C / C++ / MFC

19-Jun-11 3:32

Hi. I got the following situation,

void FunctionA(some params)
{
// do something
SOCKET sock = create new socket();
WSAEVENT hEvent = new event();
wsaeventselect(sock, event, some flags);
FunctionB(sock);
delete event();
}

void FunctionB(sock)
{
// here do something else with a socket
}

The thing is, i dont have access to FunctionA() but it calls

FunctionB()

, where i am the master and can do anything. In function B i am receiving a socket which has an event associated with it but nothing else. I got no handle to an event - nothing, just a socket. Is there any way to get this event handle having only socket descriptor? If there is no such API, windows system should store these events somewhere, right? Is there any way to dig into this place and grab events from there?
Thanks

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enhzflep19-Jun-11 4:13

enhzflep

19-Jun-11 4:13

I don't know anything at all about WSA sockets, save for the few crumbs I just picked up reading the winsock function reference at msdn.

However, I can see nothing to indicate this to be the case - nor can I think of a reason why this would necessarily be included - i.e if it's a socket you've created, then surely it is also you thas already attached said event to it - in which case you'd already have it's handle.. At least that's the way I guess it.

With that in mind, and the preparedness to go poking about in windows' memory space, I'd suggest that there is likely to be a far simpler way - unless of course FunctionA is in the kernel.
When FunctionB ends, it has to be able to return control to FunctionA, right? With this address available, you can look at the (machine) code for FunctionA, so with a little fancy footwork you can easily get the location in memory of the variable that is used to store the newly created event, and snatch the handle from there..

You know how I'd go about it? I'd write a dummy FunctionB, and put a statement something like

printf("here is my marker");

I'd then fire-up the program in OllyDbg, and search for refferences to "here is my marker". When I'd found the place that the memory containing this string was referenced, I'd know that I'd found my FunctionB. From there I would simply place a break-point inside my function, start the program running and wait for it to hit the break-point. Single-stepping would then allow me to return to FunctionA, where I could grab the required handle from the memory it was originally stored into.

Might take 30 mins to come up with working code for a real FunctionB to grab the event handle, so long as there's no bumps along the way like protected memory. I forget how to go about reading a process's memory but I've got code some where to do it.

You sure have got me curious about your project.. Thumbs Up | :thumbsup:

csrss19-Jun-11 5:43

19-Jun-11 5:43

The thing is, socket is not created by me. Well, i was able to grab a list of events using ntquerysysteminformation and ntqueryobject but there seems to be no way to find out if a current event was selected by WSAEventSelect for a specified socket. enhzflep, this rather must be "out-of-the-box-working-template-automated-appliable-to-everything" code :P
Seems like the easiest way would be to just insert inline hook, but more hooks - less stable the app become =/

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Mark Salsbery19-Jun-11 9:38

Mark Salsbery

19-Jun-11 9:38

Even if you could get the event handle, what would you do with it? I assume you also don't have control over how the event is being used so you won't have any synchronization with whatever thread(s) are already using the event...

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

csrss19-Jun-11 13:13

19-Jun-11 13:13

There are lots of things i can do with event, Mark. I can wait on it, so i dont have to deselect old event and select new one, so the original thread have a control over this original event. I can block signaling this event and signal it when i am done with my job, so original thread will be notified when it is appropriate.

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Mark Salsbery19-Jun-11 14:10

Mark Salsbery

19-Jun-11 14:10

csrss wrote:
I can wait on it, so i dont have to deselect old event and select new one, so the original thread have a control over this original event

Then do you have a race condition? Which thread calls WSAEnumNetworkEvents() first? Or is only one FD_XXX event being "listened for" so WSAEnumNetworkEvents() isn't called?

csrss wrote:
I can block signaling this event and signal it when i am done with my job

How do you block signalling of an event (just curious...I didn't know it was possible since I've never needed to)?

