Question

I’m pretty new to assembly, and I was wondering how one would write output to stderr. I know you can access C Standard Library functions, like printf, to print to the console. But I can’t figure out how to print to stderr. I was trying to use fprintf, but I’m just sort of guessing at arguments, and I have no idea how to specify stderr as the file pointer. Thanks.

Edit: As per sehe’s suggestion, I tried this:

.586
.model small,c
.stack 100h

.data
msg db 'test', 0Ah

.code
includelib MSVCRT
extrn fprintf:near
extrn exit:near

public main
main proc
    push    offset msg
    push    2       ;specify stderr
    call    fprintf ;print to stderr
    push    0
    call    exit    ;exit status code 0

main endp

end main

But it just caused my program to crash. Any other advice?

Answer 1

Are you using fprintf from the MSVCRT dll?

The first parameter is a pointer to a stream. Here is how you could use fprintf in assembly.
Also, when calling C functions from Assembly, you need to adjust the stack after each call that you push parameters to.

Also, a BIGGIE… your string is NOT NULL terminated! You must NULL terminate your strings, that is how functions find the length of the string.
Not sure what Assembler you are using, but this is how you can do it in MASM:

include masm32rt.inc

_iobuf STRUCT
    _ptr        DWORD ?
    _cnt        DWORD ?
    _base       DWORD ?
    _flag       DWORD ?
    _file       DWORD ?
    _charbuf    DWORD ?
    _bufsiz     DWORD ?
    _tmpfname   DWORD ?
_iobuf ENDS

FILE TYPEDEF _iobuf

.data
msg         db 'test', 0Ah, 0    

.data?
stdin       dd ?
stdout      dd ?
stderr      dd ?

.code
start: 

    call    crt___p__iob
    mov     stdin,eax          
    add     eax,SIZEOF(FILE)
    mov     stdout,eax          
    add     eax,SIZEOF(FILE)
    mov     stderr,eax        

    push    offset msg
    push    eax
    call    crt_fprintf
    add     esp, 4 * 2

    push    0
    call    crt_exit 

end start