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# Descriptions of multithreaded programming techniques

Article ID: 108433

Article Last Modified on 4/29/2005

APPLIES TO

- The C Run-Time (CRT), when used with:
  - Microsoft Visual C++ 1.0 Professional Edition
  - Microsoft Visual C++ 2.0 Professional Edition
  - Microsoft Visual C++ 2.1
  - Microsoft Visual C++ 4.0 Standard Edition
  - Microsoft Visual C++ 4.1 Subscription
  - Microsoft Visual C++ 5.0 Standard Edition
  - Microsoft Visual C++ 6.0 Service Pack 5

This article was previously published under Q108433

## SUMMARY

The MTDLL sample illustrates several techniques and concepts related to multithreaded programming in Win32. The sample uses a multithreaded dynamic-link library (DLL) to implement six sorting algorithms. The DLL is called into from a Microsoft Foundation Class (MFC) Libraries application that uses a dialog box as the main window. The six sorting algorithms implemented are the Bubble sort, Exchange sort, Heap sort, Insertion sort, Quick sort, and Shell sort.
Note The sample code has been built using Visual C++, version 2.0. If you are using versions 4.0 or later, you need to convert the project files to Developer Studio projects. To do so, open a workspace specifying SORTDEMO.MAK and SORTDLL.MAK file. A message box appears that prompts you for the project name and it does a conversion for you. Visual C++ 5.0 and 6.0 cannot convert the Sortdll.mak. A makefile wrapper project is generated instead.
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## MORE INFORMATION

Some of the multithreaded concepts and techniques illustrated by the sample are:

- Conditionally using __declspec(dllexport) and __declspec(dllimport) on function prototypes that reside in a header file that is included in both the DLL and the calling application.
- Using __declspec(thread) to allocate thread-local data.
- Using a critical section object to control access to the display.
- Handling termination of a currently executing thread.
- Using _beginthread() and _endthread() to handle thread creation and termination.

The calling executable and the sorting DLL in this sample also use mutual imports. That is, the executable calls routines that have been exported from the DLL, and the DLL calls routines that have been exported from the executable. The sample includes the import libraries needed to build both the DLL and the calling executable. However, if the sample is modified and additional functions are exported from both the executable and the DLL, the mutual imports could make it impossible to build either the DLL or the executable, because one must be built before the other. This situation can be worked around by using the LIB utility with the /DEF option to build an import library from a linker .DEF file. This technique is discussed in Chapter 4 of the "Programming Techniques" manual and Chapter 3 of the "Build Tools User's Guide" supplied with Visual C++ for Windows NT.
This sample does not use .DEF files to specify exported functions. It illustrates using __declspec instead. However, the sample does include .DEF files for the DLL and the executable, which can be used as starting points if at some point the LIB utility needs to be used with the /DEF option to solve the problem of mutual imports.
This sample also uses CTL3D32.DLL to implement a 3-D look for its dialog box controls. For CTL3D32.DLL to work, however, it must be copied from the multithreaded DLL sample directory to the Windows NT SYSTEM32 subdirectory, for example C:\WINNT\SYSTEM32. After copying the DLL, it must be deleted from the multithreaded DLL sample directory. If CTL3D32.DLL is not copied to the \WINNT\SYSTEM32 directory and deleted from the multithreaded DLL sample directory, a message box with the following error will be displayed when the sample is run:
This application uses CTL3D32.DLL, which has not been correctly installed.
After choosing OK to dismiss the message box, the main dialog box of the sample is displayed but it uses a white background and none of the dialog box controls will be 3-D.
You may receive a linker error of the type:
error LNK2001: unresolved external symbol _Ctl3dxxxxxxxx
This means that the Ctl3d32.lib file is not one of the library modules passed to the linker. Specify this library in the "Object/library modules:" box on the Link property page in the Project Settings dialog box.

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