Programming Forums > Application Development > Assembly

rsnd · Jul 5th, 2005, 12:15 AM

[PHP]int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{

char * msg = "128\0";
char * msg2 = "Equal\0";
char * msg3 = "NotEqual\0";
MessageBox(0,NULL,NULL,0);
__asm{
mov eax, 0x04;
push eax;
push msg;

call str2Int;

mov ebx, eax;
cmp ebx,128;

je B;

mov ecx, 0x00000000;
push ecx;
push msg3;
push msg;
push ecx;

call MessageBoxA

jmp C;
B:
mov ecx, 0x00000000;
push ecx;
push msg2;
push msg;
push ecx;

call MessageBoxA

C:
}
MessageBox(0,NULL,NULL,0);
return 0;
}[/PHP]
Gives no error messages but it stuffs up when i run it!!!

DaWei · Jul 5th, 2005, 10:08 AM

I'm compelled to say that your ability to ask a question in a sensible way sucks. If you want good answers, you need to ask good questions. In this milieu, information is key. You don't show much. For instance, you don't show "str2Int", so one can't tell what calling convention it uses. The fact that you don't clean up the stack after calling it indicates __stdcall, but the fact that your program runs off into the weeds and barfs on its shoes (invalid memory access) may be a sign that you neglected to consider the issue.

rsnd · Jul 5th, 2005, 9:04 PM

hmmm...I tent to assume im too predictable. Isn't it supposed to use C calling convension by default?
OK I, using VC++6.0...here is the complete code:

(Toggle Plain Text)

#include <windows.h>
//#include "nConsole.h"

int str2Int(char*, int);


int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
	
	char * msg = "128\0";
	char * msg2 = "Equal\0";
	char * msg3 = "NotEqual\0";
	MessageBox(0,NULL,NULL,0);
	__asm{
		mov eax, 0x04;
		push eax;
		push msg;

		call str2Int;

		mov ebx, eax;
		cmp ebx,128;


		je B;

		mov ecx, 0x00000000;
		push ecx;
		push msg3;
		push msg;
		push ecx;

		call MessageBoxA

		jmp C;
B:
		mov ecx, 0x00000000;
		push ecx;
		push msg2;
		push msg;
		push ecx;

		call MessageBoxA

C:
	}
	MessageBox(0,NULL,NULL,0);
	return 0;
}


int str2Int(char * str, int len){
    int x, y;
    byte z;
    y = 0;
    for(x=0;x<len;x++){
        *(str+x) -= 0x30;
    }
    z = *(str+x);
    y = z;
    for(x =0;x<len;x++){
        z = *(str+x);
        y *= 10;
        y+= z;
    }
    for(x=0;x<len;x++){
        *(str+x) += 0x30;    
    }
    return y;
}

Thanks

DaWei · Jul 5th, 2005, 11:16 PM

Your compiler uses the __cdecl calling convention by default. However, your original post didn't indicate whether str2Int was written in C/C++ and could be presumed to be __cdecl, or whether you had written it in assembler, also, which of course has no "calling convention."

The __cdecl convention is that the caller cleans up the stack (removes the parameters that were placed there prior to the call). The advantage to this convention is that one can use a variable number of arguments (ala "printf"); the caller knows how many he pushed, so he knows how many to remove. The called function has no idea. The disadvantage to the method is increased footprint as each caller has to perform cleanup instead of it being incorporated in the called function.

Since you are calling the function from assembly language, it is your responsibility to clean up the stack. You aren't.

(Toggle Plain Text)

        push eax;        // this is the length (args pushed right to left)
        push msg;       // this is the pointer to the message

        call str2Int;     // make the call

        mov ebx, eax;  // eax is the return value, which you are testing
        cmp ebx,128;

// You have nothing to get the parameters off the stack before you proceed
// You need to add a couple of pops

This issue is going to give you problems when you reach the end of the program because the stack pointer is not going to be pointing to the correct return address when you reach the end of main. You will get problems sooner if you add more to the code. THIS IS NOT YOUR CURRENT PROBLEM, HOWEVER.

