How to IMPLEMENT a Mutex in C++ ? - C / C++ / MFC Discussion Boards

Hi,

I have a MFC application that uses Microsoft Access tables.
The result of my program writes data into a Access database so at the end, I have a Table with the results.

I would like to know: How can I allow the user to save that RESULT TABLE where they SPECIFY the name of the table and the location of where to save it?

If anyone has any ideas, Please let me know.

Thanks in Advance

Steve

Hi Everyone,

How do I display the data in a Microsoft Access Table with an MFC application?
Can I call up a table from Access for display directly?

If anyone knows, Please let me know.

Thanks in Advance!

Steve

I'm writting an MMC utility to do some remote admin. I need to be able to connect to a computer by computer name on the local network and stop/start services. I know this can be done through Windows 2000/NT management tools but I need this capability in my MMC snap-in. Does anyone know what library or functions I can use for this task. Just give me a name and I can search on it. Thanks.

Hi there,
Could somebody help me to figure out how to restore Visual Studion (version 6) settings, of a user in WinNT.
Thus because the user keeps getting an error message: "memory cannot be read" when executing the MSDEV.EXE,
but under the admin and other accounts it's still working fine.

Thanks for any help, or idea

How to convert a BSTR into LPSTR?

#include <atlconv.h>


void convert(BSTR bstr)
{
  USES_CONVERSION;
  LPSTR sz = W2A(bstr);  // convert to a non-const string
  LPCSTR sz2 = W2CA(bstr);  // convert to a const string
}

The new strings allocated by the W2A and W2CA macros are on the stack, so they will vanish once the function returns.

I want that my application will work in english and in my language.
How can I do it?

This looks like a school project, but it's not.

Having used mutexes and semaphores a zillion times, "finally the time has arrived to write my own implementation for use in an embedded system without OS." Frown | :-(

I did some research on the matter and I found Peterson's Algorithm (for 2 processes, threads or whatever) and Lamport's bakery algorithm (for N processes, threads or whatever). Wink | ;)

There is no need to reinvent "hot water" again, an existing algorithm will do just fine.

For info,

Peterson's Algorithm:

int flag[2] = { FALSE, FALSE } // flag[i] indicates that Pi wants to
// enter its critical section.
int turn = 0; // turn indicates which process has
// priority in entering its critical
// section.

// mutexbegin:
flag[i] = TRUE;
turn = 1 - i;
while (flag[1 - i] && turn == 1 - i)
;

// mutexend:
flag[i] = FALSE;

Now Lamport's Bakery algorithm.

Assumptions:

NPROCS is the number of processes.

max(int *array) returns the maximum value in array.

Each process has a unique ID, so ties on the number chosen are broken by comparing IDs.

Replace i with the appropriate process ID.

// Global initialization:
int choosing[NPROCS] = { FALSE };
int number[NPROCS] = { 0 };

// mutexbegin:
choosing[i] = TRUE;
number[i] = max(number) + 1;
choosing[i] = FALSE;
for (j = 0; j < NPROCS; ++j)
{
while (choosing[j])
;

while (number[j] != 0 && (number[j] < number[i] ||
number[j] == number[i] && j < i) )
;
}

// mutexend:
number[i] = 0;

THE QUESTION:

in the line
number[i] = max(number) + 1;
we see the function 'max'. How do I interprete this?
I thought it meant the maximum of all the numbers in number[], but that does not seam to work.
Also, any suggestions on writing a
bool CPP_Mutex::TryEnter(unsigned int PNumber)
function ?

Any input or pointers greatly appreciated! Cool | :cool:

NOTE: I know this class produces what is called a "spinning lock", but that's OK.

I tested it with a dual processor machine with the following code:

#define NPROCS 2
#undef Max
#define Max(A,B) ((A)<(B)?(B):(A))

// Implementation of LAMPORTS Bakery Algorithm

class CPP_Mutex
{
public:
CPP_Mutex()
{
memset(m_nChoosing, 0, sizeof(int)*NPROCS);
memset(m_nNumber, 0, sizeof(int)*NPROCS);
}
~CPP_Mutex()
{

