Recently,i was reading the tutorial of winpcap. But the following words confused me.
“Why do we use pcap_next_ex() instead of the old pcap_next()? Because pcap_next() has some drawbacks. ”
Isn't ex means old?
I will be glad if someone can help me!
"ex" stands for "extended". This is a common practice in some libraries, and I think Win32 is the main notorius example thereof.
See this for reference on the Win32 case:
Adding -Ex to the name of a type in .Net
Related
I maintain a program which can be automated via COM. Generally customers use VBS to do their scripting, but we have a couple of customers who use Matlab's ActiveX support and are having trouble calling COM object methods with a NULL parameter.
They've asked how they do this in Matlab - and I've been scouring Mathworks' COM/ActiveX documentation for a day or so now and can't figure it out.
Their example code might look something like this:
function do_something()
OurAppInstance = actxserver('Foo.Application');
OurAppInstance.Method('Hello', NULL)
end
where NULL is where in another language, we'd write NULL or nil or Nothing, or, of course, pass in an object. The problem is this is optional (and these are implemented as optional parameters in most, but not all, cases) - these methods expect to get NULL quite often.
They tell me they've tried [] (which from my reading seemed the most likely) as well as '', Nothing, 'Nothing', None, Null, and 0. I have no idea how many of those are even valid Matlab keywords - certainly none work in this case.
Can anyone help? What's Matlab's syntax for a null pointer / object for use as a COM method parameter?
Update: Thanks for all the replies so far! Unfortunately, none of the answers seem to work, not even libpointer. The error is the same in all cases:
Error: Type mismatch, argument 2
This parameter in the COM type library is described in RIDL as:
HRESULT _stdcall OurMethod([in] BSTR strParamOne, [in, optional] OurCoClass* oParamTwo, [out, retval] VARIANT_BOOL* bResult);
The coclass in question implements a single interface descending from IDispatch.
I'm answering my own question here, after talking to Matlab tech support: There is no equivalent of Nothing, and Matlab does not support this.
In detail: Matlab does support optional arguments, but does not support passing in variant NULL pointers (actually, to follow exactly how VB's Nothing works, a VT_EMPTY variant, I think) whether as an optional argument or not. There is documentation about some null / pointerish types, a lot of which is mentioned in my question or in various answers, but these don't seem to be useable with their COM support.
I was given a workaround by Matlab support using a COM DLL they created and Excel to create a dummy nothing object that could be passed around in scripts. I haven't managed to get this workaround / hack working, and even if I had unfortunately I probably could not redistribute it. However, if you encounter the same problem this description might give you a starting point at least!
Edit
It is possible this Old New Thing blog post may be related. (I no longer work with access to the problematic source code, or access to Matlab, to refresh my memory or to test.)
Briefly, for IUnknown (or derived) parameters, you need a [unique] attribute for them to legally be NULL. The above declaration required Matlab create or pass in a VT_EMPTY variant, which it couldn't do. Perhaps adding [unique] may have prompted the Matlab engine to pass in a NULL pointer (or variant containing a NULL pointer), instead - assuming it was able to do that, which is guesswork.
This is all speculation since this code and the intricacies of it are several years behind me at this point. However, I hope it helps any future reader.
From the mathworks documentation, you can use the libpointer function:
p = libpointer;
and then p will be a NULL pointer. See that page for more details.
See also: more information about libpointer.
Peter's answer should work, but something you might want to try is NaN, which is what Matlab ususally uses as a NULL value.
In addition to using [] and libpointer (as suggested by Peter), you can also try {}.
The correct answer for something in VB that is expecting a Nothing argument, is to somehow get a COM/ActiveX Variant which has a variant type of VT_EMPTY. (see MSDN docs which reference marshaling behavior for Visual Basic Nothing)
MATLAB may do this with the empty array ([]), but I'm not sure.... so it may not be possible purely in MATLAB. Although someone could easily write a tiny COM library whose purpose is to create a Variant with VT_EMPTY.
But if the argument has the [optional] atttribute, and you want to leave that optional argument blank, you should not do this. See the COM/ActiveX docs on Variants which say under VT_EMPTY:
VT_EMPTY: No value was specified. If an optional argument to an Automation method is left blank, do not pass a VARIANT of type VT_EMPTY. Instead, pass a VARIANT of type VT_ERROR with a value of DISP_E_PARAMNOTFOUND.
