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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-6D8460DF-8B0F-5249-B236-92ABE0E67A14" xml:lang="en"><title>Memory

    13 Allocation Overview</title><shortdesc>Provides low-level functionality by which a Symbian platform process

    14 can access and manipulate memory areas.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    15 <section id="GUID-B6FA381B-2A42-4D5A-9511-DC634E8AF6C2"><title>Purpose</title> <p>Most client programs do not

    16 need to use this functionality directly. They are used by programs that are

    17 explicitly concerned about sharing memory areas between threads within a process,

    18 or between processes. </p> </section>

    19 <section id="GUID-39581657-1B46-436B-A8F2-A3F7AB525DA4"><title>Description</title> <p>The API has two key concepts: chunk

    20 and heap. </p> <p><b>Chunk</b> </p> <p>A

    21 chunk defines a contiguous region of virtual addresses. It represents memory

    22 that is addressable by running code. A chunk is the fundamental way in which

    23 Symbian platform allocates memory and makes it available to running code. </p> <p>The

    24 range of addressable memory is also referred to as <i>reserved</i> memory. </p> <p>Physical

    25 addresses representing either memory storage like RAM, or memory-mapped I/O

    26 devices, are mapped into a subset of a chunk's reserved memory. Such memory

    27 is said to be <i>committed</i>. </p> <p>When a process is created, it contains

    28 at least two chunks: </p> <ul>

    29 <li id="GUID-143BBF82-BAA5-5548-83AC-54DF2067C058"><p>a chunk that contains: </p> <ul>

    30 <li id="GUID-2A57D6BD-83FD-5C4B-B5AC-CA1BACDD5CB7"><p>the process executable's <codeph>.data</codeph> section

    31 (initialised global and writable static data) </p> </li>

    32 <li id="GUID-F6D7AB82-BEB5-5052-9723-B190F12A47ED"><p>the process executable's <codeph>.bss</codeph> section

    33 (zero filled data) </p> </li>

    34 <li id="GUID-DC6E22B6-9E12-5181-99DB-D6C2154C513A"><p>user-side stack space

    35 for all threads that run in that process. </p> </li>

    36 </ul> </li>

    37 <li id="GUID-73016FAD-8F84-596C-B048-CD1E1B05AA8B"><p>a chunk to contain the

    38 heap used by the main thread of the process. </p> </li>

    39 </ul> <p>If the process executable is loaded into RAM (i.e. it is not ROM

    40 resident), then there is another chunk to contain the code. </p> <p>On ARM

    41 processors up to and including those that support the ARMv5 architecture,

    42 the memory model used by Symbian platform is the <i>moving memory model</i>.

    43 To guarantee real-time behaviour using this model, each process is limited

    44 to a maximum of 16 chunks. This means that a program can create up to 14 additional

    45 chunks. Where the process executable is loaded into RAM, the chunk containing

    46 the code is effectively global and does not contribute to the 16 chunk per

    47 process limit. </p> <p>On ARM processors that support the ARMv6 architecture,

    48 the memory model used by Symbian platform is the <i>multiple memory model</i>.

    49 Using this model, there is no limit on the number of chunks per process. Where

    50 the process executable is loaded into RAM, the chunk containing the code is

    51 specific to the process. </p> <p> <xref href="GUID-DA41F070-0E54-3F8A-B301-39A0C6AFAB38.dita"><apiname>TFindChunk</apiname></xref> is used for

    52 finding a global chunk created by another process. </p> <p>The user-side interface

    53 to a chunk is provided by an instance of the <xref href="GUID-326A2F4D-0E99-31C0-A35D-E8BF45913F07.dita"><apiname>RChunk</apiname></xref> class. </p> <p><b>Heap</b> </p> <p>A heap is used for explicit dynamic allocation of memory.

    54 Symbian platform defines C++'s <codeph>new</codeph> operator to create objects

    55 on the current thread's heap. </p> <p>Heaps may be: </p> <ul>

    56 <li id="GUID-1ADAF0B8-BB72-5D07-BEA9-359DEFFC25E3"><p>monitored for memory

    57 leaks: this happens automatically for GUI applications </p> </li>

    58 <li id="GUID-0F5F3E02-F1E1-56D0-9865-D3DFA440DCF0"><p>shared between threads

    59 within a process </p> </li>

    60 <li id="GUID-ED409A70-ECD3-5EE9-9245-42FDEF5CCA42"><p>accessed and manipulated

    61 at the cell level </p> </li>

    62 </ul> <p>The heap interface is provided by the <xref href="GUID-9DB4A58C-6FC8-3292-A547-4C161BD188FC.dita"><apiname>RAllocator</apiname></xref> and <xref href="GUID-C5475104-B67A-3722-B11D-DBB930423492.dita"><apiname>MAllocator</apiname></xref> classes.

    63 This interface is abstract so that device manufacturers can implement heaps

    64 with different allocation algorithms. Symbian platform provides the <xref href="GUID-EFAFDD75-7E59-306A-882D-317F5564979E.dita"><apiname>RHeap</apiname></xref> class

    65 as a default heap implementation. In practice, there is no need to know about

    66 implementation details. </p> <p>When managing the current thread's heap, it

    67 is more convenient to use the equivalent functions provided by the System

    68 Static Functions API; this is the <xref href="GUID-C197C9A7-EA05-3F24-9854-542E984C612D.dita"><apiname>User</apiname></xref> class. For example, <xref href="GUID-C197C9A7-EA05-3F24-9854-542E984C612D.dita#GUID-C197C9A7-EA05-3F24-9854-542E984C612D/GUID-C035F07D-B6D9-3A21-A323-DAC89137280D"><apiname>User::AllocL()</apiname></xref>.

    69 The System Static Functions API also provides macros that conveniently wrap

    70 calls for monitoring heap usage for memory leaks. </p> </section>

    71 <section id="GUID-7A3524D6-AA39-49A2-9DBC-E3EAEA654A7B"><title>See also</title> <p> <xref href="GUID-506642C2-A14F-55F2-9377-43DDB14F4053.dita">Raw

    72 Memory Overview</xref>  </p> <p> <xref href="GUID-FF8F5D97-7D37-5F6B-84A3-C064E2FD53E0.dita">System

    73 Static Functions Overview</xref>  </p> <p> <xref href="GUID-5D4B86D3-20C4-5D87-A6C1-225018D32347.dita">Thread

    74 And Process Management Overview</xref>  </p> </section>

    75 </conbody></concept>