National Instruments 320685D-01 Compiled Modules that Contain Asynchronous Callbacks

Chapter 2 Using Loadable CompiledM odules

National Instruments Corporation 2-7 LabWindows/CVI Programmer Reference Manual

Windows 95/NT unloads DLLs at process termination is not well-def ined. Therefore, t he

DLL you call into might no longer be loaded. This can cause a general protection fault.

Nevertheless, when you use your DLL in a program in the LabWindows/CVI

development environment, it is often necessary to call into DLLs to release resources

after each run. To solve this dilemma, use conditional code to release resources only if

you are running in the LabWindows/CVI development environment. An example

follows.

#include <utility.h>

switch (runState)

{

case kRunState_Stop:

if (! InStandaloneExecutable())

{ /* call into other DLLs to release resources */ }

/* release resources, including unloading DLLs */

break;

}

It is always safe to call into the LabWindows/CVI Run-time Engine in a run st ate change

callback.

• If your DLL uses global v ariables that can become stale after each prog ram e xecution in

the LabWindows/CVI development environment, re-initialize the variables in response

to the kRunState_Start or kRunState_Stop message. For example, memory that

you allocate using LabWindows/CVI ANSI C functions such as malloc or calloc is no

longer valid when you restart your program. If your DLL has global variables that point

to allocated memory, set those pointers to NULL in response to the kRunState_Start

or kRunState_Stop message.

Compiled Modules that Contain Asynchronous Callbacks

A compiled module can call a source code function asynchronously. This can happen through

interrupts or signals. In Windows 95/NT, the compiled module can call the source code

function from a thread other than the main thread. The call takes place asynchronously with

respect to the normal execution of the source code in your program.

The execution and debugging system in the LabWindows/CVI development environment is

not prepared to handle this asynchronous execution. Consequently, the compiled module must

announce to LabWindows/CVI that it is calling asynchronously into source code. It does this

by calling EnterAsyncCallback before calling the function, and calling

ExitAsyncCallback after calling the function. EnterAsyncCallback and

ExitAsyncCallback have one parameter, which is a pointer to a buffer of size

ASYNC_CALLBACK_ENV_SIZE. You must pass the same buffer into ExitAsyncCallback

that you passed into EnterAsyncCallback because the buffer stores state information. The

definition of ASYNC_CALLBACK_ENV_SIZE and the prototypes for these two functions are

in libsupp.h.