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We used to have a problem like that when users would have the option to change background and foreground colors in the app. They would make the background black and the foreground white, but the app would force a white background during printing but leave the foreground white.
They didn't have print preview however, so I'm not sure if this could be related but I thought I'd post it as an idea.
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I am currently involved a project that requires that I have random access to multiple files on disk. I have a single 'file writer' object that handles writes, but I am unsure about how to proceed with reads.
The question is: If I want to be able to service multiple 'reads' at the same time to a single file should I have a single object (an fstream) that is synchronized (using a lock) or multiple fstream objects that are independent. I want to take advantage of my raid hardware as well as multiple processors throughout the application. My initial thought is that having multiple 'reader' objects leaves synchronization up to the OS, and that using some type of locking mechanism (such as a criticalsection/mutex) could slow performance.
Any help here is much appreciated.
On a side note: If I'm just writing buffers of data (or 1 byte aligned structures) does it make more sense to just use stdio functions?
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KenThompson wrote: My initial thought is that having multiple 'reader' objects leaves synchronization up to the OS
What OS and where is it documented that it performs such synchronization. And by this I assume you mean synchronization to the "write operations".
KenThompson wrote: and that using some type of locking mechanism (such as a criticalsection/mutex) could slow performance.
"would" slow performance, it's not in question. However without the OS performing synchronization, of which I am unaware, you would have to do it to avoid reading corrupted data. You could optimize by creating a far more complex mechanism like Databases do of managing what "parts" of the file are locked for writing. Which of course raises the question of why you wouldn't just just use a database because they already implement everything you require.
Also you are not clear if this is across threads or across processes. The later synchronization is far more expensive.
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I'm already synchronizing write and read operations. By this I mean that I keep track of what is currently being done to the file. Basically, the writer never goes backwards, so whatever has been written is fair game in regards to reading. The only random access is reading. A database for this application isn't acceptable.
The question remains though. What approach is the best? Have a single reader per file handling many requests. (ie. setg to the offset) or having several fstream objects created that read independently, in a shared mode.
I didn't mean that the OS, in this case Windows, prevents corruption when modifying files. I should of been more clear in my statement. I meant to say: My initial thought is that having multiple 'reader' objects is perfectly acceptable and not a performance hit. In addition, in a raid situation would it not make more sense to create multiple file streams to the same file due to the very nature of multiple disk heads? I'm not all that aware of where there is any performance to gain based on implementation. I can only assume that if I issue two reads to the same file, via two streams, that the raid controller (in my case raid 5) would out perform a setg operation. Maybe not with 2 reads, but maybe 100s of reads per second. Does this make sense?
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KenThompson wrote: Does this make sense?
Absolutely. It would seem, on the surface (not a valid assessment), that multiple readers would be much more efficient. You might consider just implementing that and then run a profiler and see if that area even stands out at all. Sometimes (usually) profiler results will surprise you. 
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Thanks, I needed some sanity here. What type of profiler do you use? I've been playing with a few intel products, but the cost sours me to them.
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QC mostly runs them (several). I think the last time I used one myself it was an Intel product. The company has lots of money so cost is not an issue.
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KenThompson wrote: A database for this application isn't acceptable.
So its better to write half of a database on your own instead of using e.g. SQLlite[^]?
KenThompson wrote: Does this make sense?
I think no. You would only ever work on buffers managed by the OS. The Harddisk-Heads would ascend and decend its cylinders all the time, doing the read/write out of order.
Though I speak with the tongues of men and of angels, and have not money, I am become as a sounding brass, or a tinkling cymbal.
George Orwell, "Keep the Aspidistra Flying", Opening words
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SQLite is designed for use with databases sized in kilobytes or megabytes not gigabytes. Therefore, it is unacceptable.
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IOCP - not just for sockets, they can be used with any IFS based HANDLE e.g. sockets, files, pipes, ...
Scatter/Gather I/O functions:
http://msdn2.microsoft.com/en-us/library/aa365472.aspx[^]
http://en.wikipedia.org/wiki/Vectored_I/O[^]
...cmk
The idea that I can be presented with a problem, set out to logically solve it with the tools at hand, and wind up with a program that could not be legally used because someone else followed the same logical steps some years ago and filed for a patent on it is horrifying.
- John Carmack
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Thank you, this suggestion has proved quite fruitful!
Ken
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Can someone please post the code (or an outline) for creating a UDP Server, just want the data to be transmitted even when the client is down - is that possible.
I get a bit confused with the calls.
Andy.
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Andy202 wrote: Can someone please post the code (or an outline) for creating a UDP Server
You mean like the MFC/C++ article here on CodeProject titled "A simple UDP time server and client for beginners"?
No, no one can do that.
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Hello comunity,
is it posible to protect environment variable in multithreaded application,
to ensure that only one thread can use this variable, for manipulations!
