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<concept xml:lang="en" id="GUID-547CF71C-6A62-57C0-A9BE-E76B4286C6D6"><title>Threads And Processes Overview</title><prolog><metadata><keywords/></metadata></prolog><conbody><section id="GUID-D894C473-2BCF-4988-BF07-435C7A779BAF"><title>Purpose</title> <p>Provides access to processes, threads, and thread-local storage.</p> <p>Processes are usually only manipulated directly by system programs. Threads can be directly manipulated by multi-threaded programs. Note however that the active objects framework provided by the Asynchronous Services API is usually preferred to multi-threading in Symbian OS programming.</p> </section> <section id="GUID-90EBDA3E-A7E1-4460-9A63-CDEC922AC58A"><title>Description</title> <p>The API has three key concepts: process, thread, and thread-local storage. </p> <p><b>Process</b> </p> <p>A program running in Symbian OS is a process, which can contains one or more conceptually concurrent threads of execution. Each user process has its own private address space. A process can create another process. </p> <p>Processes are Kernel objects and so are accessed by user programs through handles. </p> <p>The process handle interface is provided by <xref href="GUID-9DD1EA2B-DC59-315C-8E9C-CE6D9461B695.dita"><apiname>RProcess</apiname></xref>.</p> <p><xref href="GUID-71AF2BC3-5CCD-3561-8807-7B8CDC1A64E9.dita"><apiname>TFindProcess</apiname></xref> is used for finding another process.</p> <p><b>Thread</b> </p> <p>A thread is the unit of execution within a process. The scheduling of threads is pre-emptive, i.e. a currently executing thread may be suspended at any time to allow another thread to run.</p> <p>Note the following properties of threads:</p> <ul><li id="GUID-7FA506FD-3F36-51AB-87C6-0A4415ED4257"><p>every process has a primary thread that is created when the process is initialised </p> </li> <li id="GUID-3C2107B8-2979-5835-8985-AC289C75AAA6"><p>a thread can create, suspend, resume, panic and kill other threads, subject to protection</p> </li> <li id="GUID-D4298E05-F3C7-59E9-9E05-EA43BAE51D93"><p>data can be passed between threads whether those threads are in different processes or the same process</p> </li> <li id="GUID-8EF7BA10-3BBA-568B-A0BE-00A6F4B58D6C"><p>each thread has a priority: the scheduler runs the highest-priority thread that is ready to run </p> </li> </ul> <p>Threads are Kernel objects and so are accessed by user programs through handles. </p> <p>The thread handle interface is provided by <xref href="GUID-B0E661BC-4058-3256-B9C3-5A4FD52F6DE5.dita"><apiname>RThread</apiname></xref>. </p> <p><xref href="GUID-29F27759-CC53-36DF-AE92-623F061D6C96.dita"><apiname>TFindThread</apiname></xref> is used for finding another thread.</p> <p>The System Static Functions API <xref href="GUID-C197C9A7-EA05-3F24-9854-542E984C612D.dita"><apiname>User</apiname></xref> class provides a number of functions that operate on the current thread.</p> <p><b>Thread-local storage</b> </p> <p>Thread-local storage in Symbian OS supports one machine word of static data per DLL per thread. The scope of the word is the thread and is only accessible to DLL code. DLL code running on behalf of one thread does not see the same word when running on behalf of another thread.</p> <p>The thread-local storage interface is provided by <xref href="GUID-6085A635-1664-323E-997A-EA11F75FBAC3.dita"><apiname>Dll</apiname></xref>.</p> </section> <section id="GUID-33582F5A-3ADA-434C-BDD2-503AA667BAE0"><title>See also</title> <p><xref href="GUID-1294F540-FDA7-5050-BAFB-3C9888491B98.dita">Asynchronous Services Overview</xref> </p> <p><xref href="GUID-FF8F5D97-7D37-5F6B-84A3-C064E2FD53E0.dita">System Static Functions Overview</xref> </p> </section> </conbody></concept>