When you declare a string as follows:

char * msg = "128\0";

the compiler sets aside static memory for the string and stack-memory for a pointer ("msg") and puts the appropriate value in the pointer. It's strictly a favor by the compiler, at compile time, for as you know, one cannot assign a C-style string to an array using the "=" operator. (You don't need to overtly add the terminating zero when you express it as you have; the compiler does THAT for you automatically, also).

It is not universally true that the string is "const" (write-protected), but in your implementation, using your compiler, it is. You are trying to modify the string via the pointer you passed as a parameter. That is a memory access violation. The quick cure is to put the string on the stack:

char msg = "128";

"msg", since it refers to an array, does double duty as a pointer under the circumstances, so you can make the call as you're presently doing. That double duty occassionally will get one in trouble because of the false assumption that an array and a pointer are the same thing, but in this case, you're fine. You may modify it at will; just remember to observe the length requirement which you pass, so as not to trundle off the end into verboten territory.

rsnd · Jul 6th, 2005, 6:33 AM

Thanks DaWei. But what was the problem? The str2int function seems fine to me...I think the problem is with the message Box call!!
this crashes:
[php]#include <windows.h>

int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
char msg []= "128";
char msg2 []= "Equal";
char msg3 []= "NotEqual";
MessageBox(0,NULL,NULL,0);
__asm{

mov ecx, 0x00;
push ecx;
push msg3;
push msg;
push ecx;

call MessageBoxA

pop eax;
pop eax;
pop eax;
pop eax;
}
MessageBox(0,NULL,NULL,0);
return 0;
}[/php]

omega_red · Jul 6th, 2005, 6:47 AM

The problem is in a way you call MessageBox:

(Toggle Plain Text)

call MessageBoxA

At assembly level, MessageBoxA is a label to an offset in the Import Table of your exe. It points to a DWORD that contains real address of the API. You need to dereference that address:

(Toggle Plain Text)

call dword ptr [MessageBoxA]

It should work fine.

DaWei · Jul 6th, 2005, 6:54 AM

As I said, this instruction,

(Toggle Plain Text)

        *(str+x) -= 0x30;

is attempting to modify read-only memory. You're not getting far enough for the message box call to be a problem, at this point.

DaWei · Jul 6th, 2005, 7:38 AM

Incidentally, Omega is correct about your need for an indirect call, when you get that far. Big K, I tried to edit this into the above post, but there's no "EDIT" button. ???

Ooble · Jul 6th, 2005, 8:35 AM

The Edit button is disabled half an hour after you post.

rsnd · Jul 12th, 2005, 5:38 AM

Hmmm...ok...that didnot work!!!
[php]#include <windows.h>

int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
char msg []= "128";
char msg2 []= "Equal";
char msg3 []= "NotEqual";
MessageBox(0,NULL,NULL,0);
__asm{

mov ecx, 0x00;
push ecx;
push msg3;
push msg;
push ecx;

call dword ptr [MessageBoxA];

pop eax;
pop eax;
pop eax;
pop eax;
}
MessageBox(0,NULL,NULL,0);
return 0;
}
[/php]

Jul 5th, 2005, 12:15 AM	#1
rsnd Hobbyist Programmer Join Date: Jun 2005 Location: Helltown Posts: 162 Rep Power: 4	Whats wrong with this inline code? [PHP]int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) { char * msg = "128\0"; char * msg2 = "Equal\0"; char * msg3 = "NotEqual\0"; MessageBox(0,NULL,NULL,0); __asm{ mov eax, 0x04; push eax; push msg; call str2Int; mov ebx, eax; cmp ebx,128; je B; mov ecx, 0x00000000; push ecx; push msg3; push msg; push ecx; call MessageBoxA jmp C; B: mov ecx, 0x00000000; push ecx; push msg2; push msg; push ecx; call MessageBoxA C: } MessageBox(0,NULL,NULL,0); return 0; }[/PHP] Gives no error messages but it stuffs up when i run it!!! __________________ Spread your wings and fly! Chicken!