You must have intimate knowledge of the existing code to believe this is feasible Smile | :)

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

csrss20-Jun-11 5:46

20-Jun-11 5:46

This is actually more an experiment then a real life solution - i am trying to create my own layer without writing and installing one. It looks like this: some event is created in some function which sends some data over a socket:

WSAEVENT hEvent = WSACreateEvent();
::WSAEventSelect(sock, hEvent, FD_WRITE | FD_READ);
WSASend(....);

And then a thread waits for event, in fact event occurs when we got some bytes waiting for us or when we are able to write some bytes. WSASend is out of the question here, because all my job is done before WSASend gets executed. The pain is WSARecv, because after this call i have some thing to do and after i am done i can return control to a function which waits for WSARecv to complete. BTW, i am hooking these calls so when WSAsomething is called control goes to my function, where i am calling WSAsomething and additionally do some work with outgoing || incoming data.
So the thing is to prevent WSAsomething completion event signaling and signal it when i am done with my part.

I know it is wrong :P and all bad things might happen and there can be a disaster crash or something and this is not what i am going to implement in final solution. I am trying and learning how to dig into system and control it in every ways possible. About blocking event, well, i can save old event handle, select new event, do my stuff, and when done, - signal old event and delete mine, for example :P Or i can go deeper into the cave and try to hook something here and there and do some crazy stuff, you know Big Grin | :-D

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Mark Salsbery20-Jun-11 9:29

Mark Salsbery

20-Jun-11 9:29

csrss wrote:
About blocking event, well, i can save old event handle, select new event, do my stuff, and when done, - signal old event and delete mine,

Right. Problem is, when the event is signaled, WSAEnumNetworkEvents() is usually called by the waiting/polling thread to determine which event(s) occurred. Only one thread can make that call for a given event signal on a given socket (even if you duplicate the socket) because the call resets its event status.

Good luck though Smile | :)

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

csrss20-Jun-11 10:17

How can I read all files from 'Recent File List' ?

20-Jun-11 10:17

Hmmm, thanks...
Yeah, i've abandoned this idea Wink | ;)

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_Flaviu18-Jun-11 8:56

Re: How can I read all files from 'Recent File List' ?

18-Jun-11 8:56

I try to read all files ( path files ) from 'Recent File List' from registry , and I failed ... I try with RegEnumValue function , but this give me string values ( File1 , File2 etc. ) not path of this files... I read RegEnumValue remarks and I see that somehow , with LPBYTE lpData parameter I could have string values , but I have no ideea how to convert LPBYTE to CString or char ... and Google didn't help me ( I didn't find a sample appropiate to my request ) ... can you help me , please ... maybe you have front with this problem ... Thank you.

Richard MacCutchan18-Jun-11 21:42

Richard MacCutchan

18-Jun-11 21:42

Using the RegEnumValue()[^] function returns you the name of the value (File1 etc) and the content as a byte array. It also returns you a value item in lpType which tells you what type of data the value has. Assuming you check that this is a REG_SZ (null terminated string) then you can merely cast the pointer to the appropriate type and thence handle it as required, something like:

HKEY hKey // set by previous function
DWORD dwIndex = 0;
char szValueName[256];
DWORD cchValueName = 256;
DWORD dwType;
BYTE bData[4096];
DWORD cbData = 4096;
RegEnumValue(hKey, dwIndex, szValueName, &cchValueName, NULL, &dwType, bData, &cbData)
// check status etc
LPSTR pszValue = reinterpret_cast<LPSTR>(bData);

The best things in life are not things.

Re: How can I read all files from 'Recent File List' ? [modified]

_Flaviu18-Jun-11 21:45

Re: How can I read all files from 'Recent File List' ?

18-Jun-11 21:45

Thank you so much , I will try your solution and let you know what I have done .

modified on Tuesday, June 21, 2011 2:52 AM

Richard MacCutchan19-Jun-11 21:45

Richard MacCutchan

19-Jun-11 21:45

Up-voted - I have no idea what sort of moron would 1-vote what is a totally reasonable response to my post.

