C Library Example question?

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rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 23 Apr 2010, 14:58

Hey!

I've just recently looked at Vid's flibc example, and everything makes a lot of sense. Very Happy

The only thing I'm trying to comprehend now is when you're making function calls, why do you add numbers to the stack pointer after you make the call?

For example...

Code:

        ; write welcome text and prompt text
        ; puts("Hello\nPlease type a number: ")
        push    _hello
        call    puts
        add     esp, 4

Is it really necessary?

Thanks!
[/code]

23 Apr 2010, 14:58

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 23 Apr 2010, 15:01

Upon further inspection of the code, I notice you add four when you only have one parameter, and eight when you have two... Does this mean that you have to change the stack pointer yourself after you push values on the stack, so not to overwrite anything? And I thought that the stack pointer automatically changed when something was pushed on the stack...

23 Apr 2010, 15:01

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 23 Apr 2010, 15:17

Also, I'm playing around with some code I wrote based off of this (a simple "Hello World!" program), and I'm wondering... does it matter which order the sections are in? The reason I ask is because it wasn't working when I had the data sections before the code section (.data and .idata).

Also, I noticed when I tried to jmp to the imports directly, rather than making a call to a wrapper it didn't work... Why is that?

Sorry, I'm just trying to learn. Smile

23 Apr 2010, 15:17

bitshifter

Joined: 04 Dec 2007
Posts: 796
Location: Massachusetts, USA

bitshifter 23 Apr 2010, 15:34

From the looks of your code it seems to be _cdecl calling convention.

23 Apr 2010, 15:34

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 23 Apr 2010, 15:49

What's the "_cdecl calling convention"? Sorry... I'm still very new at this.

23 Apr 2010, 15:49

a115433

Joined: 05 Mar 2010
Posts: 144

a115433 23 Apr 2010, 16:10

cdecl = arguments removed by caller.

23 Apr 2010, 16:10

ManOfSteel

Joined: 02 Feb 2005
Posts: 1154

ManOfSteel 23 Apr 2010, 16:57

__cdecl

23 Apr 2010, 16:57

Tyler

Joined: 19 Nov 2009
Posts: 1216
Location: NC, USA

Tyler 23 Apr 2010, 21:47

The add esp,* is to remove the parameters from the stack. You're pushing a 4 byte pointer onto the stack as an arg and removing it after the function you call returns.

23 Apr 2010, 21:47

edfed

Joined: 20 Feb 2006
Posts: 4353
Location: Now

edfed 23 Apr 2010, 23:01

note: add esp,4 is equivalent to pop reg32, but without reg overwriting.

something fun would be to do pop dword[ss:esp], it will do exactlly the same thing, but slower because of the memory transfert from [ss:esp] to [ss:esp]...
like a move eax,eax...

23 Apr 2010, 23:01

Tyler

Joined: 19 Nov 2009
Posts: 1216
Location: NC, USA

Tyler 24 Apr 2010, 03:28

edfed wrote:

something fun would be to do pop dword[ss:esp]

A very novel idea... Although slower, it seems like it would be a lot less error prone, because of how clear it is as to what you're doing and why.

24 Apr 2010, 03:28

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 24 Apr 2010, 09:25

edfed wrote:

something fun would be to do pop dword[ss:esp], it will do exactlly the same thing, but slower because of the memory transfert from [ss:esp] to [ss:esp]...
like a move eax,eax...

You have to be careful with such claims: here is what Intel SDM wrote about pop (underline is mine):

Quote:

If the ESP register is used as a base register for addressing a destination operand in memory, the POP instruction computes the effective address of the operand after it increments the ESP register.

add esp, 4 trashes EFLAGS, though they're not used in cdecl calling convention. lea esp, [esp+4] doesn't.

24 Apr 2010, 09:25

peter

Joined: 09 May 2006
Posts: 63

peter 26 Apr 2010, 01:44

On calling conventions:

Raymond Chen's blog post (with links to MSDN)
Wikipedia
Agner Fog's manual

26 Apr 2010, 01:44

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