Re: Size of the Message loop - C / C++ / MFC Discussion Boards

Razanust24-Dec-09 7:47

24-Dec-09 7:47

In the following code when we print the list why do we do curr = curr->next ;

#include<stdlib.h>
#include<stdio.h>

struct list_el {
int val;
struct list_el * next;
};

typedef struct list_el item;

int main() {
item * curr, * head;
int i;

head = NULL;

for(i=1;i<=3;i++) {
curr = (item *)malloc(sizeof(item));
curr->val = i;
curr->next = head;
head = curr;
}

curr = head;

while(curr) {
printf("%d\n", curr->val);
curr = curr->next ;
}
}

Re: Linked Lists.

C^hris L^osinger24-Dec-09 8:58

C^hris L^osinger

24-Dec-09 8:58

Razanust wrote:
why do we do curr = curr->next ;

to get to the next item in the list... ?

image processing toolkits | batch image processing

Re: Linked Lists.

includeh1024-Dec-09 18:50

includeh10

24-Dec-09 18:50

Code should be:

int main() 
{
item *head,*prev,*curr;
int i;

for(i=0;i<3;i++)
{
  curr=(item *)malloc(sizeof(item));
  curr->val=i+1;

  if(i==0) head=curr;
  else     prev->next=curr;

  prev=curr;
}

curr->next=NULL;

......
}

But I did not test...

Re: Linked Lists.

Bram van Kampen27-Dec-09 12:56

Bram van Kampen

27-Dec-09 12:56

Hi,

In your code, 'curr' is at all times the item you are currently looking at, I assume tat that's why you called it 'curr'

curr=curr->next, advances your pointer. So, after that statement, curr points at the next item of your list.

In that way, your printf will print each value in your list, by starting at the head, and then by iterating thru the list items.

BTW Was this a School Assignment? Surely looks like one.
Smile | :)

Bram van Kampen

Size of the Message loop

vc++_fragrance23-Dec-09 23:14

vc++_fragrance

23-Dec-09 23:14

Hi,
In windows programming all the messages will be placed in the message queue for processing.

Let us assume I have a dll which continuously raises events.
These events will be placed in the Message queue.

Now my doubt is, if the message queue fills , the application will crash.

1)How to handle the scenario... so that my application should not crash.

2) How to find the size of the Message queue.

Thanks inadvance.

Re: Size of the Message loop

CPallini23-Dec-09 23:50

CPallini

23-Dec-09 23:50

From PostMessage function documentation on MSDN [^]:

There is a limit of 10,000 posted messages per message queue. This limit should be sufficiently large. If your application exceeds the limit, it should be redesigned to avoid consuming so many system resources. To adjust this limit, modify the following registry key.

HKEY_LOCAL_MACHINE
     SOFTWARE
          Microsoft
               Windows NT
                    CurrentVersion
                         Windows
                              USERPostMessageLimit

The minimum acceptable value is 4000.

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke

[My articles]

Re: Size of the Message loop

Richard MacCutchan24-Dec-09 0:01

Richard MacCutchan

24-Dec-09 0:01

Trust Microsoft to put this detail in the one function that I did not look at just now. Mad | :mad:

Re: Size of the Message loop

CPallini24-Dec-09 0:19

CPallini

24-Dec-09 0:19

You should subscribe, like I did, the
'immediately informed by Microsoft about' neewsletter. Big Grin | :-D

BTW: Merry Christmas!!!
Smile | :)

Re: Size of the Message loop

Richard MacCutchan24-Dec-09 0:59

Richard MacCutchan

24-Dec-09 0:59

Thanks for the tip, I'll give it a try. And Merry Christmas to you also.

Re: Size of the Message loop

Chris Meech24-Dec-09 2:16

Chris Meech

24-Dec-09 2:16

Changing the limit is one alternative. Another is to check the return value from PostMessage. If the queue is full and the message can not be posted, I believe PostMessage returns a boolean false to indicate failure. Just wanted to suggest another way of looking at the problem. Smile | :)

Happy Holidays to everyone.

Chris Meech
I am Canadian. [heard in a local bar]
In theory there is no difference between theory and practice. In practice there is. [Yogi Berra]

Re: Size of the Message loop

Rajesh R Subramanian24-Dec-09 2:33

Rajesh R Subramanian

24-Dec-09 2:33

Chris Meech wrote:
Changing the limit is one alternative. Another is to check the return value from PostMessage. If the queue is full and the message can not be posted, I believe PostMessage returns a boolean false to indicate failure. Just wanted to suggest another way of looking at the problem.

Good Lord, so you've worked with that sort of horrendous applications? (I'm into that club too and I ended up rewriting an app once)

BTW, Merry Christmas to you!

“Follow your bliss.” – Joseph Campbell

Re: Size of the Message loop

Chris Meech24-Dec-09 2:47

Chris Meech

24-Dec-09 2:47

I have worker thread classes that query a database and post registered messages to controls. The loop that posts always seems to run faster than the UI thread can process. So I use a lock to throttle the worker thread when this happens. It was a difficult bug to deal with. Smile | :)

Chris Meech
I am Canadian. [heard in a local bar]
In theory there is no difference between theory and practice. In practice there is. [Yogi Berra]

Re: Size of the Message loop

Richard MacCutchan23-Dec-09 23:59

Richard MacCutchan

23-Dec-09 23:59

vc++_fragrance wrote:
1)How to handle the scenario... so that my application should not crash.

Your message loop should take care of this by processing every message on the queue. Only if the loop does not handle the messages is this likely to happen.

vc++_fragrance wrote:
2) How to find the size of the Message queue.

I'm not sure if this information is available, but you could check the MSDN documentation[^] for further information.

