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<concept id="GUID-5BDE6462-E6CA-5738-A587-C7D875574789" xml:lang="en"><title>Static

Data</title><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>Symbian developers used to PC operating systems, such as Windows, may be

used to using writeable static data (typically meaning global variables) in

DLLs. On the Symbian platform, this is possible, but not recommended, because

it is expensive in memory, and has limited support in the Symbian platform

Emulator. </p>

<p>This page describes writeable static data (WSD), the alternatives to using

WSD, and the costs and issues associated with its use. </p>

<section><title>Global writeable static data</title> <p>Global writeable static

data (WSD) is any per-process variable which exists for the lifetime of the

process. In practice, this means any globally scoped data: data that is declared

outside of a function, struct, or class, and function scoped static variables. </p> <codeblock id="GUID-C7F90E06-286B-5E64-B94A-7683AA0DA0FB" xml:space="preserve">TBufC&lt;20&gt; fileName; //WSD 

void SetFileName()

 {

 static Tint iCount; //WSD ...

 }</codeblock> <p>Sometimes writeable static data appears in a non-obvious

way. It is not always the case that <codeph>const</codeph> global variables

are read-only data rather than global writeable static data. This is true

for <codeph>const</codeph> objects with trivial constructors, such as integers.

However if a <codeph>const</codeph> class has a non-trivial constructor, the <codeph>const</codeph> object

will require a real variable and must be stored as WSD. For example: </p> <codeblock id="GUID-897D2E68-5AF6-5C69-A5AA-276A9FB89DEC" xml:space="preserve">const TInt myVariable=…; //OK – truly const

const TPtrC KSomeConstPtr=...; //NOT OK – non trivial constructor 

const TRgb KSomeConstCol=...; //NOT OK – non trivial constructor</codeblock> </section>

<section><title>Support for global writeable static data on Symbian</title> <p>The

Symbian platform supports global writeable static data in EXEs on all versions

and handsets. </p> <p>Versions of Symbian OS based on the EKA2 kernel (8.1b

and later) support WSD in DLLs on target hardware. Versions 8.1a and earlier,

based on the EKA1 kernel, do not support global WSD in DLLs. </p> </section>

<section><title>Clean up of global writeable static data</title> <p>Your program

must make sure that global writeable static data that the program allocates

is cleaned up. </p> <p>The clean up rules are as follows: </p> <ul>

<li id="GUID-1D25BA8E-1A8E-5502-8FD9-7F74DBB39EF4"><p>Your program must clean

up WSD objects that are defined in the process EXE or its statically-loaded

DLLs. When a process exits, the Symbian platform does not automatically call

the destructors of these WSD objects. </p> </li>

<li id="GUID-7F1EB6D6-2EAD-5318-A066-DC9128A48EAF"><p>Your program does not

need to clean up WSD objects defined in DLLs that the process has dynamically

loaded using <xref href="GUID-25327159-83D6-3507-B187-09EA4BB3727F.dita"><apiname>RLibrary</apiname></xref>. The Symbian platform automatically

calls destructors of these objects. </p> </li>

</ul> </section>

<section><title>Alternatives to using global writeable static data</title> <p>Native

Symbian platform C++ code rarely uses WSD. </p> <p>However, code ported from

other operating systems may contain large amounts of static data. For example,

code written in the C programming language often makes use of WSD as the "glue"

between C function calls. </p> <p>In EKA1, WSD is not supported, so there

is no choice but to use the alternative mechanisms provided by the Symbian

platform to port such code. Even in EKA2 where global data is supported, Symbian

recommends that it only be used as a last resort. </p> <p>The following sections

describe the alternatives that can be used to port code that makes use of

global writeable static data. </p> <p><b>Use

thread-local storage (TLS)</b> </p> <p>Thread Local Storage (TLS) is a single

per-thread word that can be used to simulate global writeable static data. </p> <p>All

the static data in the DLL is grouped into a single struct or class. On creation

of the thread, an instance of the thread is allocated on the heap and a pointer

to this data is saved to TLS (using <codeph>Dll::SetTls()</codeph>). On destruction

of the thread, the data is destroyed. Throughout the DLL the code references

the TLS data (using <codeph>Dll::Tls()</codeph>) rather than the original

global writeable static data. </p> <p><b>Wrap

in a server</b> </p> <p>The Symbian platform supports writeable global static

data in EXEs. A common porting strategy is therefore to wrap the code in a

Symbian server (which is an EXE), and expose its API as a client interface. </p> <p><b>Move global variables into your classes</b> </p> <p>With relatively small

amounts of code, it may be possible to move most global data inside classes. </p> </section>

