National Instruments 320685D-01 ユーザーズマニュアル

Chapter 3

Windows 95/NT Compiler/Linker Issues

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 National Instruments Corporation

3-3

LabWindows/CVI Programmer Reference Manual

callback in a DLL, you must take special care. Refer to the 

 section in Chapter 2, 

, of this manual, for a detailed 

discussion of run state change callbacks. 

Each DLL can have a 

DllMain

 function, except that the Borland compiler uses 

DllEntryPoint

 as the name. The operating system calls the 

DllMain

 function with various 

messages. To generate the template for a 

DllMain

 function, use the Insert Constructs 

command in the Edit menu of a Source window. 

Use caution when inserting code in the 

PROCESS_ATTACH

 and 

PROCESS_DETACH

 cases. In 

particular, avoid calling into other DLLs in these two cases. The order in which 
Windows 95/NT initializes DLLs at startup and unloads them at process termination is not 
well-defined. Thus, the DLLs you want to call might not be in memory when your 

DllMain

 

receives the 

PROCESS_ATTACH

 or 

PROCESS_DETACH

 message.

It is always safe to call into the LabWindows/CVI Run-time Engine in a run state change 
callback, as long as you do so before calling 

CloseCVIRTE

.

When a program terminates, the operating system disposes resources your DLL allocates. If 
your DLL remains loaded throughout program execution, it does not need to dispose 
resources explicitly when the system unloads it at program termination. However, if the 
program unloads your DLL during program execution, it is a good idea for your DLL to 
dispose of any resources it allocates. It can release resources in the 

DllMain

 function in 

response to the 

PROCESS_DETACH

 message. The DLL can also release resources in a function 

that it registers with the ANSI C 

atexit

 function. The system calls the function you register 

when the DLL receives the 

PROCESS_DETACH

 message.

If your DLL calls into the LabWindows/CVI Run-time Engine DLL, it can allocate resources 
such as user interface panels. If a program unloads your DLL during execution, you might 
want to dispose these resources by calling functions such as 

DisposePanel

 in the 

LabWindows/CVI Run-time Engine. On the other hand, as explained in the previous section, 
it is generally unsafe to call into other DLLs in response to the 

PROCESS_DETACH

 message. 

To solve this dilemma, you can use the 

CVIRTEHasBeenDetached

 function in the 

Utility Library. It is always safe to call the 

CVIRTEHasBeenDetached

 function. 

CVIRTEHasBeenDetached

 returns FALSE until the main Run-time Engine DLL, 

cvirte.dll

, receives the 

PROCESS_DETACH

 message. Consequently, if 

CVIRTEHasBeenDetached

 returns FALSE, your DLL can safely call functions in 

LabWindows/CVI Run-time Engine to release resources. 

00ProRef.book : 06chap03.fm  Page 3  Monday, March 9, 1998  3:23 PM