CODEWITHSUNDEEP
linuxassemblyionasm32-bit
user3348294
user3348294

Reputation: 33

Writing integer to console gives Segmentation Fault

Going from high to low lever language I got to assembly. Now at the very beginning, I wrote a simple age program (I'm not clear how to get system time yet so I just used another input). I get Segmentation Fault (core dumped) after I enter the final input. Here is my code:

section .text
global _start       

_start:                     

mov edx, lenask
mov ecx, ask
mov ebx, 1
mov eax, 4
int 0x80

mov edx, 5
mov ecx, input
mov ebx, 2
mov eax, 3
int 0x80

mov edx, lenask2
mov ecx, ask2
mov ebx, 1
mov eax, 4
int 0x80

mov edx, 5
mov ecx, input2
mov ebx, 2
mov eax, 3
int 0x80

mov eax, input2
mov ebx, input
sub eax, ebx

push eax

mov edx, lenanswer
mov ecx, answer
mov ebx, 1
mov eax, 4
int 0x80

pop eax

mov edx, 7
mov ecx, eax
mov ebx, 1
mov eax, 4
int 0x80

section .data

ask db "What is your age?"
lenask equ $-ask
ask2 db "What is today's year?"
lenask2 equ $-ask2
answer db "The age you were born was: "
lenanswer equ $-answer

section .bss 

input resb 5
input2 resb 5

An example of what happens:

What is your age?45
What is today's year?2015
The age you were born was: Segmentation fault

It should have done:

What is your age?45
What is today's year?2015
The age you were born was: 1970

Upvotes: 0

Views: 101

Answers (1)

Markian
Markian

Reputation: 342

The problem is that int 0x80 with eax set to 4 calls the kernel's sys_write function (i.e. a write system call) which expects a pointer to a string. By providing a integer to the function call the kernel will think that the integer is a pointer to a memory location. Because 1970 is not a valid pointer it will raise a -EFAULT. To bypass this you need to code a ToString function to convert the number to a string and then pass the pointer to the converted string.

The Segmentation Fault itself is caused by not having a sys_exit call. The reason for this is that the program will continue past the end of your code (usally into a bunch of 00 00)

Upvotes: 1

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