Author Topic: nasm & printf, segmentation fault  (Read 14157 times)

Offline clzola

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nasm & printf, segmentation fault
« on: March 13, 2014, 01:41:39 PM »
I wrote this code ant it gives me "Segmentation fault (core dumped)" after I input a number.

test.asm
Code: [Select]
%include "asm_io.inc"
extern printf
global asm_main

section .data
    msg db "Value: %d",10,0
   
section .text
    asm_main:
        enter 0, 0   
        pusha
       
        call read_int ; this function is from asm_io.asm
                          ; It reads an integer and stores it in EAX register.

        push msg    ; switching next two lines prints message on screen with correct value
        push eax     ; but still getting Segmentation fault error.
        call printf
       
        popa
        mov eax, 0
        leave
        ret

driver.c
Code: [Select]
int main()
{
    int ret_value;
    ret_value = asm_main();
    return ret_value;
}

asm_io.inc and asm_io.asm are files provided with book PC Assembly Language. I am trying to use printf function as I would do it in C.

Code: [Select]
printf("Value: %d", x); // this x should be in eax.

to compile: (Linux x86)
Code: [Select]
nasm -f elf test.asm
gcc asm_io.o test.o driver.o -o test

Offline encryptor256

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Re: nasm & printf, segmentation fault
« Reply #1 on: March 13, 2014, 01:54:38 PM »
Hi!

Try this, maybe works:
Code: [Select]
%include "asm_io.inc"
extern printf
global asm_main

section .data
    msg db "Value: %d",10,0
   
section .text
    asm_main:
        enter 0, 0   
        pusha
       
        call read_int ; this function is from asm_io.asm
                          ; It reads an integer and stores it in EAX register.

        push eax
        push msg
        call printf
add esp,2*4
       
        popa
        mov eax, 0
        leave
        ret
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Offline clzola

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Re: nasm & printf, segmentation fault
« Reply #2 on: March 13, 2014, 02:02:55 PM »
Yes. It works.
Why do I need that "add esp, 2*4"?

Offline encryptor256

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Re: nasm & printf, segmentation fault
« Reply #3 on: March 13, 2014, 02:05:24 PM »
You need to understand what is calling convetions,
that was the error in this case.

In this case it was CDECL, because printf used CDECL calling convention,
and you need to clear stack for every variable you pushed onto stack, before
you called printf, read more here:

Wiki: x86 calling conventions - CDECL

Wiki: x86 calling conventions - Caller clean-up

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Offline encryptor256

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Re: nasm & printf, segmentation fault
« Reply #4 on: March 13, 2014, 02:12:11 PM »
Yes. It works.
Why do I need that "add esp, 2*4"?

You pushed two times before you called printf.

Code: [Select]
        push eax
        push msg
        call printf

printf is CDECL, so you need to restore or pop pushed items of the stack.

1. Restore stack pointer after CDECL procedure like printf was called:
Code: [Select]
        add esp, 2*4

OR this also should do the trick, if eax is not needed:

2. Restore stack pointer after CDECL procedure like printf was called:
Code: [Select]
        pop eax
        pop eax

OR

3. Restore stack pointer after CDECL procedure like printf was called:
Code: [Select]
         add esp,4
         add esp,4

* Error in early code was caused because you didn't knew that printf is CDECL.

So, read, find info about x86 calling convetions (CDECL/STDCALL/Other) or use my links above.

Bye!
« Last Edit: March 13, 2014, 02:14:20 PM by encryptor256 »
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Offline clzola

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Re: nasm & printf, segmentation fault
« Reply #5 on: March 13, 2014, 05:46:50 PM »
Thank you, I understand now.