How to call win32 APIs in inlined at&t asm?

[reverser]

hi all

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char * msg = "Hello, World!\n";
char * wMsg = "Content of the window..";
char * wCaption = "Window title";

int main (){
  std::cout << "before call" << "\n";

   asm(
"movl _msg,%eax\n\t"
"pushl %eax\n\t"
"calll _printf\n\t"
"popl %eax\n\t"
"movl $0, %eax\n\t"
"movl _wCaption, %ebx\n\t"
"movl _wMsg, %ecx\n\t"
"movl $0, %edx\n\t"
"pushl %eax\n\t"
"pushl %ebx\n\t"
"pushl %ecx\n\t"
"pushl %edx\n\t"
"calll MessageBoxA\n\t"
"popl %edx\n\t"
"popl %ecx\n\t"
"popl %ebx\n\t"
"popl %eax\n\t"
      );

    std::cout << "after call" << "\n";
}

when i want to compile, i get:
undefined reference to `MessageBoxA'

can someone please tell me what's wrong?

[ollydbg]

Just a guess: do you link the correct library?

[reverser]

Just a guess: do you link the correct library?

i don't think so
can you please tell me how? you have no idea how much i goggled it.

[stahta01]

Link that might help https://wiki.codeblocks.org/index.php/FAQ-Compiling_(general)#Q:_I_would_like_to_compile_a_project_using_some_non-standard_libraries._How_can_I_indicate_to_CodeBlocks_that_these_libraries_and_include_files_exist.3F

It is best to follow the "For your project :" part of the FAQ.

Edit: Google/MS says the library is user32

Tim S.

[ollydbg]

This is the test code I use(I'm using 64bit GCC compiler)

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#include <iostream>

using namespace std;

char * msg = "Hello, World!\n";
char * wMsg = "Content of the window..";
char * wCaption = "Window title";



int main (){
  std::cout << "before call" << "\n";

    int src = 1;
    int dst;

    asm ("mov %1, %0\n\t"
        "add $1, %0"
        : "=r" (dst)
        : "r" (src));

    printf("%d\n", dst);


   asm(
    "movq $0, %%rax\n\t"
    "movq $0, %%rax\n\t"
    "add $48, %%rax\n\t"
    "movq $0, %%rbx\n\t"
    "movq $0, %%rcx\n\t"
    "movq $0, %%rdx\n\t"
    "push %%rax\n\t"
    "push %%rbx\n\t"
    "push %%rcx\n\t"
    "push %%rdx\n\t"
    "call MessageBoxA\n\t"
    :
    : "r" (msg), "r" (wMsg) , "r" (wCaption)
    : "cc");

    std::cout << "after call" << "\n";
}



The MessageBoxA function can be called.

But I have no idea how to pass the string to this function.

See those links as reference:

http://asm.sourceforge.net/articles/rmiyagi-inline-asm.txt

https://stackoverflow.com/questions/66323292/messagebox-program-in-x86-assembly

https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#GotoLabels

https://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html


BTW: you need an export on assembly language for help. Our forum is not the right forum to ask.

EDIT:

To reference the input variables, you need

Code Select Expand

%0,  %1 and %2 like string in the assembly code.

[ollydbg]

OK, I just notice that the call convension is different in 64bit and 32bit Windows.

So, here is the modified code for 64bit Windows

Code Select Expand

#include <iostream>

using namespace std;

char * msg = "Hello, World!\n";
char * wMsg = "Content of the window..";
char * wCaption = "Window title";



int main (){
  std::cout << "before call" << "\n";


   asm(
    "movq $0, %%rcx\n\t"
    "movq %2, %%rdx\n\t"
    "movq %1, %%r8\n\t"
    "movq $0, %%r9\n\t"
    "callq MessageBoxA\n\t"
    :
    : "r" (msg), "r" (wMsg) , "r" (wCaption)
    : "cc");

    std::cout << "after call" << "\n";
}

The important change is: for 64bit function call, the first 4 arguments are not pushed in the stack, but put in the registers, see some references:

https://en.wikipedia.org/wiki/X86_calling_conventions#Microsoft_x64_calling_convention

https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170

https://stackoverflow.com/questions/42488273/call-a-function-with-inline-asm

[reverser]

OK, I just notice that the call convension is different in 64bit and 32bit Windows.

thanks a lot. it executed the call but Args are not passed correctly as message box is titled "Error" and is empty.
can you help me with 32bit version too; in x32, it shows the same error i mentioned. i cant make it work. if you cant (as you said here is not the right forum), please PM me the right place of asking this.

regards.

[ollydbg]

OK, I just notice that the call convension is different in 64bit and 32bit Windows.

thanks a lot. it executed the call but Args are not passed correctly as message box is titled "Error" and is empty.
can you help me with 32bit version too; in x32, it shows the same error i mentioned. i cant make it work. if you cant (as you said here is not the right forum), please PM me the right place of asking this.

regards.

I'm not sure, you can just Google for some asm forum(maybe masm32.com or others). Maybe stackoverflow site is also OK.

[reverser]

OK, I just notice that the call convension is different in 64bit and 32bit Windows.

thanks a lot. it executed the call but Args are not passed correctly as message box is titled "Error" and is empty.
can you help me with 32bit version too; in x32, it shows the same error i mentioned. i cant make it work. if you cant (as you said here is not the right forum), please PM me the right place of asking this.

regards.

to resolve this problem:
in x86 mode, win32 APIs must be called by their decorated names, preceded by a under score e.g calll _messageBox@16 (16 is the number of bytes that the functions parameters use on stack)