<section><title>Enabling global writeable static data</title> <p>In order

to enable global writeable static data, simply add a statement <codeph>epocallowdlldata</codeph> (case

insensitive) to the project's MMP file: </p> <codeblock id="GUID-0E673C33-569E-5F7B-9E51-55C2FC233D70" xml:space="preserve">TARGET my.dll 

TARGETTYPE dll 

EPOCALLOWDLLDATA 

… </codeblock> </section>

<section><title>Costs and limitations</title> <p><b>Emulator

only allows a DLL with WSD to load into a single process</b> </p> <p>The Symbian

platform EKA2 Emulator only allows a DLL with WSD to be loaded into a single

process. </p> <p>This is a very serious restriction. If you have a shared

DLL with WSD, then the second process that attempts to load it in the emulator

will fail with <codeph>KErrNotSupported</codeph>. </p> <p><b>Emulator allows WSD by default</b> </p> <p>The Emulator will allow WSD

in DLLs even if <codeph>epocallowdlldata</codeph> is not declared in the mmp

file. However the data will be truly global: there will be one copy for the

entire emulator, rather than one copy for each emulated process. The only

restriction is that if the data's initialisers call any Symbian platform kernel

functions (i.e. executive calls), then the emulator will fault. </p> <p><b>RAM usage for WSD data chunk </b> </p> <p>When a process loads its first

DLL containing WSD, it creates a single chunk to store the data. The data

for subsequent WSD-enabled DLLs is loaded into the same chunk. </p> <p>The

data chunk will occupy at least one 4K RAM page (the smallest possible RAM

allocation), irrespective of how much static data is required. Any memory

not used for WSD will be wasted. Since the memory is per-process, the memory

wastage on the machine is: </p> <codeblock id="GUID-005105EF-A09A-5FF7-8056-7316AFCE1894" xml:space="preserve">(4Kb - WSD Bytes) × number-client-processes</codeblock> <p>It is very easy for a developer to add a few words of WSD to their DLL thinking

that's all the memory that they are using. However, the cost is actually 4K

for every process if a DLL with WSD has not already been loaded into the process.

If for example the DLL is used by 4 processes, that's potentially an "invisible"

cost of 16K. </p> <p><b>Chunks

are a finite resource</b> </p> <p>EKA2 has a hard coded limit of 16 chunks

per process; a limit that is required to ensure real-time behaviour. Every

process loading WSD-enabled DLLs uses a chunk to hold the data, reducing the

number of chunks available for other uses. </p> <p><b>ARM architecture 4 and 5 specific costs and limitations</b> </p> <p>There

are other significant costs that apply only to DLLs that link against "fixed

processes". Fixed processes are a feature of ARM v4 or v5 architecture only;

the following behaviour does not apply to devices based on the ARMv6 architecture. </p> <p> <b>Case

1: Non Execute-in-place DLLs</b>  </p> <p>For non-execute-in-place (non XIP)

DLLs, an additional code chunk is required for every fixed process which links

against the DLL. </p> <p>So imagine a 20Kb DLL (with a few bytes of WSD) that

is loaded into 4 normal "non-fixed" processes, and 2 fixed processes. The

(static) memory consumed is: </p> <ul>

<li id="GUID-57F9041D-240C-5B75-BC0C-EFA66AB411B6"><p>Code chunk shared by

all moving processes = 20 Kb </p> </li>

<li id="GUID-1FCC1ED4-0A60-5983-AF5A-2F7C8622BC80"><p>Code chunk for each

fixed process loading the DLL = 40 Kb </p> </li>

<li id="GUID-6CD0DBDD-E406-5F7D-9207-8E55B4043779"><p>Data chunk for each

process loading the DLL = 6×4Kb = 24 Kb </p> </li>

</ul> <p>So to allow a few bytes of WSD there is a 64Kb increase in consumed

memory. Note that the 20Kb code chunk shared by all processes is consumed

whether or not WSD is enabled. </p> <p> <b>Case 2: Execute-in-place DLLs</b>  </p> <p>For

XIP DLLs, there is no additional RAM cost other than the size of the WSD itself,

rounded up to the next multiple of 4KB. However: </p> <ul>

<li id="GUID-8B1EDEA4-8082-5DB1-87A8-E7B0FE5BB802"><p>An XIP DLL can be loaded

by any non-fixed process, OR it may be loaded by a single fixed process (and

therefore cannot be loaded by any other processes whatsoever) </p> </li>

<li id="GUID-BA2D82B1-A036-5281-95EB-A46A7976605B"><p>The ROM build fails

if a DLL with static data links to a fixed process <i>and</i> any other process. </p> </li>

