CODEWITHSUNDEEP
stringassemblystdinnasm
terrygarcia
terrygarcia

Reputation: 477

Remove Trailing Line-Breaking Characters from Read Input in Assembly

I have recently been teaching myself assembly, and I decided that the NASM assembler and its syntax are the most efficient and easiest to use. I am currently working with standard input and output; however, I am at a loss as I need to remove line-breaking characters (carriage return, form-feed, newline, - 0xd, 0xc and 0xa, respectively) from a string that I'm reading. Consider the following:

    section .data
;; ...
    section .bss
input:  resb    255
 .len:  equ     $ - input

    section .text
    global  _start

_start:
    ;; Display message prompting for input, then...
    mov     edx, input.len
    mov     ecx, input
    mov     ebx, 0
    mov     eax, 3
    int     0x80

Presently, I wish to strip trailing line-breaking characters. Consider the following pseudo-code:

if the last character in `input` is 0xa or 0xc or 0xd:
    subtract the last character from `input`
repeat until false

I have most likely made myself clear, but here is a Python equivalent to the above pseudo-code:

while input[-1] is "\r" or input[-1] is "\f" or input[-1] is "\n":
    input = input[:-1]

Upvotes: 2

Views: 2811

Answers (1)

jcomeau_ictx
jcomeau_ictx

Reputation: 38442

this isn't particulary elegant or efficient, but it might provide a starting point:

jcomeau@intrepid:/tmp$ cat test.nasm ; nasm -f elf -o test.o test.nasm; ld -o test test.o; ./test
    section .bss
input:  resb    255
 .len:  equ     $ - input

    section .text
    global  _start

_start:
    ;; Display message prompting for input, then...
    mov     edx, input.len
    mov     ecx, input
    mov     ebx, 0
    mov     eax, 3
    int     0x80  ;read in from stdin
    call    rstrip
    mov     edx, eax  ;count
    mov     ebx, 1  ;stdout
    mov     eax, 4
    int     0x80  ;write out
    mov     eax, 1
    xor     ebx, ebx
    int     0x80
rstrip:
    dec     eax  ;convert 1-based length to 0-based pointer
.loop:
    cmp     byte [ecx + eax], 0xa
    je      .chop
    cmp     byte [ecx + eax], 0xc
    je      .chop
    cmp     byte [ecx + eax], 0xd
    je      .chop
.done:
    inc     eax  ;convert pointer back to length
    ret
.chop:
    mov     byte [ecx + eax], 0
    dec     eax
    jns     .loop
    jmp     .done
this is a test
this is a testjcomeau@intrepid:/tmp$

Upvotes: 2

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