LabVIEW调用C/C++ DLLs

什么是DLL封装，什么时候需要使用DLL封装？

解决方案

所谓封装就是指为另一种软件提供兼容性接口的程序。由于第三方DLL的开发设计通常是为了从C语言（或者其它类似的底层语言）中调用，而不从是LabVIEW中调用，因此当使用LabVIEW进行应用程序开发时，往往会用到封装。例如，有的DLL会返回指针或者其它复杂的数据结构，而这在LabVIEW中却无法很容易的实现。

编写一个DLL封装，可以类比在C语言环境下，按照该DLL原始开发者设计的调用DLL方式，来编写一个完全独立的程序。反过来，这个封装程序也是专门针对在LabVIEW中调用DLL所设计的。也就是说，这个新的用C编写的“封装” 程序将原来的C程序（DLL）封装起来，形成一个接口层。使用封装的好处在于，不再需要使用原来的DLL的源代码，也不需要对这些源代码进行任何改动。

Calling C/C++ DLLs ContainingSimple and Complex Datatypes from LabVIEW

Overview

LabVIEWdevelopers often have to call C/C++ DLLs from LabVIEW and because of thedifferences between LabVIEW and C/C++ datatypes, passing and receiving datafrom these DLLs can be intimidating at first.

This exampleseeks to show developers how to pass and receive different types of dataranging from simple datatypes like numerics (int, float, double, etc), arraysand strings to more complex data types like pointers, struct (cluster), arraysof structs, and even arrays of structs that have arrays of structs in them.

In addition,this example seeks to show developers the different ways to pass and receivedata in LabVIEW with a DLL – when functions pass data by value, return datausing a return statement, or return data using pass by reference.

This examplecovers the following use cases:

Attached Files

• LabVIEWWrapper.zip: LabVIEW Library (.lvlib)     containing examples on how to call each individual function.

• PassingDataSampleDLL.zip: Source code for the ANSI C     DLL as well as the compiled DLL. Written in CVI, but tested to compile in     Visual Studio as well.

• CTestApplication.zip: Examples of calling the C DLL     from ANSI C. Written in Visual Studio.

Example Hierarchy

The majority ofVIs in the example were automatically generated using the Import Shared Library Wizard. The wizarddoes not cover all cases, and could not generate VI wrappers for somefunctions, and some VI wrappers were incomplete. However, it gives us a reallygood starting point for calling our DLL.

For an exampleon using the Import Shared Library Wizard, refer to:
Tutorial: Creating Wrapper VIs for C/C++ DLL functionsusing the Import Shared Library Wizard

For anexplanation on what the Import Shared Library Wizard missed, refer to thesection titled Caveats with the Import Shared Library Wizard.

The VIs in theexample are arranged in the following virtual folders in the library:

• Import Shared Library Wizard Generated VIs: These are the     auto-generated VIs that were the output of the Import Shared Library     Wizard

• Dereferencing Pointers: VIs that assist     with dereferencing pointers. They use either MoveBlock or     GetValueByPointer. See:
  Dereferencing Pointers from C/C++ DLLs in LabVIEW

• Custom Controls For Structs: Strict Typedef     controls that represent the struct datatypes in the DLL

• Completed, Corrected and Added VIs: Additional VIs that     were added in order to work around the caveats encountered using the     Import Shared Library Wizard

Caveats with the Import SharedLibrary Wizard

The followingcases were not completely handled by the Import Shared Library Wizard:

VI Not Generated

In the followingcase, VIs were not generated at all:

• When a function returns anything other than a     numeric, string or void. Pointers are fine because they are treated as     numerics.
       In these cases, the data should be treated as a pointer and the data type     in LabVIEW for the return value should be set as Unsigned Pointer-Sized     Integer

In the examplelvlib, these VIs were manually created and given the suffix “Added” to thename.

Incomplete VIs

In the followingcases, the VIs that were generated were incomplete in some fashion:

• When a function returned a pointer and the pointer     wasn’t automatically dereferenced.
       In these cases, the pointer needs to be dereferenced using either     MoveBlock or GetValueByPointer. See:
  Dereferencing Pointers from C/C++ DLLs in LabVIEW
  

In the examplelvlib, these VIs completed in a new VI that has the suffix “Complete” in thename.

Incorrect VIs

In the followingcases, the VIs that were generated had some incorrect behavior:

• When a function returned a pointer to a pointer     (strings and 2D Arrays)
       These must be dereferenced twice.

• When a function took a struct (cluster) with an     array/string in it
       The individual elements should be dereferenced individually.

In the examplelvlib, these VIs were corrected in a new VI and given the suffix “Corrected” tothe name.

List of Functions and VIs (byData Type and Call Type)

The following isa list of functions exposed by the C DLL as well as the VI that shows how toproperly call the particular function.

