offers a
way of making a handle, which is specific to a thread, known to all the threads
in a process.</p> <p>This does not work for all handles but <codeph>RLibrary</codeph> is
an example of one where it can make sense to promote an <codeph>RLibrary</codeph> thread-relative
handle to a process-relative handle because this ensures that the library
will not be automatically unloaded when the original thread terminates. For
example:</p> <codeblock id="GUID-E02BE012-EE3F-53A0-A3E0-7B57E03B9D28" xml:space="preserve">...
_LIT(KMyLib,"MYLIB");
...
RLibrary threadLibrary;
err=threadLibrary.Load(KMyLib);        // Load up a DLL, this gives
                                       // a thread-specific handle.
RLibrary aDup=threadLibrary;           // Copies handle without
                                       // doing Open().
err=threadLibrary.Duplicate(RThread());// Overwrite the original.
if (err==KErrNone)
    {
       aDup.Close();                      // Close the original.
    }
...
</codeblock> </section>
</conbody></concept>