Matlab should (but probably does not) provide methods to create these objects (a "nothing" and an "optional blank") so you can interface correctly with COM objects.
I need some help with a LINQ query in VB.Net, please.
I have this simple group statement:
Dim drivers = From d In DriversOwners _
Group d By Key = d.UnitNumber Into Group _
Select Key, DriverGroup = Group
This works, and returns me the data I need to work with. Now I want to iterate through the groups, using a For Each construct. like this:
For Each x In drivers
Next
However, the compiler is barking at me, telling me that the
"'x' is not accessible in this context because it is 'Friend'."
Anyone know what I am doing wrong here?
Thanks in advance.
After digging and digging, I finally found the answer to this problem. Talk about obtuse!
Enabling LINQ in a .NET Framework 3.5 Project
When you move a project to .NET
Framework 3.5, a reference to
System.Core and a project-level import
for System.Linq (in Visual Basic only)
are added automatically. If you want
to use LINQ features, you must also
turn Option Infer on (in Visual Basic
only) [my emphasis].
When I changed the target framework from 2.0 to 3.5, Visual Studio automatically added the System.Core assembly, and automatically imported the System.Linq namespace. Now why in the world did it not also set Option Infer to "On" as well?
http://msdn.microsoft.com/en-us/library/bb398197.aspx
Looks to me like the variable x was declared earlier - as a class field, a method parameter, or a local variable.
Am I right?
I'm not sure if you've solved this or not. I had the exact same problem today and what ended up working for me was to not use a single-letter variable in the For Each loop. My code was the same as yours:
For Each x In a
...
Next
When I changed the code to the following it worked:
For Each retVal in a
...
Next
I also found the same 'Friend' error behavior for any single-letter variable.
I have no idea why it behaves this way, but I thought I'd pass this along in case this question is still out there.
This question is a little silly, but sometimes it's tough to figure out how to name things correctly. The conversion will parse a config file into XML and vice versa. I want to call the program MyCompany.Config2Xml, but the program also needs to be able to "Xml2Config".
I propose: ConfigParser
In keeping with SqlDataReader, TextReader, XmlReader etc I'd just call it ConfigReader and ConfigWriter.
Or, you could just go the serialization approach and then not have to worry about naming conventions.
CC for short:
ConfigConverter ?
Rather than ConfigParser as proposed by jeffamaphone (+1 for nice username), make it a verb:
parse-config
This makes it read nicely in scripts:
if ! parse-config < config-file > config.xml; then
exit 1
fi
I think it helps a lot to think about the verbs (methods) you intend to use with the class and the role the class plays in the application.
In other words if you envision the operation to be {class}.Get() or {class}.Load() then ConfigParser might be a good choice.
If on the other hand you have a corresponding {Class}.Set() or {class}.Save() operation then something like ConfigManager would be a better choice, particularly if the class will be used to isolate the application from the persistence of its configuration.
If the role of the class is nothing more than part of a standalone application or a step in a longer running process then I would would lean more towards class and method pairs that are more like Convert.ToXml() Convert.ToConfig() or Translate.FromXml() Translate.FromConfig().
General term seems like it would be format convertor, or transformatter (by analogy with transcoder). In terms of the specific names you discuss, I think I'd go with ConfigConvertor.
DaTransmogrifier
UberConvertPlus
Xml2Config2Xml
ConfiguratorX
'XConTrans'
or simply 'Via'
ConfXmlSwitcher :P
I'm trying to store the names of some variables inside strings. For example:
Dim Foo1 as Integer
Dim Foo1Name as String
' -- Do something to set Foo1Name to the name of the other variable --
MessageBox.Show(Foo1Name & " is the variable you are looking for.")
' Outputs:
' Foo1 is the variable you are looking for.
This would help with some debugging I'm working on.
Well, you can clearly just set Foo1Name = "Foo1" - but I strongly suspect that's not what you're after.
How would you know which variable you're trying to find the name of? What's the bigger picture? What you want may be possible with reflection, if we're talking about non-local variables, but I suspect it's either not feasible, or there's a better way to attack the problem in the first place.
Does this example from msdn using reflection help?
One solution would be to use an associative array to store your variables. Once, I did this in .Net, but I think I wrote a custom class to do it.
myArray("foo1Name") = "foo1"
Then, you can just store a list of your variable names, or you can wrap that up in the same class.
if( myArray(variableName(x)) == whatImLookingFor ) print variableName(x) & "is it"
I think this really depends on what you are trying to debug. Two possible things to look at are the Reflection and StackTrace classes. That said when your program is compiled, the compiler and runtime do not guarantee that that names need to be consistent with the original program.