1.     Numeric (Integer)   

a.     Returning a value (return statement)
Function: int ReturningAValue_Integer (void);
VI: Returning A ValueInteger.vi
Auto Generated VI Status: Working     

b.     Returning a pointer (return statement)
Function: int*ReturningAValue_PointerToInteger (void);
VI: Returning A Value Pointer To Integer Complete.vi
Auto Generated VI Status: Incomplete     

c.     Passing a parameter (Pass by value)
Function: int PassingParameters_Integer(int x, int y);
VI: Passing Parameters Integer.vi
Auto Generated VI Status: Working     

d.     Returning values by reference (pass by ref)
Function: voidReturningValuesByReference_Integer (int x, int y, int *sum);
VI: Returning ValuesBy Reference Integer.vi
Auto Generated VI Status: Working     

2.     Array of Numerics   

a.     Returning a value (return statement)
Function: int*ReturningAValue_ArrayOfIntegers (int length);
VI: Returning A ValueArray Of Integers Complete.vi
Auto Generated VI Status: Incomplete     

b.     Returning a pointer (return statement)
Function: N/A (arrays variables are already pointers)
VI: N/A
Auto Generated VI Status: N/A     

c.     Passing a parameter (Pass by value)
Function: intPassingParamters_ArrayOfIntegers (int x[], int length);
VI: Passing ParamtersArray Of Integers.vi
Auto Generated VI Status: Working     

d.     Returning values by reference (pass by ref)
Function: void ReturningValuesByReference_ArrayOfIntegers (int *x, int length,int **newArray, int *newLength);
VI: Returning ValuesBy Reference Array Of Integers Complete.vi
Auto Generated VI Status: Incomplete     

3.     Strings   

a.     Returning a   value (return statement)
Function: char*ReturningAValue_String (void);
VI: Returning A ValueString.vi
Auto Generated VI Status: Working     

b.     Returning a pointer (return statement)
Function: N/A (arrays variables are already pointers)
VI:N/A
Auto Generated VI Status: N/A     

c.     Passing a parameter (Pass by value)
Function: intPassingParamters_String (char *str);
VI: Passing ParamtersString.vi
Auto Generated VI Status: Working     

d.     Returning values by reference (pass by ref)
Function: voidReturningValuesByReference_String (char *str, char **newString);
VI: Returning ValuesBy Reference String Corrected.vi
Auto Generated VI Status: Incorrect     

4.     2 Dimensional Array   

a.     ReturningAValue_2DArrayOfIntegers
Function: int**ReturningAValue_2DArrayOfIntegers (int rows, int cols);
VI: Returning A Value2D Array Of Integers Complete.vi
Auto Generated VI Status: Incomplete     

b.     Returning a pointer (return statement)
Function: N/A (arrays variables are already pointers)
VI: N/A
Auto Generated VI Status: N/A     

c.     Passing a parameter (Pass by value)
Function: intPassingParamters_2DArrayOfIntegers (int *x, int rows, int cols);
VI: Passing Paramters2D Array Of Integers Corrected.vi
Auto Generated VI Status: Incorrect     

d.     Returning values by reference (pass by ref)
Function: voidReturningValuesByReference_2DArrayOfIntegers (int rows, int cols, int***newArray);
VI: Returning ValuesBy Reference 2D Array Of Integers Complete.vi
Auto Generated VI Status: Incomplete     

5.     Simple Struct (with basic datatypes)   

a.     Returning a   value (return statement)
Function: structsimpleStructCircle ReturningAValue_SimpleStruct(void);
VI: Returning A ValueSimple Struct Added.vi
Auto Geerated VI Status: Not Generated     

b.     Returning a pointer (return statement)
Function: structsimpleStructCircle* ReturningAValue_PointerToSimpleStruct(void);
VI: Returning A ValuePointer To Simple Struct Added.vi
Auto Generated VI Status: Not Generated     

c.     Passing a parameter (Pass by value)
Function: floatPassingParamters_SimpleStruct (struct simpleStructCircle circle);
VI: Passing ParamtersSimple Struct.vi
Auto Generated VI Status: Working     

d.     Returning values by reference (pass by ref)
Function: voidReturningValuesByReference_SimpleStruct (struct simpleStructCircle circle,struct simpleStructCircle *largerCircle);
VI: Returning ValuesBy Reference Simple Struct.vi
Auto Generated VI Status: Working     

e.     Returning array of struct
Function: voidReturningValuesByReference_ArrayOfSimpleStruct (struct simpleStructCircle**circleArray, int length);
VI: Returning ValuesBy Reference Array Of Simple Struct Complete.vi
Auto Generated VI Status: Incomplete  

6.     Complex Struct (with structs and arrays)   

a.     Returning a   value (return statement)
Function: structcomplexStructPolygon ReturningAValue_ComplexStruct (void);
VI: Returning A ValueComplex Struct Added.vi
Auto Generated VI Status: Not Generated     

b.     Returning a pointer (return statement)
Function: structcomplexStructPolygon* ReturningAValue_PointerToComplexStruct (void);
VI: Returning A ValuePointer To Complex Struct Added.vi
Auto Generated VI Status: Not Generated     

c.     Passing a parameter (Pass by value)
Function: intPassingParamters_ComplexStruct (struct complexStructPolygon triangle);
VI: Passing ParamtersComplex Struct Corrected.vi
Auto Generated VI Status: Incorrect     

d.     Returning values by reference (pass by ref)
Function: voidReturningValuesByReference_PointerToComplexStruct (struct complexStructPolygon*triangle);
VI: Returning ValuesBy Reference Pointer To Complex Struct Corrected.vi
Auto Generated VI Status: Incorrect     

e.      Returning array of struct
Function: void ReturningValuesByReference_ArrayOfComplexStruct(struct complexStructPolygon **triangles, int length);
VI: Returning ValuesBy Reference Array Of Complex Struct Corrected.vi
Auto Generated VI Status: Incorrect         

需要说明的是，上述的例程和文档，都是可以下载的，双击即可打开，其中压缩文件是可以采用粘贴复制的方式，拷贝到硬盘上。这不是图片，各位小伙伴看到后尝试一下，这个问题就不用加微信咨询了。有关LabVIEW编程、LabVIEW开发等相关项目，可联系们。