I want to protect one vector to be sure that only one thread can remove item from this vector?
I think on something like this:
//
LockVector;
DeleteItemFromVector;
UnlocVector;
From here allow all access to my vector! :->
thanks in advance
termal
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EnterCriticalSection();<br />
LeaveCriticalSection();
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termal wrote: ...to ensure that only one thread can use this variable, for manipulations!
Have you considered a synchronization object such as a critical section or a mutex?
"A good athlete is the result of a good and worthy opponent." - David Crow
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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Hello,
thanks for reply,yes i use critical section, but only in one function,
is it posible to protect one variable programwide,
so wenn one function use vector other functions have to wait until this vector is free?
Example:
FunktionOne() // use vector
FunktionTwo() // must wait because vector is in use by FunktionOne()
Is that posible or i want to much?
with best regards
termal
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termal wrote: is it posible to protect one variable programwide,
so wenn one function use vector other functions have to wait until this vector is free?
Example:
FunktionOne() // use vector
FunktionTwo() // must wait because vector is in use by FunktionOne()
Is that posible or i want to much?
That almost sounds like an event.
"A good athlete is the result of a good and worthy opponent." - David Crow
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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termal wrote: is it posible to protect environment variable in multithreaded application
Absolutely. The standard answer, is a mutex. Mutual exclusion prevents anything other than one thread from executing through a region of code. A primary way is with CriticalSections. However, it is important to note that you will be accessing the region of code in linear space. That is, you will reduce the code execution for that task to single threaded operation, so you want to make sure it is not your primary occupation of time.
There are other ways, google lock-free algorithms. A good CAS or CAS2 will give you the base structure to build a non-blocking synchronization system, but you will have to think on it. Qt uses something called "implicit sharing" meaning you want to share read access with all threads all the time, but you want to lock it for writing.
You can also use record locking, if you understand how to use a CAS or CAS2 step properly which will allow you to add/change a single vector item and block that one item from reading until the update step is completed, but allow all other reads to all other vector items to continue without delays.
You can also just pull down Intel's thread building blocks and use their threadsafe vector and be done with it all.  http://www.threadingbuildingblocks.org/[^]
_________________________
Asu no koto o ieba, tenjo de nezumi ga warau.
Talk about things of tomorrow and the mice in the ceiling laugh. (Japanese Proverb)
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Hi,
I want to extract a zipped file which is stored in Oracle DB table as HUGEBLOB. I want to modify that one and put it back. The file extension is .xml.gz. I want to bring this .xml.gz file using my application based on MFC.
Can any body give me any suggestion/help on how to get the file? I dont want to get te data into any buffer. I want to get the physical zipped file.
Can anyone help me out?
Thanks
Sourav
-- modified at 15:40 Monday 10th September, 2007
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Found this article
<ahref="http: www.codeproject.com="" useritems="" codecave.asp#introduction0"="">http://www.codeproject.com/useritems/codecave.asp#Introduction0[^]
Which had me very excited because I thought it was the answer to my problem.
But soon I realized I just dont have the skill to implement it.
I am lookin (Praying) for someone to take pity on me and show me how to do it or do it for me (PLEASE).
Here are the details (Sry its lenghty).
I co-own a gaming Halo clan.
We operate 16 dedicated online servers.
When naming the servers we are permited 64 spaces.
Thru time people have found that by using Alt Codes in the game name they can move to the top of the online listing as it precedes the alpha numerical listing.
Now some people have found a way to circumvent that as well. By deleteinfg the naming of the server in the .ini file the games .exe file defaults to a generic "HALO" nameing. Then by hex editing that loacation they can rename the server.
The problem in this method is that we are now limited to onlt 26 spaces for naming. I need more. The names and descriptions of our serves are about 40 spaces.
I have been able to identify the starting and ending byte of the location in the .exe file. There is alos plenty of unused space in the program. I was hoping to add a call and return to one of those locations so I can store the longer name.
won't someone please help me?
-- modified at 10:40 Wednesday 5th September, 2007
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selkov wrote: ...or do it for me (PLEASE).
On a volunteer site? Most of us here have paying jobs, and only provide help when we have time.
"A good athlete is the result of a good and worthy opponent." - David Crow
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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Well I have exahusted all the options I have. If you have any other ideas please feel free to share them with me.
I might be wrong but I assume if a person knew how to do this it would not be a difficult project.
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Hi,
Ultimate Grid supports Multifonts OnSetup().
But how can I use Editing cell (multiline + multifont)as
CRichEditCtrl class with user-defined actions?
Thanks
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I included Window Mouse Hook in my app, when compiled I get this:
"error C2065: 'WH_MOUSE_LL' : undeclared identifier"
???
g_hMouseHook = SetWindowsHookEx(WH_MOUSE_LL,MouseProc,GetModuleHandle(NULL),0);
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