}
void Enter(unsigned int PNumber)
{
m_nChoosing[PNumber] = 1;
m_nNumber[PNumber] = MAX(m_nNumber) + 1;
m_nChoosing[PNumber] = 0;
for (unsigned int teller = 0; teller < NPROCS; teller++)
{
while (m_nChoosing[teller])
;
while ( m_nNumber[teller] != 0
&& ( m_nNumber[teller] < m_nNumber[PNumber] ||
m_nNumber[teller] == m_nNumber[PNumber] && teller < PNumber)
)
;
}
}
void Leave(unsigned int PNumber)
{
m_nNumber[PNumber] = 0;
}
protected:
int MAX(int* numbers)
{
int toReturn = 0;
for (unsigned int teller = 0; teller < NPROCS; teller++)
{
toReturn = Max(toReturn, *(numbers+teller));
}
return toReturn;
}
private:
int m_nChoosing[NPROCS];
int m_nNumber[NPROCS];
};

unsigned int value = 0;
unsigned int prevvalue = 0;
bool stop = false;

CPP_Mutex theTestMutex;

void IncreaseValue(unsigned int PNumber)
{
// if (theTestMutex.TryEnter(PNumber))
{
theTestMutex.Enter(PNumber);
prevvalue = value;
value++;
assert(prevvalue == value-1);
// cerr << PNumber << ":" << value << "\t";
theTestMutex.Leave(PNumber);
}
}

DWORD WINAPI ThreadProc( LPVOID lpParameter )
{
while (!stop)
{
IncreaseValue((unsigned int) lpParameter);
}
return 0;
}

int main(int argc, char* argv[])
{
HANDLE hThread1 = NULL;
HANDLE hThread2 = NULL;
DWORD dwThreadID1 = 0;
DWORD dwThreadID2 = 0;

hThread1 = ::CreateThread(NULL, 0, ThreadProc, (void*) 0, 0, &dwThreadID1);
hThread2 = ::CreateThread(NULL, 0, ThreadProc, (void*) 1, 0, &dwThreadID2);

WaitForSingleObject(hThread1, 60000);

stop = true;

WaitForSingleObject(hThread1, INFINITE);
WaitForSingleObject(hThread2, INFINITE);

CloseHandle(hThread1);
CloseHandle(hThread2);

return 0;
}

Out of curiosity, how are you getting multiple threads if you don't have an OS?

Also, if this is an x86 processor, you can use the SMP correct incrementers in x86 assembler. These are instructions which guarantee that they will execute fully before allowing access. IIRC they are instructions such as XADD or something similar.

A Problem with the previously mentioned algorithms is that they assume "atomic" read and write operations.

I also found this solution which is, like you say is dependent on "undividable" processor instructions.

// ALGORITHM
////////////
// wait(s)
// { set s = 0 and if old value of s was 0 then
// repeat
// 'wait'
// until set s = 0 and old value of s was 1 }
// signal(s)
// { s = 1; Tell OS if appropriate }

Concerning, "how are you getting multiple threads if you don't have an OS?".

From my perspective (WinNT/W2K backend programmer, now engaged in building software for a NO-OS device), the device we are building has two "tasks" ("Threads").
It has a slow main loop (task 1) which can be 'interrupted' ("pre-empted") by the driver level (task 2..N).
The idea is to have the driver level handle the Hardware dependent stuff, e.g. detect if a keypad button that was pressed, and put it in an event queue for the slow main loop to process.
If my main loop can get interrupted at any time, I need some protection (locking) on the queue, otherwise it will get corrupted. (if it is locked by the main loop, I could put the event in a temporary queue and put it in the real queue some time later (ASAP of course) )

The standard way of avoiding corruption is:

mutex_begin()
{
disable_interrupts();
}

mutex_end()
{
enable_interrupts();
}

Which of course could lead to "event loss".

The algorithm above (based on TestAndSet) is an improvement over this, but it needs to be supported by processor instructions.
Since I am not a whizz kid in assembler (let's be honest, I never had to use it before so I never did), I am gratefull for any pointers or assembler crash courses that anyone can give me about these undividable SMP safe (i386) instructions I could use to implement a Test And Set function.

// MSDN example
int power2( int num, int power )
{
__asm
{
mov eax, num ; Get first argument
mov ecx, power ; Get second argument
shl eax, cl ; EAX = EAX * ( 2 to the power of CL )
}
/* Return with result in EAX */
}

// my would-be function that sets a value (nValue) on the target address (pTargetAddress) and returns the previous value (that was found in pTargetAddress)
int testandset(int* pTargetAddress, int nValue)
{
__asm
... // help !
}

Best regards,

Gert.

