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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 Notification of Changes in Run
State section in Chapter 2, Using Loadable Compiled Modules, of this manual, for a detailed
discussion of run state change callbacks.
DllMain
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.
Releasing Resources when a DLL Unloads
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