atl under the hood - part 4.mht

大家知道wtl是window UI库

      to set the stack frame. </P><PRE lang=3Dasm>	<SPAN =
class=3Dcpp-keyword>push</SPAN>	ebp
	<SPAN class=3Dcpp-keyword>mov</SPAN>	ebp, esp
	<SPAN class=3Dcpp-keyword>sub</SPAN>	esp, <SPAN =
class=3Dcpp-literal>12</SPAN>					<SPAN class=3Dcpp-comment>; =
0000000cH</SPAN>
</PRE>
      <P>This code is called prolog code. And in the same way the code =
inserted=20
      at the end of function is called Epilog code. In the same program =
the=20
      Epilog code generated by the compiler is </P><PRE lang=3Dasm>	=
<SPAN class=3Dcpp-keyword>mov</SPAN>	esp, ebp
	<SPAN class=3Dcpp-keyword>pop</SPAN>	ebp
	<SPAN class=3Dcpp-keyword>ret</SPAN>	<SPAN class=3Dcpp-literal>0</SPAN>
</PRE>
      <P>Now take a look at the function with naked attribute =
</P><B>Program=20
      64</B> <PRE><SPAN class=3Dcpp-keyword>extern</SPAN> <SPAN =
class=3Dcpp-string>"C"</SPAN> <SPAN class=3Dcpp-keyword>void</SPAN> =
_declspec(naked) fun() {
	<SPAN class=3Dcpp-keyword>_asm</SPAN> ret
}

<SPAN class=3Dcpp-keyword>int</SPAN> main() {

	fun();
	<SPAN class=3Dcpp-keyword>return</SPAN> <SPAN =
class=3Dcpp-literal>0</SPAN>;
}
</PRE>
      <P>The code of the function fun, which is generated by the =
compiler, is=20
      something like this. </P><PRE lang=3Dasm>	_asm <SPAN =
class=3Dcpp-keyword>ret</SPAN>
</PRE>
      <P>Means there is no prolog and epilog code in this function. In =
fact,=20
      there are rules of naked function, i.e. you can't declare =
automatic=20
      variable in naked function, because for this compiler have to =
generate the=20
      code for you and in naked function compiler wont generate any code =
for=20
      you. In fact you have to write the ret statement yourself =
otherwise=20
      program will be crash. You even can't write return statement in =
the naked=20
      function. Why? Because when you return something from the =
function, then=20
      compiler puts its value in <CODE>eax</CODE> register. So it means =
compiler=20
      have to generate the code for your return statement. Let's take a =
look at=20
      this simple program to understand the working of return value from =
the=20
      function. </P><B>Program 64</B> <PRE><SPAN =
class=3Dcpp-preprocessor>#include &lt;cstdio&gt;</SPAN>

<SPAN class=3Dcpp-keyword>extern</SPAN> <SPAN =
class=3Dcpp-string>"C"</SPAN> <SPAN class=3Dcpp-keyword>int</SPAN> =
sum(<SPAN class=3Dcpp-keyword>int</SPAN> a, <SPAN =
class=3Dcpp-keyword>int</SPAN> b) {
	<SPAN class=3Dcpp-keyword>return</SPAN> a + b;
}

<SPAN class=3Dcpp-keyword>int</SPAN> main() {

	<SPAN class=3Dcpp-keyword>int</SPAN> iRetVal;
	sum(<SPAN class=3Dcpp-literal>3</SPAN>, <SPAN =
class=3Dcpp-literal>7</SPAN>);
	<SPAN class=3Dcpp-keyword>_asm</SPAN> mov iRetVal, eax
	printf(<SPAN class=3Dcpp-string>"%d\n"</SPAN>, iRetVal);
	<SPAN class=3Dcpp-keyword>return</SPAN> <SPAN =
class=3Dcpp-literal>0</SPAN>;
}
</PRE>
      <P>The output of this program is "10". Here we haven't directly =
use the=20
      return value of the function, instead of this we copy the value of =

      <CODE>eax</CODE> in the variable just after calling the function. =
</P>
      <P>Now write our whole function naked with prolog and epilog code =
which=20
      return the value of two variables after return it. </P><B>Program =
65</B> <PRE><SPAN class=3Dcpp-preprocessor>#include =
&lt;cstdio&gt;</SPAN>

<SPAN class=3Dcpp-keyword>extern</SPAN> <SPAN =
class=3Dcpp-string>"C"</SPAN> <SPAN class=3Dcpp-keyword>int</SPAN> =
_declspec(naked) sum(<SPAN class=3Dcpp-keyword>int</SPAN> a, <SPAN =
class=3Dcpp-keyword>int</SPAN> b) {