Jul 5th, 2005, 10:08 AM	#2
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	I'm compelled to say that your ability to ask a question in a sensible way sucks. If you want good answers, you need to ask good questions. In this milieu, information is key. You don't show much. For instance, you don't show "str2Int", so one can't tell what calling convention it uses. The fact that you don't clean up the stack after calling it indicates __stdcall, but the fact that your program runs off into the weeds and barfs on its shoes (invalid memory access) may be a sign that you neglected to consider the issue. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Jul 5th, 2005, 9:04 PM	#3
rsnd Hobbyist Programmer Join Date: Jun 2005 Location: Helltown Posts: 162 Rep Power: 4	hmmm...I tent to assume im too predictable. Isn't it supposed to use C calling convension by default? OK I, using VC++6.0...here is the complete code: (Toggle Plain Text) #include <windows.h> //#include "nConsole.h" int str2Int(char, int); int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) { char msg = "128\0"; char * msg2 = "Equal\0"; char * msg3 = "NotEqual\0"; MessageBox(0,NULL,NULL,0); __asm{ mov eax, 0x04; push eax; push msg; call str2Int; mov ebx, eax; cmp ebx,128; je B; mov ecx, 0x00000000; push ecx; push msg3; push msg; push ecx; call MessageBoxA jmp C; B: mov ecx, 0x00000000; push ecx; push msg2; push msg; push ecx; call MessageBoxA C: } MessageBox(0,NULL,NULL,0); return 0; } int str2Int(char * str, int len){ int x, y; byte z; y = 0; for(x=0;x<len;x++){ (str+x) -= 0x30; } z = (str+x); y = z; for(x =0;x<len;x++){ z = (str+x); y = 10; y+= z; } for(x=0;x<len;x++){ (str+x) += 0x30; } return y; } #include <windows.h> //#include "nConsole.h" int str2Int(char, int); int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) { char * msg = "128\0"; char * msg2 = "Equal\0"; char * msg3 = "NotEqual\0"; MessageBox(0,NULL,NULL,0); __asm{ mov eax, 0x04; push eax; push msg; call str2Int; mov ebx, eax; cmp ebx,128; je B; mov ecx, 0x00000000; push ecx; push msg3; push msg; push ecx; call MessageBoxA jmp C; B: mov ecx, 0x00000000; push ecx; push msg2; push msg; push ecx; call MessageBoxA C: } MessageBox(0,NULL,NULL,0); return 0; } int str2Int(char * str, int len){ int x, y; byte z; y = 0; for(x=0;x<len;x++){ (str+x) -= 0x30; } z = (str+x); y = z; for(x =0;x<len;x++){ z = (str+x); y = 10; y+= z; } for(x=0;x<len;x++){ *(str+x) += 0x30; } return y; } Thanks __________________ Spread your wings and fly! Chicken!

