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<concept id="GUID-BB39DE14-B314-59CB-A8EC-BBD2A5C1BCD9" xml:lang="en"><title>GUI App: Porting
an Engine for Use in a Symbian Application</title><shortdesc>This section describes the steps to port an engine for use in a
Symbian application.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>The application uses the FreeBSD <codeph>cksum</codeph> utility as its
engine. The checksum example is an application with a user interface and
an engine written in C.</p>
<p>The first stage in porting this program is to split the project into
the engine (GUIAppEng) and the application (GUIApp).</p>
<section id="SECTION_1B097EC0E8EC43BE82B88BE0D04DC5F8"><title>The engine</title> <p>The
engine is written in C. At its core is the <codeph>crc()</codeph> function.
This function accepts a file descriptor and returns the checksum and the
file size.</p> <p>The engine's project specification file (<filepath>GUIAppEng.mmp</filepath>)
is as follows:</p><codeblock xml:space="preserve">TARGET GUIAppEng.dll
TARGETTYPE dll
UID 0x1000008d 0x01000a02
VENDORID 0x70000001
SOURCEPATH .
SOURCE crc.c
SYSTEMINCLUDE \epoc32\include\libc \epoc32\include
LIBRARY estlib.lib euser.lib
#if defined(WINS)
deffile GuiAppEngWINS.def
#else if defined(ARM)
deffile GuiAppEngARM.def
#endif
nostrictdef</codeblock> <p> The output file is <filepath>GUIAppEng.dll</filepath>,
whose import library will be included by the UI. </p> <p>
The first UID specified (<codeph>0x1000008d</codeph>) is the standard UID
for an interface DLL. The second UID (<codeph>0x01100a02</codeph>) is
unique to the <filepath>GUIApp</filepath> project. </p> <p>
Splitting the project into engine and UI means that the definition of
<codeph>crc()</codeph> in <filepath>crc.c</filepath> must be marked <codeph>EXPORT_C</codeph>
because this function will be exported from the engine's DLL. </p>
<p> For more information on DLLs, see <xref href="GUID-4A56B285-790E-5171-88F3-8C40B2AA9699.dita">DLLs</xref>.
</p> </section>
<section id="GUID-52000200-5CFD-4FE1-B80E-0A5F4C7E050D"> <title>Porting checksum - the GUI app</title> <p> The implementation
of checksum (<filepath>GUIApp.app</filepath>) limits the user to the selection
of a single file at a time and uses the default algorithm <codeph>crc()</codeph>,
defined in <filepath>crc.c</filepath>, to produce a 32-bit value. </p>
<p> The user interface provides two main menu commands; <codeph>Calculate
checksum</codeph> (invokes a file selection dialog) and <codeph>View
checksums</codeph>. When a file is selected and <codeph>OK</codeph> is
pressed, the selected file name is retrieved and opened as shown in the
following code: </p> <codeblock xml:space="preserve">const TUint16* fn = iFileName->PtrZ();
int fd = wopen( ( const wchar_t* )fn, O_RDONLY, 0 ); </codeblock> <p>
This code fragment is taken from <filepath>examples\stdlib\GUIApp.cpp.</filepath>
</p> <p> <codeph>open()</codeph> returns a file descriptor which
the engine's <codeph>crc()</codeph> function uses to identify the file.
The checksum is calculated (unless an error occurred in attempting to
open or read the file), and is displayed. The file is closed. </p>
<p> The file name and checksum are appended to an array, the contents
of which may be viewed by selecting <codeph>View checksums</codeph>.
</p> </section>
<section id="SECTION_7A8BA355A42C45C3A14EDF1756AB1126"> <title>Linking
to STDLIB - The project specification</title> <p> The application
program includes several STDLIB header files, located in <filepath>\epoc32\include\libc\</filepath>.
At link time, the program includes <filepath>estlib.lib</filepath> and
the engine DLL's <filepath>.lib</filepath> (<filepath>GUIAppEng.lib</filepath>).
Unlike the previous examples, this application does not link to <filepath>ecrt0.lib</filepath>.
