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POGCC's assembly output of an empty program on x86, win32
 

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    <p>I write empty programs to annoy the hell out of stackoverflow coders, NOT. I am just exploring the gnu toolchain.</p> <p>Now the following might be too deep for me, but to continuie the empty program saga I have started to examine the output of the C compiler, the stuff GNU as consumes.</p> <pre><code>gcc version 4.4.0 (TDM-1 mingw32) </code></pre> <p><em>test.c:</em></p> <pre><code>int main() { return 0; } </code></pre> <p><em>gcc -S test.c</em></p> <pre><code> .file "test.c" .def ___main; .scl 2; .type 32; .endef .text .globl _main .def _main; .scl 2; .type 32; .endef _main: pushl %ebp movl %esp, %ebp andl $-16, %esp call ___main movl $0, %eax leave ret </code></pre> <p>Can you explain what happens here? Here is my effort to understand it. I have used the <code>as</code> manual and my minimal x86 ASM knowledge:</p> <ul> <li><code>.file "test.c"</code> is the directive for the logical filename.</li> <li><code>.def</code>: according to the docs <em>"Begin defining debugging information for a symbol name"</em>. What is a symbol (a function name/variable?) and what kind of debugging information?</li> <li><code>.scl</code>: docs say <em>"Storage class may flag whether a symbol is static or external"</em>. Is this the same <em>static</em> and <em>external</em> I know from C? And what is that '2'?</li> <li><code>.type</code>: stores the parameter <em>"as the type attribute of a symbol table entry"</em>, I have no clue.</li> <li><code>.endef</code>: no problem.</li> <li><code>.text</code>: Now this is problematic, it seems to be something called section and I have read that its the place for code, but the docs didn't tell me too much.</li> <li><code>.globl</code> <em>"makes the symbol visible to ld."</em>, the manual is quite clear on this.</li> <li><code>_main:</code> This might be the starting address (?) for my main function</li> <li><code>pushl_</code>: A long (32bit) push, which places EBP on the stack</li> <li><code>movl</code>: 32-bit move. Pseudo-C: <code>EBP = ESP;</code></li> <li><code>andl</code>: Logical AND. Pseudo-C: <code>ESP = -16 &amp; ESP</code>, I don't really see whats the point of this.</li> <li><code>call</code>: Pushes the IP to the stack (so the called procedure can find its way back) and continues where <code>__main</code> is. (what is __main?)</li> <li><code>movl</code>: this zero must be the constant I return at the end of my code. The MOV places this zero into EAX.</li> <li><code>leave</code>: restores stack after an ENTER instruction (?). Why?</li> <li><code>ret</code>: goes back to the instruction address that is saved on the stack</li> </ul> <p><strong>Thank you for your help!</strong></p>
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    1. COGood question. :)
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    2. COSounds like an excellent exercise for a true geek.
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    3. COI found the COFF specification. This should give some references to what "32" in ".type" means etc: http://www.microsoft.com/whdc/system/platform/firmware/PECOFFdwn.mspx
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