	<SPAN class=3Dcpp-comment>// prolog code</SPAN>
	<SPAN class=3Dcpp-keyword>_asm</SPAN> push ebp
	<SPAN class=3Dcpp-keyword>_asm</SPAN> mov ebp, esp

	<SPAN class=3Dcpp-comment>// code for add two variables and =
return</SPAN>
	<SPAN class=3Dcpp-keyword>_asm</SPAN> mov eax, dword ptr [ebp + <SPAN =
class=3Dcpp-literal>8</SPAN>]
	<SPAN class=3Dcpp-keyword>_asm</SPAN> add eax, dword ptr [ebp + <SPAN =
class=3Dcpp-literal>12</SPAN>]
=09
	<SPAN class=3Dcpp-comment>// epilog code</SPAN>
	<SPAN class=3Dcpp-keyword>_asm</SPAN> pop ebp
	<SPAN class=3Dcpp-keyword>_asm</SPAN> ret
}

<SPAN class=3Dcpp-keyword>int</SPAN> main() {

	<SPAN class=3Dcpp-keyword>int</SPAN> iRetVal;
	sum(<SPAN class=3Dcpp-literal>3</SPAN>, <SPAN =
class=3Dcpp-literal>7</SPAN>);
	<SPAN class=3Dcpp-keyword>_asm</SPAN> mov iRetVal, eax
	printf(<SPAN class=3Dcpp-string>"%d\n"</SPAN>, iRetVal);
	<SPAN class=3Dcpp-keyword>return</SPAN> <SPAN =
class=3Dcpp-literal>0</SPAN>;
}
</PRE>
      <P>The output of this program is "10" i.e. the sum of two =
parameter 3 and=20
      7. </P>
      <P>This attributed is used in ATLBASE.H file to implement the =
member of=20
      <CODE>_QIThunk</CODE> structure. This is structure is used to =
debug=20
      reference counting the ATL program when =
<CODE>_ATL_DEBUG_INTERFACES</CODE>=20
      are defined. </P>
      <P>I hope to explore some other mysterious of ATL in next article. =
</P><!-- Article Ends --></DIV>
      <H2>About Zeeshan Amjad</H2>
      <TABLE width=3D"100%" border=3D0>
        <TBODY>
        <TR vAlign=3Dtop>
          <TD class=3DsmallText noWrap><BR></TD>
          <TD class=3DsmallText width=3D"100%">Software Engineer at =
SoftPak=20
            Financial Systems<BR><A=20
            =
href=3D"http://www.softpak.com/">http://www.softpak.com/</A><BR><BR>And=20
            sometimes teaching Object Oriented programming and C++ in =
University=20
            of Karachi as a Visiting Faculty Member.<BR><A=20
            href=3D"http://www.csku.edu.pk/">http://www.csku.edu.pk/</A>
            <P class=3DsmallText>Click <A=20
            =
href=3D"http://www.codeproject.com/script/profile/whos_who.asp?vt=3Darts&=
amp;id=3D5890">here</A>=20
            to view Zeeshan Amjad's online =
profile.</P></TD></TR></TBODY></TABLE><BR>
      <H2>Other popular ATL articles:</H2>
      <UL>
        <LI><A=20
        =
href=3D"http://www.codeproject.com/atl/shellfoldertree.asp">ShellFolderTr=
ee</A><BR><SPAN=20
        class=3Dsmalltext>Mimicking and extending the shell=92s =
folder-tree control=20
        functionality</SPAN>
        <LI><A href=3D"http://www.codeproject.com/atl/atlserver.asp">ATL =
Server -=20
        Web Application/Web Service</A><BR><SPAN class=3Dsmalltext>Web=20
        Application/Web Service development using ATL Server =
classes</SPAN>
        <LI><A =
href=3D"http://www.codeproject.com/atl/newinatl7.asp">What's new in=20
        ATL7</A><BR><SPAN class=3Dsmalltext>Overview of new classes in =
ATL7</SPAN>
        <LI><A=20
        =
href=3D"http://www.codeproject.com/atl/ietoolbartutorial.asp">Internet=20
        Explorer Toolbar (Deskband) Tutorial</A><BR><SPAN =
class=3Dsmalltext>A=20
        tutorial on Using RBDeskband and CWindowImpl ATL Object Wizards =
to=20
        create an Internet Explorer(IE) Toolbar.</SPAN></LI></UL>
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