CODEWITHSUNDEEP
assemblyx86nasm
AmirSojoodi
AmirSojoodi

Reputation: 1340

Writing to stdout in nasm

In this code I managed to print an array with syscalls:

%include 'syscalls32.inc'
section .bss
section .data
    array dd 1,2,3,4,5
    arraylen equ ($ - array) / 4            ; array length * 4 = number of elements
section .text
    global _start:
_start:
    mov     esi, 0                      ; get pointer to array
    mov     edi, arraylen - 1               ; edi = number of array elements
.PrintArray:
    mov     edx, 1                          ; print 1 byte                 
    mov     ecx, [array + 4]                    ; get current array element
    add     ecx, 48                         ; add 48 to convet to ASCII
    push    ecx                             ; push to stack since we need an address of item to print
    mov     ecx, esp                        ; mov address of char to ecx
    mov     ebx, stdout
    mov     eax, sys_write
    int     80h                             ; now print it
    pop     ecx                             ; balance stack
    add     esi, 4                          ; get next element, 4 because it's an array of dwords
    dec     edi                             ; decrease loop counter
    jns     .PrintArray                     ; if edi ! -1 continue loop
.PrintLineFeed:
    sub     esp, 4
    mov     byte [esp], 10
    mov     edx, 1
    mov     ecx, esp
    mov     ebx, stdout
    mov     eax, sys_write
    int     80h
    add     esp, 4                          ; not needed since next call is exit, but learn good techniques.
exit:
    mov     ebx, 0
    mov     eax, sys_exit
    int 80H

I found this grep trick to make syscalls32.inc:

grep __NR /usr/include/asm/unistd_32.h | grep define | sed -e 's/\#/\%/' -e 's/__NR_/sys_/' > syscalls32.inc

How can I do such a thing to make sth like std.inc, because now my code has this error: symbol stdout undefined?

By the way if I set stdout to 0 in .data section: stdout equ 0 it works fine. But I want to know if an include file exists or can be made or not?

Upvotes: 0

Views: 2854

Answers (1)

Gunner
Gunner

Reputation: 5884

Down here in Assembly land, if it doesn't exist, you make it.

Nothing is wrong with: stdout equ 0

Nor is: %define stdout 0

Add one of them to the top of your source, outside of any sections or add one to an include file.

Upvotes: 3

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