C / C++ / MFC

5-Mar-07 1:15

Is reading/writing a 32-bit integer on Win32 guaranteed to be atomic ? I could not find that information anywhere.

wbr Brainley

Mr.Brainley5-Mar-07 1:28

5-Mar-07 1:28

Ok, sometimes i'm posting over-eagerly. Here is the solution :

Interlocked Variable Access[^]

In short : the answer is yes.

wbr Brainley

Stephen Hewitt5-Mar-07 1:30

Stephen Hewitt

5-Mar-07 1:30

No. See here[^] for information on how to do it safely. If your programming x86 machine code you could also look up the lock prefix[^].

Steve

Mr.Brainley5-Mar-07 6:12

5-Mar-07 6:12

I'm writing a procedure where two values, wich always make up a pair, can be changed from one thread, but read from many. I only want to make shure that a read-operation always returns a valid pair (writing is actually limited by a CRITICAL_SECTION). So i use a checksum. The thread reads the variables and then checks the sum against a checksum. If one of the values got changed somewhere in between, the checksum will not match and it will try again. This works, since simple read/write operations on 32-bit Integers are atomic on 32-bit systems. I chose this way because it is much faster that using a lock mechanism, since write operation happen a lot less often (> factor 20) than write-operations.

Mark Salsbery5-Mar-07 6:41

Mark Salsbery

5-Mar-07 6:41

How can it get changed if only one thread ever changes it? How can you be sure it doesn't get
changed while computing the checksum? Is a critical section really that slow?

"Great job, team. Head back to base for debriefing and cocktails."
(Spottswoode "Team America")

Mr.Brainley5-Mar-07 6:59

5-Mar-07 6:59

Here a scenarion. Thread A reads, Thread B writes, the pair consists of V1 and V2 of type int.

A reads V1

B writes V1
B writes V2

A reads V2

So now A has a pair that does not belong together. Now consider this :

A reads V1

B writes Checksum V1+V2
B writes V1
B writes V2

A reads V2

A checks the Sum -> does not match, so A tries the read again.

There are a lot more cases, but they all work out.
Using a CRITICAL_SECTION or any other kind of synchronization would mean traps into the OS and context-switches on the wait-operations. In cases where such an optimistic approach is possible, that means the probability of an error, and thus a second or even third read is very low, the performance is higher this way.

James R. Twine5-Mar-07 8:17

James R. Twine

5-Mar-07 8:17

   The spin count on a CS object can be utilitized to handle situations like this, but you really need to be on a multi-CPU/multi-core machine to really make use of it.

   If the CS is released during the spin count, no context-switch will take place. Given that simple operations are being done, I would bet that you would hit that situation more often than not.

   Peace!

Mark Salsbery5-Mar-07 10:16

Mark Salsbery

5-Mar-07 10:16

I just hijacked the thread for my own personal learnin'...

Can you explain what the heck you're talking about? Smile | :)

*edit* Google is my friend: Critical Section Objects[^] Roll eyes | :rolleyes:

Thanks!
Mark

"Great job, team. Head back to base for debriefing and cocktails."
(Spottswoode "Team America")

accessing registry using vc++

Stephen Hewitt5-Mar-07 11:37

Stephen Hewitt

5-Mar-07 11:37

Looks like you should do some research on "readers writer locks".

Steve

siddhesh13835-Mar-07 1:14

siddhesh1383

5-Mar-07 1:14

hi i need to read and write the registry values using vc++.
if anyone has any idea please forword itto me.

thanks.

Rajesh R Subramanian5-Mar-07 1:15

Rajesh R Subramanian

5-Mar-07 1:15

Have a look at CRegKey on MSDN.

And please delete your previous message.

Glory follows virtue as if it were its shadow.
- Marcus Tullius Cicero

Joan M5-Mar-07 1:36

Joan M

5-Mar-07 1:36

Here you have some samples...

CRegistry is a class that I use often in order to work with the registry... it's something that we had developed some time ago.

<br />
bool	CRegistry::SetKey(HKEY hKey, CString csSubkey, CString csValName, CString csVal)<br />
{<br />
	if(hKey != HKEY_CURRENT_USER)<br />
		return false;<br />
<br />
	if(csSubkey.IsEmpty() || csValName.IsEmpty() || csVal.IsEmpty())<br />
		return false;<br />
<br />
	HKEY hWkKey;<br />
	LONG lRetorn = RegOpenKeyEx(hKey,csSubkey,0,KEY_ALL_ACCESS,&hWkKey);<br />
	if(lRetorn == ERROR_SUCCESS)<br />
	{<br />
		int iLength = csVal.GetLength();<br />
		lRetorn = RegSetValueEx(hWkKey,<br />
									csValName,<br />
									0, <br />
									REG_SZ, <br />
									(BYTE *)csVal.GetBuffer(0),<br />
									iLength);<br />
<br />
		RegFlushKey(hWkKey);<br />
		RegCloseKey(hWkKey);<br />
<br />
		if( lRetorn == ERROR_SUCCESS)<br />
			return true;<br />
	}<br />
	return false;<br />
}<br />
<br />
bool	CRegistry::SetKey(HKEY hKey, CString csSubkey, CString csValName, int iVal)<br />
{<br />
	if(hKey != HKEY_CURRENT_USER)<br />
		return false;<br />
<br />
	if(csSubkey.IsEmpty() || csValName.IsEmpty())<br />
		return false;<br />
<br />
	HKEY hWkKey;<br />
	LONG lRetorn = RegOpenKeyEx(hKey,csSubkey,0,KEY_ALL_ACCESS,&hWkKey);<br />
	if(lRetorn == ERROR_SUCCESS)<br />
	{<br />
		int iLength = sizeof(iVal);<br />
		lRetorn = RegSetValueEx(hWkKey,<br />
									csValName,<br />
									0, <br />
									REG_DWORD, <br />
									(BYTE *)&iVal,<br />
									iLength);<br />
<br />
		RegFlushKey(hWkKey);<br />
		RegCloseKey(hWkKey);<br />
<br />
		if( lRetorn == ERROR_SUCCESS)<br />
			return true;<br />
	}<br />
	return false;<br />
}

Hope this helps...

Michael Dunn5-Mar-07 12:39

Michael Dunn

5-Mar-07 12:39

Never use KEY_ALL_ACCESS unless you want the code to be usable only by administrators.

--Mike--
Visual C++ MVP Cool | :cool:

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Joan M5-Mar-07 19:43

Joan M

5-Mar-07 19:43

Yes, you are right... Blush | :O

Sometimes this becomes a problem when I share code... as I make machine prototypes, and I prevent people who use my apps to reach the OS, then normally I don't have this kind of problems, I always work as an administrator.