I have to implement zero copy method in a program in Qt C++,I read about it and got transferto() method.as described below:
public void transferTo(long position, long count, WritableByteChannel target);
but i can't understand how it works.It is written that Internally, it depends on the underlying operating system's support for zero copy.
So I tried to write a function but i didn't get the meaning of "position" and "channel" here.
Can anyone help me?
There is no cross-platform way to do zero copy in C++, and there is no zero-copy API in Qt. You can implement it for your target OS using provided API:
Linux supports zero copy through system calls such as sys/socket.h's
sendfile, sendfile64, and splice. Windows supports zero copy through
the TransmitFile API.
Source
Related
I am newbie in Qt, I have dll from 3rd party (NFC reader), this is C Win32 library (I don have header file, only dll file) so in C# i call it by using DLLImport Attribute. Now I want to using Qt to make the application and I don know how to do it. I hope somone will help me. Please explain clearly because I am very very fresh with Qt.
Thank you so much
Use QLibrary to load the library - it's a bit easier than using the native API (LoadLibrary etc.).
Use resolve("symbolname") on the QLibrary instance to get a pointer to a particular function. You then need to cast that pointer to the signature of the function you're actually going to call, and call via this cast pointer.
The documentation for your NPC Reader's DLL should describe the signatures of the functions, you needed that information for C# anyway.
I've developed a program and I am trying to make this program work with a controllable light source manufactured by some other company. I've emailed the company and they have agreed to send me their external library as a DLL.
I have developed all of my software using Qt 4.8.1 and it has been compiled using MSVC2008.
The controllable light's DLL was compiled in Visual Studio 2008 and was written in either C++ or C# (the manufacturer is uncertain). All I have been given is the DLL and a text file saying that I must:
Add the DLL as a reference to my project
Add using LightName; to the top of the class
Instantiate an instance of the object like so: LightName *ln = new LightName();
Call function void turnOn() with the newly created LightName instance.
Firstly, I find it odd that an external library requires me to instantiate an instance of their object especially when its for a simple piece of hardware.
Secondly, the other company has provided me with no interface files.
My question is:
How can I possibly link to a C++ DLL and expose the functions nested in this library without having an interface header file in a Qt environment? Is there someway to make an interface for an external library?
I have already attempted using QLibrary and doing the following:
QLibrary myLib("mylib");
typedef void (*MyPrototype)();
MyPrototype myFunction = (MyPrototype) myLib.resolve("mysymbol");
if (myFunction)
myFunction();
However, this doesn't work because the DLL I was given was not a C DLL and I have no interface so Qt doesn't have a clue about what symbols it needs to resolve.
I've attempted to display all the definitions exported from my DLL using the dumpbin /EXPORTS command. Unfortunately this was unable to produce anything. I was hoping that I would get some sort of mangled C++ from this that I could then unscramble to make my own header.
I've attempted to use the dependency walker(very useful tool) however it couldn't resolve any symbols to give me some function definitions.
QLibrary only helps you if the library has functions exported as C symbols. If that is C++ you can dump the symbol table and look if that is sufficient for you. Names must be demangled: try to look for dumpbin or similar. It is however possible that you can't do this, it depends on how the symbols have been defined. In that case you'll have to ask for the header: read this.
Well it's absolutely legal to ask you for "instantiating an instance of their object". It's been simply their design decision to make the dll interface object oriented (as contrary to plain extern "C"). QtCore.dll is just someone else's DLL too, and you are instantiating their objects all the time.
But it also means that you will have harder time to call the DLL. The symbols are not "C" symbols (you can't export class that way) so QLibrary can't do anything for you. You can try dumpbin /EXPORTS from the DLL and then tediously unmangle them to reconstruct the class declaration. Luckily there are tools to help you (even online)
But not providing a header file for such DLL is completely dumb in the first place.
I am writing a Qt application that calls QProcess::startDetached("wscript.exe script.vbs") to show the delete confirmation dialog in Windows.
this is the script:
Set objShell = CreateObject("Shell.Application")
Set objFolder = objShell.Namespace("-")
Set objFolderItem = objFolder.ParseName("-")
objFolderItem.InvokeVerb("Delete")
the arguments for Namespace and ParseName are from the arguments passed to the script.
This may be inefficient because it opens an external application first before running the script. I was wondering if i can run VBScripts in a Qt application.
If not, what alternatives can i do?
My VBScript is very weak, so I'm not 100% sure I understand what you are trying to do. My assumption is that you are trying to delete a folder, but want to give the user the normal confirmation box and animation while the action is occurring. If that is not correct, please let me know and I will remove this answer.
A few ideas:
You could call the Windows API directory within your C++ code to do this. I believe the correct call would be to use IFileOperation (Vista and later) or SHFileOperation (pre-Vista)
Qt already has message box dialogs. Although you might not get the exact same functionality as the native shell, you could use this (QMessageBox::warning) and then delete the folder using QDir. This would also be cross-platform portable.
If you stick with the VBScript, I doubt you would see any performance issues unless this is being called many, many times in a loop or something. You know, the old "premature optimization is the root of all evil" thing.
You should read up on the IActiveScript COM interface. You can create an instance of an interpreter that implements IActiveScript to provide a runtime for evaluating scripts. VBScript and JScript can both be used for this and a number of other third-party scripting languages also provide IActiveScript support.
The overview for working with this is you create a language runtime (an instance of VBScript for instance) then add some custom objects to it. Typically if you are embedding an interpreter into your application then exposing an Application object is a good place to start. This can be just an IDispatch interface or something more concrete with an IDL generated typelibrary and all the trimmings. Once you have added the necessary named items into the runtime you load one or more scripts. Any public functions or subroutines declared in the scripts now get exposed via the IDispatch interface of the live runtime once you switch its state to active or running. To actually run the script program, I invoke the Main function for my stuff - you could choose some other scheme as applicable to your environment.
The nice thing about ActiveScripting, is to change language you just change the runtime CLSID. So if people prefer Perl they can use PerlScript or PythonScript etc. Your Application object remains the same hence you don't have to write additional code to support the new languages. The only requirement is that everything is COM.
I'm using Qt.
I could not find the function or class to start application at booting time.
In windows, for example, add register to HKMU/Software/Microsoft/Windows/CurrentVersion/Run.
I wanna know the function/class like above example.
You can use QSettings in order to set a value at Windows Registry.
QSettings settings("HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\CurrentVersion\\Run",
QSettings::NativeFormat);
and then use the setValue function.
However this will work only in Windows so it is better to use an #ifdef before calling it.
The only thing I can think of is the QtService framework. It is available for all the major platforms (look here). It might not be exactly what you were looking for but it may help.
It is also important to mention that a Windows service has no direct access to the graphical interface. This means that if the application is graphical, you'll probably have to redesign it.
There is no such API in Qt, you have to search for 3rdparty library or write whole code on your own.
I need to make so that my application can have only one instance running at a time. Also when it's launched with a command line parameter ( like when registered to open certain file types ) it should pass the parameter to an existing instance and quit immediately without displaying graphical interface. You all probably know what I mean. The framework used is Qt 4, and it seems like it must have some facilities for that. Any ideas?
There is a Qt Solutions project doing this:
Qt Single Application
There are several ways to do inter process communication. Examples can be found in Qt's examples section.
It's also possible to implement a this sort of class oneself using QSharedMemory
(see QSharedMemory::attach() ). It's capable of being used for both determining whether other instance is already running and communicating/sending messages. Except some pointer magic and memory copying it's quite straightforward.