I think this is it, unless I'm mistaken.

int TestAndSet(int* pTargetAddress, int nValue)
{
__asm
{
mov edx, dword ptr [pTargetAddress]
mov eax, nValue
xchg eax, dword ptr [edx]
}
}

Anyone know of any good face recogition software? I've got two bitmaps, and I'd like to compare them to see if they are the same person.

Hello!

I have been trying to set the color of a slider I created with MFC. I managed to color it's background but didn't manage to change the slider "knob" or "path" color, how is this done?

I also have a Combobox wich I would like to change the color of as well, but it didn't work out either. I tried to check the "owner drawn and with strings on the Combobox, but I still couldn't change the colors and over the text was the picture of my icon, hmm...

Then there's the frame of the window, I beieve I should override OnNcPaint() function for this but either I'm wrong or doig it wrong. =)

And changing the color of the entire application menu would be very nice if I could do this too.

I tried to override OnPaint, but it didn't work... I could use some samples of code, since I seem to be doing something wrong when I don't get it to work. =) I heard something about "changeing the thumb and track colors using custom draw." how is this done in code?

Thanks,

/Fredrik

Try

http://www.codeproject.com/miscctrl/cfctrl.asp

Not exactly what you want, but you could work from here...

HTH

Jerry

Thanks, that helped with the colors in the ComboBox, but my real problem is the color of the little thin line around the ComboBox and the little arrow for klicking on the combobox to bring down the choices in it. Does anyone know how to override the dialog from getting these colors from the system and how to set them? I have tried a lot of things but none seems to work.

Also, how to paint the frame of the dialog window and how to override the menu colors, not just the fields in the menu, but I want contol over the whole menu and what color it has.

I really need help with this, this problem is really killing me! Smile | :)

Simple just paint over it!

Have a look here...

http://www.codeproject.com/combobox/combobox_flatcombo.asp

HTH

Jerry

I have a dialog-based application. I would like to hide the main dialog and display an icon only in the system tray. I've tried to use ShowWindow(SW_HIDE), but the dialog still appears.
In "ApplicationName.cpp" file and in "InitInstance" module, my codes look like the following:

CFwuTestDlg dlg; //1 //the main dialog
m_pMainWnd = &dlg; //2
m_pMainWnd->ShowWindow(SW_HIDE);//3
m_pMainWnd->UpdateWindow();//4
int nResponse = dlg.DoModal();//5
return FALSE;//6

I've tried to delete line 5, but Windows complains that .exe file generate error.
Could anyone help me? Thank you.

First, you do not need to hide the dialog. The way that the system tray works is that windows are minimized to be not shown.

http://www.codeproject.com/shell/systemtray.asp
http://home.socal.rr.com/samhobbs/VC/TaskbarStatusIcon.html

You certainly do not want to remove the DoModal (line 5) but you do want to remove the ShowWindow and UpdateWindow (lines 3 and 4).

Is it possible to store pointers to objects in the STL collection classes and have the collection automatically destroy the objects as the collection elements are destroyed? e.g.:

class A
{
int var1;
};

vector<a*> v;

A* obj1 = new A;
A* obj2 = new A;

v.push_back(obj1);
v.push_back(obj2);

v.clear();

The last statement here will remove all elements from the vector but won't delete the actual objects, *obj1 and *obj2. Can vector be specialised so it will delete the objects its elements point to? (This can be done fairly easily in MFC so I am hoping the same is true for STL)

No. It's not possible, at least not without modifying the STL classes (which you shouldn't do anyways, since people will expect their behavior to be consistent with what the standard dictates).

Just write a small for loop which iterates over the pointers and delets them, then call clear. Or, don't store pointers in your vector, but the actual objects themselves.

No. The trick is not to specialize the container, but to specialize the pointers you store in the container.
Use smart pointers in the collection instead of ordinary pointers.
A smart pointer will delete the object it points to, if the smart pointer gets deleted. Otherwise it behaves just like an ordinary pointer.
A smart pointer implementation that can be used as STL container element can be found at http://www.boost.org. Just look there for the class shared_ptr in the smart_ptr library.

I'm having trouble finding information on how to customize the Windows file open dialog to add, for example, a preview pane for images or something like that. I know I have to subclass the common dialog and I've found a few simple examples but nothing like adding other controls to the dialog. Can anyone point me to any resources or give me a general idea what I need to do? Thanks in advance.

Cheers,
Farzad

I want to know if I press a icon on the Toolbar, how can I run the code??? I mean like if I click a icon, and what I want to do is run a function!!!! How?
Thanks

Here's a link to an MSDN document to get you started.