Wednesday, October 5, 2022

[SOLVED] Assembly 32-bit - Calling a program from another program

Issue

I'm currently learning Assembly, I'm not very new but maybe since 1 - 2 months back. I am using 32-bit Assembly, NASM to assemble it into a .o file and MinGW/gcc to compile to a .exe file. but It takes so much time to type "nasm -f elf " in the nasm directory and then do "gcc -o " all the time. so I wanted to create a quick program where I just start it. and all that gets executed.

So I want the program to 1st start the nasm assembler. then wait till it's done (like when you call another function), and after that, I want to start the gcc compiler, and wait till that is done. I know it's quite easy to make in batch files, but I still want to learn how to do it in assembly, incase I need to do it someday. So the question is, how do I start a program (with start/console parameters), and then also wait for it to exit.

Thanks!


Solution

You can start a program with CreateProcessA and wait for its end with WaitForSingleObject. The GCC-linker (LD) needs also a suffix with the number of the pushed bytes (e.g. "@4", one dword = four bytes). That is one reason to use another linker.

Here's a simple example to start a shell (cmd.exe) with a "dir"-command:

exec_dir.asm:

STRUC _STARTUPINFO                      ; https://msdn.microsoft.com/library/windows/desktop/ms686331.aspx
    .cb:                resd 1
    .lpReserved:        resd 1
    .lpDesktop:         resd 1
    .lpTitle:           resd 1
    .dwX:               resd 1
    .dwY:               resd 1
    .dwXSize:           resd 1
    .dwYSize:           resd 1
    .dwXCountChars:     resd 1
    .dwYCountChars:     resd 1
    .dwFillAttribute:   resd 1
    .dwFlags:           resd 1
    .wShowWindow:       resw 1
    .cbReserved2:       resw 1
    .lpReserved2:       resd 1
    .hStdInput:         resd 1
    .hStdOutput:        resd 1
    .hStdError:         resd 1
ENDSTRUC

STRUC _PROCESS_INFORMATION              ; https://msdn.microsoft.com/library/windows/desktop/ms684873.aspx
    .hProcess:      resd 1
    .hThread:       resd 1
    .dwProcessId:   resd 1
    .dwThreadId:    resd 1
ENDSTRUC

section .data
    startupinfo:
        istruc _STARTUPINFO
        iend
    procinfo:
        istruc _PROCESS_INFORMATION
        iend
    app_fullpath: db "C:\Windows\System32\cmd.exe",0
    params: db "/c dir", 0
    msg: db `\n\nok.\n`,0

global _main
EXTERN _ExitProcess@4, _CreateProcessA@40,_WaitForSingleObject@8
EXTERN _puts, _fflush

section .text
_main:

    ; CreateProcess(app_fullpath,params,0,0,false,0,0,0,&startupinfo,&procinfo)
    push procinfo
    push startupinfo
    push 0
    push 0
    push 0
    push 0
    push 0
    push 0
    push params
    push app_fullpath
    call _CreateProcessA@40         ; https://msdn.microsoft.com/library/windows/desktop/ms682425.aspx

    ; WaitForSingleObject( procinfo.hProcess, INFINITE );
    push -1         ; INFINITE
    push dword [procinfo + _PROCESS_INFORMATION.hProcess]
    call _WaitForSingleObject@8

    ; puts ("ok."), fflush to flush the stdout-buffer
    push msg
    call _puts
    mov dword [esp], 0
    call _fflush
    add esp, 4

    ; return 0
    push 0
    call _ExitProcess@4

In a (Windows) Command Prompt:

PATH C:\MinGW\bin;<Path\to\nasm.exe>
nasm.exe -fwin32 exec_dir.asm
gcc.exe -m32 -o exec_dir.exe exec_dir.obj
exec_dir.exe


Answered By - rkhb
Answer Checked By - Clifford M. (WPSolving Volunteer)
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