In this application there is no <codeph>main()</codeph> and the Symbian
platform provides its own <codeph>E32Main()</codeph>. </p> <p>
For information on Symbian applications, their project specification,
and resource files, see <xref href="GUID-81835322-5EF7-5839-9DC1-9A3FD396BD36.dita">How
to build GUI applications</xref>. </p> </section>
<section id="GUID-39A0C47A-38DC-4487-981D-D8BBC50D6DCF"> <title>Some potential issues</title> <ul id="UL_632D231449CE4474996ACD148CB9754D">
<li> <p><b>Removing writeable static</b></p> <p> The PETRAN stage
of building may report a message similar to the following: </p>
<codeblock xml:space="preserve">WARNING: Dll 'SLSUMENG[0x01000a02].DLL' has initialised data.</codeblock>
<p> This warning, which is not reported when building for the Emulator,
indicates that the DLL contains non-const static data. This is not allowed
in ARM builds. If it is not obvious where the problem occurs, the associated
<filepath>.map</filepath> file (<filepath>epoc32\release\<target>\urel\<dllname>.map</filepath>)
contains information which can help to track down the source file involved.
A search for <codeph>from *(.bss)</codeph> (to find uninitialised data)
or <codeph>from *(.data)</codeph> (to find initialised data) in <filepath>GUIAPPEng.map</filepath> will
reveal the file in which the problem occurs, and the names of the offending
variables, although static variables will not be named. </p> </li>
<li id="LI_BAD062FD16EC4CC8A11F13D0ACEE8378"><p><b>Include file clashes</b></p> <p>
In C++ source files which use STDLIB routines, the Symbian platform
C++ include files must be included before any of the STDLIB files. Failure
to do this will result in the following warning: </p> <codeblock xml:space="preserve">'NULL' : macro redefinition"</codeblock> </li>
<li> <p><b>Mixing C and C++</b></p> <p> C and C++ have different views
about the names of functions. If you refer to a C function from C++, ensure
that its prototype is declared as <codeph>extern "C"</codeph>. If there
are several such function declarations, it may be more convenient to enclose
them within the following: </p> <codeblock xml:space="preserve">#ifdef __cplusplus
extern "C"
{
#endif
...
#ifdef __cplusplus
}
#endif</codeblock> <p> See for example <filepath>examples\stdlib\GUIApp\extern.h</filepath>.
</p> </li>
<li id="LI_68EE8B3B25594B48864D5183BA8155AB"> <p><b>Stack usage</b></p> <p>
Some programs produce the following error: </p> <codeblock xml:space="preserve">unresolved external symbol __chkstk</codeblock>
<p> unless the amount of stack they use is reduced. Symbian threads
have only 8k stack by default. </p> </li>
<li> <p><b>Resource cleanup</b></p> <p> The Symbian platform
has a requirement that all resources which were allocated by an application
must be cleaned up by the time the program terminates. On the Emulator,
in debug builds, failure to do this will cause a panic from the <codeph>__UHEAP_MARKEND</codeph> macro.
</p> <p> Because the data allocated in the thread-local storage
for STDLIB's DLL (the <codeph>_reent</codeph> structure) is not automatically
cleaned up when the environment is destroyed, it must be cleaned up by
the user of STDLIB. </p> <p> The function to achieve this is <codeph>CloseSTDLIB()</codeph>.
To use this function, file <filepath>epoc32\include\libc\sys\reent.h</filepath> must
be included in the project. Call <codeph>CloseSTDLIB()</codeph> after
the point at which it is known that code in STDLIB's DLL will no longer
be called and its thread-local storage no longer needed. </p>
<p> For example, see the destructor for <codeph>CExampleDocument</codeph> in
<filepath>examples\stdlib\GUIApp\GUIApp.cpp</filepath>. </p> </li>
</ul> </section>
</conbody><related-links>
<link href="GUID-4A56B285-790E-5171-88F3-8C40B2AA9699.dita"><linktext>DLLs</linktext>
</link>
<link href="GUID-81835322-5EF7-5839-9DC1-9A3FD396BD36.dita"><linktext>How to build
GUI applications</linktext></link>
</related-links></concept>