The best things in life are not things.

Re: How can I read all files from 'Recent File List' ?

_Flaviu20-Jun-11 21:28

Re: How can I read all files from 'Recent File List' ?

20-Jun-11 21:28

Take away bad intention from various people, you do here best job, it' always a pleasure to talk here with so many good programmers that post here from altruism. Kindly thank you.

P.S. I didn't try your code yet, catch with other things.

_Flaviu22-Jun-11 20:24

Re: How can I read all files from 'Recent File List' ?

22-Jun-11 20:24

Great , does function ! Thanks again !

Richard MacCutchan22-Jun-11 21:56

Richard MacCutchan

22-Jun-11 21:56

Happy to help.

The best things in life are not things.

Best Sound API for Game Development?

AutoBot122518-Jun-11 5:04

AutoBot1225

18-Jun-11 5:04

I've been interested in game development and have been using C++ for the past 3 years now, as well as OpenGL just recently. But does anyone here know of a good sound API? I've seen OpenAL, which has a basic extent of features, but I'd like something a little more flexible if possible. I've also taken a look at fmod and liked the looks of it until I saw the price tags for publishing a game with it (ranged from $500 - $9000 per platform).

Thanks for any suggestions!

I had this interview question

Software200718-Jun-11 4:26

Software2007

18-Jun-11 4:26

A senior C#/C++ developer asked me this question during the interview. I thought I share it with you guys.

int x= 0;
int y= 0;

thread1
{
while (x==0) { just sit here};
printf("%d",y);
}

thread2
{
y=42;
x=1;
}

He said Assume that we know for sure Thread 1 had started before thread2:

Q1) what will printf print for answer? I said 42, that was correct.
Q2) Then he said, but suppose the answer was y=0, do you know how could that happen knowing thd1 started before thd2?

I did not get the answer correct. I did think threading, mutexes,..etc it wasn't what he was trying to get at I guess! I do know the answer, but I would like to see how you might answer.

Philippe Mori18-Jun-11 5:14

Philippe Mori

18-Jun-11 5:14

Well, I think, the main reason would be that because y is not marked as volatile, the compiler might have read y (into a register) before the the while loop not knowing that the variable could be modified externally.

Without that clue, I would have probably assumed that it always works... And it probably does in many cases (platform, compiler and compiler options).

Philippe Mori

Software200718-Jun-11 15:17

Software2007

18-Jun-11 15:17

Well his answer was that this wouldn't happen on a single processor, but would happen on multi CPU ssytems... He said that the instructions at may get out of order as the compiler might re-arrange instructions for optimization for example, so it might load x before it does y... he said this could be avoided obviously by locks which I mentioned to him, but he said that there is also this idea of "memory fence" where you tell it by some commad "asm something..", which tells the compiler to put a fence between the instructions so the instructions don't get re-arranged...

Chuck O'Toole18-Jun-11 15:59

Chuck O'Toole

18-Jun-11 15:59

This is a classic case and should be something you learn to watch for in any multithreaded application.

You have multiple variables (x, y) that must be in a consistent state (when x != 0, y must have the correct value) in order for the relationship between them to be correctly interpreted.

The only way to ensure this state is to use some form of mutual exclusion around the Setter and the Tester so that both variables are looked at as a single atomic item.

As Philippe said, it might work on some platforms and some compilers but if you are ever to take your multithreaded programming skills to other platforms then you need to learn how to avoid cases like this in the general case and use the tools in the multithreaded API to control access.

Software200719-Jun-11 13:53

Software2007

19-Jun-11 13:53

I am not sure if you understood my post. I did answer that I would use mutex to synchrnonize and avoid wrong results. But, his question was explain how I can get the wrong result in this case and this had nothing to do with synchronization. He wanted me to see if I understood "memory fence" which I clearly never heard of to not allow instructions to be out of order while compiler optimization. Smile | :)