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Timeout long running scripts

To prevent long running scripts to freeze the application it may be necessary to add a way to timeout the execution. This article presents two different ways to do so.

# With the line callback

The line callback feature can be used to perform some special treatment during execution of the scripts. The callback is called for every script statement, which makes it possible to verify if the script has
executed for too long time and if so suspend the execution to be resumed at a later time.

Before calling the context's Execute method, set the callback function like so:

int ExecuteScriptWithTimeOut(asIScriptContext *ctx)
{
// Define the timeout as 1 second
DWORD timeOut = timeGetTime() + 1000;
// Set up the line callback that will timout the script
ctx->SetLineCallback(asFUNCTION(LineCallback), &timeOut, asCALL_CDECL);
// Execute the script
int status = ctx->Execute();
// If the status is asEXECUTION_SUSPENDED the script can
// be resumed by calling this function again.
return status;
}
// The line callback function is called by the VM for each statement that is executed
void LineCallback(asIScriptContext *ctx, DWORD *timeOut)
{
// If the time out is reached we suspend the script
if( *timeOut < timeGetTime() )
ctx->Suspend();
}

Take a look at the sample Events to see this working.

Observe that if the script is compiled with asEP_BUILD_WITHOUT_LINE_CUES, the line callback will be invoked less frequently, though it is guaranteed to be invoked at least for every loop or function call.

# With a secondary thread

A second thread can be set up to suspend the execution after the timeout. This thread can then be put to sleep so that it doesn't impact performance during the execution. When the thread wakes up it should call the context's Suspend method.

The below shows some possible code for doing this. Note that the code for setting up the thread is fictive, as this is different for each target OS.

// The variables that are shared between the threads
asIScriptContext *threadCtx;
int threadId;
// This function will be executed in the secondary thread
void SuspendThread()
{
// Put the thread to sleep until the timeout (1 second)
Sleep(1000);
// When we wake-up we call the context's Suspend method
ctx->Suspend();
}
// This function sets up the timeout thread and executes the script
int ExecuteScriptWithTimeOut(asIScriptContext *ctx)
{
// Set the shared context pointer before creating the thread
threadCtx = ctx;
// Create the thread that will immediately go to sleep
threadId = CreateThread(SuspendThread);
// Execute the script
int status = ctx->Execute();
// Destroy the secondary thread before releasing the context
DestroyThread(threadId);
// Clear the global variables
threadId = 0;
threadCtx = 0;
// If the status is asEXECUTION_SUSPENDED the script can
// be resumed by calling this function again.
return status;
}

Observe that this way of doing it is safe even if the AngelScript library has been built without multithread support.

asIScriptContext
The interface to the virtual machine.
Definition: angelscript.h:2654
asIScriptContext::Execute
virtual int Execute()=0
Executes the prepared function.
asFUNCTION
#define asFUNCTION(f)
Returns an asSFuncPtr representing the function specified by the name.
Definition: angelscript.h:673
asIScriptContext::SetLineCallback
virtual int SetLineCallback(asSFuncPtr callback, void *obj, int callConv)=0
Sets a line callback function. The function will be called for each executed script statement.
asIScriptContext::Suspend
virtual int Suspend()=0
Suspends the execution, which can then be resumed by calling Execute again.
asCALL_CDECL
A cdecl function.
Definition: angelscript.h:226