Unique instance with a Mutex not working.

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rohagymeg

Joined: 19 Aug 2011
Posts: 77

rohagymeg 23 Aug 2011, 12:10

When I run this code CreateMutexA always returns zero no matter what I do. What is the problem? I'm on win7 32bit.

Code:

format PE GUI 4.0
include "win32ax.inc"

;Only one instance

invoke CreateMutexA, 0, 0, "Global\\{12345678-1234-1234-1234-123456789ABC}"
cmp eax, 0
jne another_instance_not_found
invoke MessageBoxA, 0, "Another instance found!", "Test", MB_OK
invoke ExitProcess, 1
another_instance_not_found:
invoke MessageBoxA, 0, "Another instance NOT found!", "Test", MB_OK
invoke ExitProcess, 0
data import

library kernel32,'KERNEL32.DLL',user32,'USER32.DLL'
                                                      
 import kernel32,\                                    
        ExitProcess,'ExitProcess', CreateMutexA,'CreateMutexA',\
                                                     
 import user32,\                                      
        MessageBoxA,'MessageBoxA'


end data

Thanks for help!

23 Aug 2011, 12:10

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20425
Location: In your JS exploiting you and your system

revolution 23 Aug 2011, 12:18

Have you read this?:

Win32 Doc wrote:

lpName
Points to a null-terminated string specifying the name of the mutex object. The name is limited to MAX_PATH characters and can contain any character except the backslash path-separator character (\). Name comparison is case sensitive.

Why do you have two backslashes there?

23 Aug 2011, 12:18

rohagymeg

Joined: 19 Aug 2011
Posts: 77

rohagymeg 23 Aug 2011, 12:24

revolution wrote:

Have you read this?:
Win32 Doc wrote:
lpName
Points to a null-terminated string specifying the name of the mutex object. The name is limited to MAX_PATH characters and can contain any character except the backslash path-separator character (\). Name comparison is case sensitive.
Why do you have two backslashes there?

Amazing! I couldn't figure that out myself! It's there because in c++ it worked for me that way.
Somewhere there was a random mutex name generator and I just pasted it there and it completely worked in code::blocks

23 Aug 2011, 12:24

AsmGuru62

Joined: 28 Jan 2004
Posts: 1660
Location: Toronto, Canada

AsmGuru62 23 Aug 2011, 13:06

Pretty cool!

MSDN says that "Global\" is a valid prefix:
http://msdn.microsoft.com/en-us/library/ms682411(v=VS.85).aspx

In FASM a double backslash will produce a double backslash!
In C++, however, this turned into a single one, which is valid!

That shows that paths and names can't be "ported" into FASM just by copying stuff.

btw: URL tag is not working. Mad

Edit by revolution: Fixed url tag

23 Aug 2011, 13:06

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20425
Location: In your JS exploiting you and your system

revolution 23 Aug 2011, 13:14

AsmGuru62 wrote:

MSDN says that "Global\" is a valid prefix

Indeed, but not compatible with older Windows versions like Win95 (if this is ever an issue for anyone anymore).

23 Aug 2011, 13:14

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 23 Aug 2011, 13:20

You also could have checked the error code from GetLastError.

23 Aug 2011, 13:20

rohagymeg

Joined: 19 Aug 2011
Posts: 77

rohagymeg 23 Aug 2011, 13:36

vid wrote:

You also could have checked the error code from GetLastError.

I sent this pm to revolution but I'll put it here, too:

If I comment out the getlasterror line CreateMutexA doesn't work. Otherwise it works. Is getlasterror a requirement for this function to return a value?

Code:

format PE GUI 4.0
include "win32ax.inc"

invoke CreateMutexA, 0, 1, mutex_name
invoke GetLastError     ;this!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

cmp eax, 0
je another_instance_not_found
invoke MessageBoxA, 0, "Another instance found!", "Test", MB_OK
invoke ExitProcess, 1

another_instance_not_found:
invoke MessageBoxA, 0, "Another instance NOT found!", "Test", MB_OK
invoke ExitProcess, 0
data import

library kernel32,'KERNEL32.DLL',user32,'USER32.DLL'
                                                      
 import kernel32,\                                    
        ExitProcess,'ExitProcess', CreateMutexA,'CreateMutexA',\
        GetLastError,'GetLastError'
                                                      
 import user32,\                                      
        MessageBoxA,'MessageBoxA'


end data

mutex_name db "d41d8cd98f00b204e9800998ecf8427e",0

23 Aug 2011, 13:36

rohagymeg

Joined: 19 Aug 2011
Posts: 77

rohagymeg 23 Aug 2011, 13:56

Everything is fine, I just understood that it returns with GetLastError, and CreateMutexA returns the mutex handle so it always showed the same return value because I checked eax after CreateMutexA, not after GetLastError, which returns ERROR_ALREADY_EXISTS, and that's what I was looking for to ensure that only one instance is running.

23 Aug 2011, 13:56

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