How to read input string from keyboard using assembly language - linux

Question

I'm trying to write a simple program that takes string from the keyboard, then prints it to the screen. So far I couldn't make it work.
Here's the code:
.section .rodata
output: .string "you entered %s\n"
input: .string "%s"
.text
.globl main
.type main, #function
main:
pushl %ebp
movl %esp, %ebp
subl $100, %esp
pushl $input
call scanf
movl %ebp, %esp
subl $100, %esp
pushl $output
call printf
xorl %eax, %eax
movl %ebp, %esp
popl %ebp
ret
When I execute it, the output is you entered (null) for any given input.
when I set the offset of subl $100, %esp command (the one before call print) to subl $104, %esp I get you entered %s, and when the offset is set to 108 I get you entered *gibberish*.
I feel like it's a game where I need to guess where scanf saved the string on the stack (why isn't it where it should be?).
I am using IA32 instruction set.
Any help would be greatly appreciated.

Answer 1

Basically there are 3 problems in your program:
subl $100, %esp
pushl $input
# Error 1:
# As Frank Kotler already wrote at this point
# only $input is stored on the stack; however
# the address of the buffer must also be on
# the stack (see Frank Kotler's comment)
call scanf
movl %ebp, %esp
# Error 2:
# Now the buffer is below ESP.
# Because interrupts use the kernel stack they
# will not overwrite the memory below ESP.
# However signals will destroy the memory below
# ESP!!
#
# Instead of the lines:
# movl %ebp, %esp
# subl $100, %esp
#
# You should use something like this:
# add $8, %esp
# or:
# lea -100(%ebp), %esp
#
subl $100, %esp
# Error 3:
# As for "scanf" the second argument
# is missing on the stack
pushl $output
call printf

Answer 2

[org 0x0100]
jmp start
;=======Data===================================
s1: db 'Enter String: $'
s2: db 'Reversed String:$'
linefeed: db 10, '$'
;=======Read string and store into stack=======
start: mov dx, s1
mov ah, 09h
int 21h
mov dx, linefeed
mov ah, 09h
int 21h
mov si,0
mov bx,0
again: mov ah, 01h
int 21h
cmp al,byte 0dh
je endread
Page 8 of 10
mov bl,al
push bx
inc si
cmp si,15
je endread
jmp again
endread: Push si
;=======Read from stack and print to screen=====
mov dx, linefeed
mov ah, 09h
int 21h
mov dx, s2
mov ah, 09h
int 21h
mov dx, linefeed
mov ah, 09h
int 21h
pop si
r1: pop bx
mov dl,bl
mov ah, 02h
int 21h
dec si
cmp si,0
jne r1
mov dx, linefeed
mov ah, 09h
int 21h
mov ax, 0x4c00 ;terminate program
int 21h