National Instruments 320685D-01 DllMain , Releasing Resources when a DLL Unloads

Chapter 3 Windows95/NT Compiler/Linker Issues

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 CompiledModules, 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

Windows95/NT initializes DLLs at startup and unloads them at proces s 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 pr evi ous sectio n,

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.