fasmg on Windows 7 bug: display fails on long output

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Grom PE

Joined: 13 Mar 2008
Posts: 114
Location: i@grompe.org.ru

Grom PE 20 Sep 2016, 20:51

Code:

bug = 0
;bug = 1 ; <== Uncomment this to see the bug:
;fasmg display command becomes silent when there's more data than this

head:
repeat 15682+bug
  dd '.'
end repeat
tail:

p = head
while p < tail
  load x dword from p
  display x
  p = p + 4
end while
display 'hello'

Last edited by Grom PE on 21 Sep 2016, 10:29; edited 1 time in total

20 Sep 2016, 20:51

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8359
Location: Kraków, Poland

Tomasz Grysztar 21 Sep 2016, 06:33

I tested on Windows and Linux and with either "bug = 0" and "bug = 1", and I see the "hello" message every time. Perhaps there is some additional condition to reproduce the bug? What version of fasmg have you tested?

21 Sep 2016, 06:33

Grom PE

Joined: 13 Mar 2008
Posts: 114
Location: i@grompe.org.ru

Grom PE 21 Sep 2016, 07:48

To reproduce this bug, I used
fasmg 0.96.1449399753
fasmg 0.98.1474441044
on Windows 7, 64-bit

fasmg 0.98.1474441044
on Windows XP 32-bit, Windows 2000: fine with 13303 repeats, triggers with 13304 repeats

21 Sep 2016, 07:48

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8359
Location: Kraków, Poland

Tomasz Grysztar 21 Sep 2016, 07:57

On Windows 10 I went up to 1000000 and still saw nothing suspicious. Perhaps this is a problem of the console? If you redirect the output to the file (like "fasm test.asm nul > test.out") is the display in file cut off too?

21 Sep 2016, 07:57

Grom PE

Joined: 13 Mar 2008
Posts: 114
Location: i@grompe.org.ru

Grom PE 21 Sep 2016, 08:10

You're right, redirected output is fine.
Still, quite surprising behavior while fasm worked as expected...

Edit: clarified the topic, also see a similar bug report in Python.

21 Sep 2016, 08:10

Grom PE

Joined: 13 Mar 2008
Posts: 114
Location: i@grompe.org.ru

Grom PE 21 Sep 2016, 11:42

I fixed it by changing source/windows/system.inc, the end of the display_string function:

Code:

   write_string_to_stdout:
        cmp     ecx, 51200
        jg      write_long_string_to_stdout
        invoke  WriteFile,[stdout],esi,ecx,bytes_count,0
        retn
   write_long_string_to_stdout: 
        push    esi
        push    ebx
        mov     edi, ecx
        mov     ebx, 51200
   next_write_to_stdout:
        invoke  WriteFile,[stdout],esi,ebx,bytes_count,0
        add     esi, ebx
        sub     edi, ebx
        cmp     edi, ebx
        jg      next_write_to_stdout
        invoke  WriteFile,[stdout],esi,edi,bytes_count,0
        pop     ebx
        pop     esi
        retn

Tested on Windows 2000 and 7, works fine. Attached diff file for convenience.

Description:	Patch for fasmg that fixes WriteFile bug with long strings on earlier Windows versions	Download
Filename:	writefile.diff.txt
Filesize:	585 Bytes
Downloaded:	786 Time(s)

21 Sep 2016, 11:42

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