Jul 5th, 2005, 11:16 PM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Your compiler uses the __cdecl calling convention by default. However, your original post didn't indicate whether str2Int was written in C/C++ and could be presumed to be __cdecl, or whether you had written it in assembler, also, which of course has no "calling convention." The __cdecl convention is that the caller cleans up the stack (removes the parameters that were placed there prior to the call). The advantage to this convention is that one can use a variable number of arguments (ala "printf"); the caller knows how many he pushed, so he knows how many to remove. The called function has no idea. The disadvantage to the method is increased footprint as each caller has to perform cleanup instead of it being incorporated in the called function. Since you are calling the function from assembly language, it is your responsibility to clean up the stack. You aren't. (Toggle Plain Text) push eax; // this is the length (args pushed right to left) push msg; // this is the pointer to the message call str2Int; // make the call mov ebx, eax; // eax is the return value, which you are testing cmp ebx,128; // You have nothing to get the parameters off the stack before you proceed // You need to add a couple of pops push eax; // this is the length (args pushed right to left) push msg; // this is the pointer to the message call str2Int; // make the call mov ebx, eax; // eax is the return value, which you are testing cmp ebx,128; // You have nothing to get the parameters off the stack before you proceed // You need to add a couple of pops This issue is going to give you problems when you reach the end of the program because the stack pointer is not going to be pointing to the correct return address when you reach the end of main. You will get problems sooner if you add more to the code. THIS IS NOT YOUR CURRENT PROBLEM, HOWEVER. When you declare a string as follows: char * msg = "128\0"; the compiler sets aside static memory for the string and stack-memory for a pointer ("msg") and puts the appropriate value in the pointer. It's strictly a favor by the compiler, at compile time, for as you know, one cannot assign a C-style string to an array using the "=" operator. (You don't need to overtly add the terminating zero when you express it as you have; the compiler does THAT for you automatically, also). It is not universally true that the string is "const" (write-protected), but in your implementation, using your compiler, it is. You are trying to modify the string via the pointer you passed as a parameter. That is a memory access violation. The quick cure is to put the string on the stack: char msg = "128"; "msg", since it refers to an array, does double duty as a pointer under the circumstances, so you can make the call as you're presently doing. That double duty occassionally will get one in trouble because of the false assumption that an array and a pointer are the same thing, but in this case, you're fine. You may modify it at will; just remember to observe the length requirement which you pass, so as not to trundle off the end into verboten territory. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Jul 6th, 2005, 6:33 AM	#5
rsnd Hobbyist Programmer Join Date: Jun 2005 Location: Helltown Posts: 162 Rep Power: 4	Thanks DaWei. But what was the problem? The str2int function seems fine to me...I think the problem is with the message Box call!! this crashes: [php]#include <windows.h> int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) { char msg []= "128"; char msg2 []= "Equal"; char msg3 []= "NotEqual"; MessageBox(0,NULL,NULL,0); __asm{ mov ecx, 0x00; push ecx; push msg3; push msg; push ecx; call MessageBoxA pop eax; pop eax; pop eax; pop eax; } MessageBox(0,NULL,NULL,0); return 0; }[/php] __________________ Spread your wings and fly! Chicken!

Jul 6th, 2005, 6:47 AM	#6
omega_red Newbie Join Date: Jun 2005 Location: Poland Posts: 5 Rep Power: 0	The problem is in a way you call MessageBox: (Toggle Plain Text) call MessageBoxA call MessageBoxA At assembly level, MessageBoxA is a label to an offset in the Import Table of your exe. It points to a DWORD that contains real address of the API. You need to dereference that address: (Toggle Plain Text) call dword ptr [MessageBoxA] call dword ptr [MessageBoxA] It should work fine. __________________ Vulnerant omnes, ultima necat.

Jul 6th, 2005, 6:54 AM	#7
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	As I said, this instruction, (Toggle Plain Text) (str+x) -= 0x30; (str+x) -= 0x30; is attempting to modify read-only memory. You're not getting far enough for the message box call to be a problem, at this point. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Jul 6th, 2005, 7:38 AM	#8
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Incidentally, Omega is correct about your need for an indirect call, when you get that far. Big K, I tried to edit this into the above post, but there's no "EDIT" button. ??? __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Jul 6th, 2005, 8:35 AM	#9
Ooble I eat cake for breakfast. Join Date: Jul 2004 Location: In my box. Posts: 4,434 Rep Power: 9	The Edit button is disabled half an hour after you post. __________________ Me :: You :: Them

Jul 12th, 2005, 5:38 AM	#10
rsnd Hobbyist Programmer Join Date: Jun 2005 Location: Helltown Posts: 162 Rep Power: 4	Hmmm...ok...that didnot work!!! [php]#include <windows.h> int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) { char msg []= "128"; char msg2 []= "Equal"; char msg3 []= "NotEqual"; MessageBox(0,NULL,NULL,0); __asm{ mov ecx, 0x00; push ecx; push msg3; push msg; push ecx; call dword ptr [MessageBoxA]; pop eax; pop eax; pop eax; pop eax; } MessageBox(0,NULL,NULL,0); return 0; } [/php] __________________ Spread your wings and fly! Chicken!

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Programming Forums > Application Development > Assembly

Whats wrong with this inline code?