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Index > Linux > How to print a float in 64-bit program?

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gAr



Joined: 18 Apr 2010
Posts: 5
gAr
I'm trying a simple program to print a floating point value in 64-bit linux distro.
What I have done is that I wrote a C program and got the disassembly and translated to fasm syntax, using an example in fasm 1.70.03 for linux.

This is the C program I used
Code:
#include <stdio.h>
/* compiled in gcc with options -g -Wall */
int main(){
    float a=1.1;
    printf("%lf\n",a);
    return 0;
}
    


and this is the fasm translated code:
Code:
format ELF64 executable 3
entry start

include 'import64.inc'

interpreter '/lib64/ld-linux-x86-64.so.2'
needed 'libc.so.6'
import printf,exit

segment readable executable

start:
push   rbp
mov    rbp,rsp
sub    rsp,0x10
mov    eax,0x3f8ccccd  ; = 1.1
mov    DWORD  [rbp-0x4],eax
movss  xmm0,DWORD  [rbp-0x4]
cvtps2pd xmm0,xmm0
mov    eax,pf
mov    rdi,rax
mov    eax,0x0  ; This was 0x1 in the disassembly, but that caused SIGSEGV
call   [printf]
mov    eax,0x0  
call [exit]

segment readable writeable
pf db '%lf',0xa,0
    


Instead of printing 1.1, it prints 0.00000
Can anyone help with this, please?
Post 22 Jan 2014, 15:29
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HaHaAnonymous



Joined: 02 Dec 2012
Posts: 1180
Location: Unknown
HaHaAnonymous
[ Post removed by author. ]


Last edited by HaHaAnonymous on 28 Feb 2015, 18:27; edited 2 times in total
Post 22 Jan 2014, 16:09
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Melissa



Joined: 12 Apr 2012
Posts: 71
Melissa
Stack has to be 16 byte aligned for printf (when float arguments
are used)...

Code:
format elf64 executable 3
include 'import64.inc'

interpreter '/lib64/ld-linux-x86-64.so.2'
needed 'libc.so.6'
import printf,exit

segment readable executable
entry start

start:
push   rbp
mov    rbp,rsp
sub    rsp,0x8 ; stack has to be 16 byte aligned for printf
mov    eax,0x3f8ccccd  ; = 1.1 
mov    DWORD  [rbp-0x4],eax
movss  xmm0,DWORD  [rbp-0x4]
cvtps2pd xmm0,xmm0
mov    eax,pf
mov    rdi,rax
mov    eax,1  ; eax -> number of float arguments
call   [printf]
xor    edi,edi
call [exit]

segment readable writeable
pf db '%lf',0xa,0

    
Post 22 Jan 2014, 16:10
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17278
Location: In your JS exploiting you and your system
revolution
You can also use the 1.1 value directly:
Code:
mov eax,1.1    
Post 22 Jan 2014, 16:21
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HaHaAnonymous



Joined: 02 Dec 2012
Posts: 1180
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HaHaAnonymous
[ Post removed by author. ]


Last edited by HaHaAnonymous on 28 Feb 2015, 18:27; edited 1 time in total
Post 22 Jan 2014, 16:24
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17278
Location: In your JS exploiting you and your system
revolution
HaHaAnonymous wrote:
Maybe he is just paranoid like me and want to be extremely sure the value is correct.
How do you know that 0x3f8ccccd is "correct"? Are you just trusting the C compiler to always give you "correct" values? Why trust it more than fasm, or Windows calculator?
Post 22 Jan 2014, 16:31
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HaHaAnonymous



Joined: 02 Dec 2012
Posts: 1180
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HaHaAnonymous
[ Post removed by author. ]


Last edited by HaHaAnonymous on 28 Feb 2015, 18:26; edited 1 time in total
Post 22 Jan 2014, 16:52
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gAr



Joined: 18 Apr 2010
Posts: 5
gAr
Thanks everybody for your replies, both of the solutions worked great.

So, alignment was the problem. I was going mad about why the very same code by the compiler wasn't working with fasm!

Quote:

How do you know that 0x3f8ccccd is "correct"? Are you just trusting the C compiler to always give you "correct" values? Why trust it more than fasm, or Windows calculator?

I just translated the code to fasm as I said, wasn't sure whether 1.1 would work. Thanks for letting me know!
Post 22 Jan 2014, 16:52
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