Re: Size of the Message loop

Rajesh R Subramanian24-Dec-09 2:21

Rajesh R Subramanian

24-Dec-09 2:21

Either SendMessage can be used, or if you're afraid of the sender being taken into a deadlock, consider SendMessageTimeout[^]

But more importantly, why is it that your DLL needs to continuously raise events and clobber the message queue? May be if you could explain your scenario, someone here might be able to suggest a better alternative.

I say it because if you increase the message queue size on one computer, your program will still not work on another computer whose message queue size is the default! This is not a pretty sight.

“Follow your bliss.” – Joseph Campbell

error "a required resource was unavailable".

Le@rner23-Dec-09 22:41

Le@rner

23-Dec-09 22:41

HI all,

i m receiving this error,when i print records with large number of pages like 50 or above.

when i debug the code i m note able to find out this error.

please help me for this.

thanks in advance.

To accomplish great things, we must not only act, but also dream;
not only plan, but also believe.

passing a variable inside thread

Le@rner23-Dec-09 18:46

Le@rner

23-Dec-09 18:46

Hi all,

I want to pass a CString variable to my worker thread.

How can i do it...

Thanks in advance

To accomplish great things, we must not only act, but also dream;
not only plan, but also believe.

Re: passing a variable inside thread

Adam Roderick J23-Dec-09 18:59

Adam Roderick J

23-Dec-09 18:59

Change Allocation to Heap if scope of thread is unknown
pass the TCHAR*.
CString csTemp( "Sample data" );
LPTSTR lpszData = new TCHAR[csTemp.GetLength()+1];
_tcscpy(lpszData , csTemp)

Величие не Бога может быть недооценена.

Re: passing a variable inside thread

Rajesh R Subramanian23-Dec-09 21:57

Rajesh R Subramanian

23-Dec-09 21:57

Adam Roderick J 09 wrote:
Change Allocation to Heap if scope of thread is unknown
pass the TCHAR*.
CString csTemp( "Sample data" );
LPTSTR lpszData = new TCHAR[csTemp.GetLength()+1];
_tcscpy(lpszData , csTemp)

While a pointer to the CString object on the heap itself can be passed to the thread, why bother creating a character array on the heap and deal with allocation size manually? Smile | :)

Adam Roderick J 09 wrote:
LPTSTR lpszData = new TCHAR[csTemp.GetLength()+1];

That won't work as expected with Unicode strings.

“Follow your bliss.” – Joseph Campbell

Re: passing a variable inside thread

CPallini23-Dec-09 22:25

CPallini

23-Dec-09 22:25

Rajesh R Subramanian wrote:
Adam Roderick J 09 wrote:
LPTSTR lpszData = new TCHAR[csTemp.GetLength()+1];

That won't work as expected with Unicode strings.

Why not?

Re: passing a variable inside thread

Rajesh R Subramanian23-Dec-09 22:32

Rajesh R Subramanian

23-Dec-09 22:32

CPallini wrote:
csTemp.GetLength()+1

Because of the +1 part, which makes a wrong assumption of the size of an Unicode character! It could have been multiplied by sizeof(TCHAR), that would make it work, but is not very clean (IMHO).

There's generally no reason to write C-Style code with manual memory allocation (deciding the size, etc.,) while using MFC, or even C++. Using a CString instead is much more elegant.

“Follow your bliss.” – Joseph Campbell

Re: passing a variable inside thread

Adam Roderick J23-Dec-09 22:39

Adam Roderick J

23-Dec-09 22:39

I think, there is a small misunderstanding.
for MBCS
LPTSTR lpszData = new TCHAR[csTemp.GetLength()+1];
Here we are allocating for TCHAR so it will allocate 2 bytes for every new operator.
so if
csTemp = _T("ab" )
size allocation with above allocation will be 4 +2 =6 bytes.
Smile | :)

Величие не Бога может быть недооценена.

Re: passing a variable inside thread

CPallini23-Dec-09 23:05

CPallini

23-Dec-09 23:05

Rajesh R Subramanian wrote:
Because of the +1 part, which makes a wrong assumption of the size of an Unicode character! It could have been multiplied by sizeof(TCHAR), that would make it work, but is not very clean (IMHO).

Nope:

LPTSTR lpszData = new TCHAR[csTemp.GetLength()+1];

Looks correct to me, whatever CString is of wide characters or not.

Rajesh R Subramanian wrote:
It could have been multiplied by sizeof(TCHAR)

It is already taken into accout by the new operator.
Smile | :)

Re: passing a variable inside thread

Adam Roderick J23-Dec-09 22:34

Adam Roderick J

23-Dec-09 22:34

For first case the reason is performance, CString is a bit heavy Smile | :)

2.That won't work as expected with Unicode strings.
Why it wont work?
Can you please give me the reason. Wink | ;)

Величие не Бога может быть недооценена.

Re: passing a variable inside thread

Rajesh R Subramanian24-Dec-09 1:38

Rajesh R Subramanian

24-Dec-09 1:38

With #2, my apologies. I overlooked the whole thing.

But I don't think you need optimise for performance by not using CString (C'mon, you're already using MFC). There will be absolutely no usable performance gain.

“Follow your bliss.” – Joseph Campbell

Re: passing a variable inside thread

Rajesh R Subramanian23-Dec-09 21:49

Rajesh R Subramanian

23-Dec-09 21:49

The variable you pass to the thread should be put on the heap. You could use it from within the thread and delete if it is not needed any more.

UINT ThreadFunc(LPVOID p)
{
  CString *pStr = (CString*)p;
  //use pStr here
  //delete pStr if it should be.
  return FALSE;
}

CMyDlg::SpawnThread()
{
  CString *pSz = new CString(_T("Some value"));
  AfxBeginThread(ThreadFunc, pSz);
}

“Follow your bliss.” – Joseph Campbell

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.