</ul> <p> <b>A few specific DLLs cannot have WSD</b>  </p> <p>DLLs that are

required to initialise the file server cannot have WSD, e.g. HAL.DLL, EUSER.DLL,

EFSRV.DLL. </p> </section>

<section><title>Frequently asked questions</title> <p><b>How

does Symbian's WSD implementation work ?</b> </p> <p> <b>Case 1. For moving

processes:</b>  </p> <p>The Symbian platform supports a moving memory model,

in which the data for a process is moved into a fixed virtual address space

(the "run section") when the process is active, and then back into the processes

"home" section when another process is active: </p> <ul>

<li id="GUID-26E98921-9094-5260-BA99-2F7FA4F7D6A7"><p>The data of processes

in the "home section" is uniquely addressed both in terms of physical RAM

pages, and in the MMU virtual address space. The data is protected from other

processes, except for the Kernel. </p> </li>

<li id="GUID-BBE7C17A-6FF7-5329-91C5-DEAF88A61B69"><p>The data of processes

in the run section occupies the same virtual address space as all other processes

when they run (the MMU moves the physical RAM pages into the appropriate virtual

address space). </p> </li>

</ul> <p>When a DLL with WSD is loaded (or at ROM build time), its code is

fixed to point to static data at specific addresses. In order to ensure that

only a single copy of the DLL code is required, Symbian's moving-process WSD

implementation ensures that the virtual addresses of static data is the same

across every running process. </p> <p>The way this works is: </p> <ol id="GUID-66A040C2-7B0F-56E0-A634-501D3E133A12">

<li id="GUID-964A78E1-D86E-50D0-8AA0-D3D9346A43E7"><p>A specific address space

is reserved for a per-process static data chunk. This chunk is used to hold

all the static data for all the DLLs loaded into the process. </p> </li>

<li id="GUID-0A528F28-55B2-5B8B-A337-8B2377241CF0"><p>At ROM build time the

kernel reserves specific addresses within the static data chunk for all the

WSD in all the ROM loaded DLLs. The addresses for ROM based DLLs are reserved

from the top of the static data chunk address space to the bottom. Note that

static data addresses for RAM based DLLs are reserved when the DLL is first

loaded into any process. In this case, addresses are reserved from the bottom

of the static data chunk address space. </p> </li>

<li id="GUID-1898478C-2E24-53B4-BDEB-765FEEDC539C"><p>When the first DLL with

WSD is loaded into a process, a static data chunk is created to hold the static

data for all DLLs that are loaded into the process. </p> </li>

<li id="GUID-7A88C9EB-31F9-559E-8DBD-221848CA2A46"><p>Any global static data

in the DLL is written to its specific reserved addresses. Note that addresses

are reserved for that particular static data across all processes; if the

DLL is loaded into another process, any static data will get the same virtual

address. </p> </li>

</ol> <p> <b>Case 2. For fixed processes:</b>  </p> <p>A fixed process is

one in which the process data does not move; code is run on process data stored

in the process "home section". </p> <p>Since the static data for every fixed

process is uniquely addressed, and a DLL can only point to a single address

for its data, the implication is that a separate copy of the DLL code is required

for every fixed process that loads the DLL. </p> <p>The restrictions listed

above for ARM architecture 4 and 5 directly result. </p> <ul>

<li id="GUID-125EDB37-4540-5EDB-9C19-6F92A307627F"><p>For XIP based devices

the DLL code chunk address is fixed at ROM build time, and there can only

be one copy of the DLL code. Therefore the DLL code can address the data in

either a single fixed process or the virtual address used by all moveable

addresses. </p> </li>

<li id="GUID-BC66EA5A-252D-5E37-80F6-BEC82C03DABA"><p>For non-XIP based devices

the DLL is run from RAM, and the loader is able to fix-up the address that

a DLL expects its data at load time (rather than ROM build time). Therefore

in this case the loader creates a separate copy of DLL code for each fixed

process that loads the DLL and a single copy shared by all moving processes. </p> </li>

</ul> <p>Notes: Fixed processes are not supported or required on ARM v6 architectures.