This is especially the case with debug vs. release builds. The point of the .PDB files (symbols) in the debug version are to include more information about the original program. For native C/C++ applications it is strongly recommended that you generate symbols for every build (debug+release) of your application to help with debugging. In .NET this is less of an issue since there are features like Reflection. IIRC John Robbins recommends that you always generate symbols for .NET projects too.
You might also find Mike Stall's blog useful and the managed debugger samples.
For finding the variable name, see: Finding the variable name passed to a function
This would apply to VB.Net as well.
Does anyone here use VB.NET and have a strong preference for or against using IsNothing as opposed to Is Nothing (for example, If IsNothing(anObject) or If anObject Is Nothing...)? If so, why?
EDIT: If you think they're both equally acceptable, do you think it's best to pick one and stick with it, or is it OK to mix them?
If you take a look at the MSIL as it's being executed you'll see that it doesn't compile down to the exact same code. When you use IsNothing() it actually makes a call to that method as opposed to just evaluating the expression.
The reason I would tend to lean towards using "Is Nothing" is when I'm negating it becomes "IsNot Nothing' rather than "Not IsNothing(object)" which I personally feel looks more readable.
I find that Patrick Steele answered this question best on his blog: Avoiding IsNothing()
I did not copy any of his answer here, to ensure Patrick Steele get's credit for his post. But I do think if you're trying to decide whether to use Is Nothing or IsNothing you should read his post. I think you'll agree that Is Nothing is the best choice.
Edit - VoteCoffe's comment here
Partial article contents: After reviewing more code I found out another reason you should avoid this: It accepts value types! Obviously, since IsNothing() is a function that accepts an 'object', you can pass anything you want to it. If it's a value type, .NET will box it up into an object and pass it to IsNothing -- which will always return false on a boxed value! The VB.NET compiler will check the "Is Nothing" style syntax and won't compile if you attempt to do an "Is Nothing" on a value type. But the IsNothing() function compiles without complaints. -PSteele – VoteCoffee
You should absolutely avoid using IsNothing()
Here are 4 reasons from the article IsNothing() VS Is Nothing
Most importantly, IsNothing(object) has everything passed to it as an object, even value types! Since value types cannot be Nothing, it’s a completely wasted check.
Take the following example:
Dim i As Integer
If IsNothing(i) Then
' Do something
End If
This will compile and run fine, whereas this:
Dim i As Integer
If i Is Nothing Then
' Do something
End If
Will not compile, instead the compiler will raise the error:
'Is' operator does not accept operands of type 'Integer'.
Operands must be reference or nullable types.
IsNothing(object) is actually part of part of the Microsoft.VisualBasic.dll.
This is undesirable as you have an unneeded dependency on the VisualBasic library.
Its slow - 33.76% slower in fact (over 1000000000 iterations)!
Perhaps personal preference, but IsNothing() reads like a Yoda Condition. When you look at a variable you're checking its state, with it as the subject of your investigation.
i.e. does it do x? --- NOT Is xing a property of it?
So I think If a IsNot Nothing reads better than If Not IsNothing(a)
I agree with "Is Nothing". As stated above, it's easy to negate with "IsNot Nothing".
I find this easier to read...
If printDialog IsNot Nothing Then
'blah
End If
than this...
If Not obj Is Nothing Then
'blah
End If
VB is full of things like that trying to make it both "like English" and comfortable for people who are used to languages that use () and {} a lot.
And on the other side, as you already probably know, most of the time you can use () with function calls if you want to, but don't have to.
I prefer IsNothing()... but I use C and C#, so that's just what is comfortable. And I think it's more readable. But go with whatever feels more comfortable to you.
I'm leaning towards the "Is Nothing" alternative, primarily because it seems more OO.
Surely Visual Basic ain't got the Ain't keyword.
I initially used IsNothing but I've been moving towards using Is Nothing in newer projects, mainly for readability. The only time I stick with IsNothing is if I'm maintaining code where that's used throughout and I want to stay consistent.
Is Nothing requires an object that has been assigned to the value Nothing. IsNothing() can take any variable that has not been initialized, including of numeric type. This is useful for example when testing if an optional parameter has been passed.