Re: [Nasm-users] How to call Win32 printf from Nasm?

[Frank K. <fbk...@zy...>]

oh...@co... wrote:
> Hi,
> 
> Can anyone tell me how to call Win32 printf from a Nasm program?
> 
> I've been trying:
> 
> mov eax, msg
> push eax
> call _printf
> .
> .
> .
> msg: db 'this is a test', 10, 0
> 
> and that seems to work, but the stack seems to be messed up after that, so i'm wondering if there needs to be a "pop eax" after the call?

Probably. As you know, the usual API is stdcall - callee cleans up 
stack. But printf, or anything that takes a variable number of 
parameters really *can't* work that way... So you'll have to do it - 
"pop eax", or "add esp, 4", or "lea esp, [esp + 4]"... even in Windows.

> Also, what other parameters can be used, e.g., can I provide printf with a format string, and if so, how?

Well, here's an example for Linux...

; nasm -f elf hwso.asm
; ld -I/lib/ld-linux.so.2 -lc -s -o hwso hwso.o

global _start
extern printf

section .text
_start:

     fld qword [anumber]
     sub esp, 8
     fstp qword [esp]
     push msg
     push fmt
     call printf
     add esp, 16

     mov eax, 1
     xor ebx, ebx
     int 80h

;section .data
     fmt db "%s%f", 10, 0
     msg db "The answer is: ", 0
     anumber dq 42.0

For Windows, you'll want an underscore on "_printf". Nasm will do that 
for ya - "nasm -f win32 --PREFIX _ myprog.asm" or so (puts an underscore 
on all "extern" or "global" variables). The "int 80h" won't work, of 
course - ExitProcess instead... Note that printf expects double 
precision floats for "%f"! (*not* a "Linux thing", AFAIK). (important to 
know, since printing floats is the only excuse for printf :) I don't 
know what the command line to the linker needs to be - "whatever you're 
using" should be okay.

New Nasm feature: you can use "\n" and the like, if you enclose the 
string in "back apostrophes" '`' instead of ' or ". (under the ~ on my 
keyboard)

format_string db `hello\n`, 0

Thanks for the question, Jim! First message I've been able to approve in 
ages! :)

Best,
Frank