This discussion only applies to devices based on ARMv4 or v5. On ARM architecture

6 each DLL with WSD has a reserved address, similar to the ARMv5 moving process

case. However there is no 'home section' and memory is not relocated between

low and high addresses on a context switch. Instead, each process uses its

own set of page tables for the bottom half of virtual address space. </p> <p><b>Why does the EKA2 Emulator only allow a DLL with WSD to load into a single

process?</b> </p> <p>The Symbian platform permits a DLL with global data to

be loaded into only one Symbian platform process on the emulator. This limitation

is a result of the way the Emulator is implemented on top of the Windows process

model. </p> <p>On EKA2, separate Symbian platform processes are emulated within

a single Windows process. To preserve Symbian platform semantics there should

be one copy of the global data for each emulated process. However, this is

not possible since a DLL on the emulator is just a Windows DLL; Windows gives

it a single copy of any global data it needs when it is loaded. </p> <p><b>What happens if epocallowdlldata isn't declared for a DLL with WSD?</b> </p> <p> <b>Case

1: On the EKA2 Emulator:</b>  </p> <p>Most constant data should be treated

by the compiler as read-only rather than writeable static data (the exception

is when the const data has a non-trivial constructor, so a real variable is

required during initialisation). </p> <p>Unfortunately, different compilers

sometimes treat const data as WSD. For example, CodeWarrior puts it in writeable

data and initialises it at run time. MSVC generally puts it into read-only

data, but occasionally puts it into writeable data. </p> <p>As most DLLs have

const data, this means that the compilers have "accidentally" created WSD

in almost every DLL. Symbian cannot therefore rigorously enforce the "single

process can load a DLL with WSD" rule, as the Emulator would not work. </p> <p>On

the EKA2 Emulator, the workaround Symbian have implemented is to recognise

two types of DLL global data: </p> <ul>

<li id="GUID-6AD2E89F-6361-5F1F-8EC1-A203801BCC8F"><p>'Deliberate' global

data is where the programmer specifies that they want DLL global data (using

the <codeph>epocallowdlldata</codeph> keyword in the MMP file. In this case

any global data in the DLL is assumed to be deliberate, and the "DLL loaded

into one-emulated-process" rule applies. </p> </li>

<li id="GUID-225E44BE-434E-5C2A-8503-318E31B17EF0"><p>'Accidental' global

data is the data introduced by the compiler with no encouragement from the

programmer. If <codeph>epocallowdlldata</codeph> is absent, global data is

assumed to be accidental and the rule does not apply. Note that the global

data includes both const and non-const static data; there is no way to tell

the compilers to only apply it to non-const data - if we could do that then

we could force correct handling of const static data. </p> </li>

</ul> <p>In order to prevent abuse of this workaround, there are restrictions

on what can be done with accidental global data; specifically, the Emulator

will fault if any of the global data's initialisers attempt to call the Symbian

platform kernel functions (i.e. executive calls). </p> <p>Note that there

is only one copy of the global data. Therefore it is possible for two processes

to write to the same 'accidental' global data (causing undefined behaviour).

The "DLL loaded into one-emulated-process" rule prevents this being a problem

for deliberate global data. </p> <p> <b>Case 2: On the EKA1 Emulator</b>  </p> <p>The

EKA1 emulator has no concept of separate Symbian platform processes. Global

data is allowed on the Emulator as there is only one copy of any global data. </p> <p>Symbian

developers often have problems when porting to real hardware, which does not

support this type of data. This is discussed in the following section. </p> <p> <b>Case

3: On native target builds</b>  </p> <p>On either EKA1 or EKA2, target compilers

will fail the build with an error indicating that the code has initialised

or un-initialised data. </p> <p>The error message does not specify the symbol(s)

that are causing the problem. The <filepath>.map</filepath> files output by

the linker can however be helpful in finding the problem symbol. The RVCT

map files provide a list of global symbols and the object files in which they

occur. FAQ-0329 on the <xref href="http://developer.symbian.org/wiki/index.php/Symbian_C++_Knowledgebase_Q&amp;As#How_can_I_find_the_.22uninitialised_data.22_in_my_DLL.3F" scope="external">DevNet knowledgebase</xref> works through an example of using

a map file using the GCC compiler used on older Symbian platform releases. </p> <p><b>Can Kernel DLLs have WSD ?</b> </p> <p>Yes, WSD is supported for kernel

DLLs in both EKA1 and EKA2 (through alternative mechanisms to those described

here). </p> <p>Of course, kernel DLLs are guaranteed to be loaded into only

one process, so the per-process multiplication of RAM usage does not apply.

EKA2 will work correctly with global data in any kernel DLL. However EKA1

does not call constructors for global C++ objects in kernel extensions or

device drivers, and does not call destructors for global C++ objects in device

drivers at driver unload time. </p> </section>

</conbody></concept>