What are Thread-local variables used for?

[holly7787]

Hello everyone, maybe it's a simple question but i don't get it... usually when i do multithread application i create local variable like this

/// HIFN Document your function here.
/// HIFN You may use multiple lines for documentation.
/// HIRET Document return value here.
/// HIRET You may use multiple lines for documentation.
/// HIPAR functionData/Document parameter here.
int CVICALLBACK ThreadFunction(void *functionData)
{
	int MyLocalVariable = {0};
	
	while(quit)
	{
		// do something
	}

}

and every thread have their MyLocalVariable that can contain different value, so why NI suggest to do this?

int CVICALLBACK ThreadFunction (void *functionData);
int tlvHandle;
int gSecondaryThreadTlvVal;

int main (int argc, char *argv[])
{
    int functionId;
    int *tlvPtr;

    if (InitCVIRTE (0, argv, 0) == 0)
        return -1; /* out of memory */
    CmtNewThreadLocalVar (sizeof(int), NULL, NULL, NULL, &tlvHandle);
    CmtScheduleThreadPoolFunction (DEFAULT_THREAD_POOL_HANDLE, ThreadFunction, 0, &functionId);
    CmtWaitForThreadPoolFunctionCompletion (DEFAULT_THREAD_POOL_HANDLE, functionId, 0);
    CmtGetThreadLocalVar (tlvHandle, &tlvPtr);
    (*tlvPtr)++;
    // Assert that tlvPtr has been incremented only once in this thread.
    assert (*tlvPtr == gSecondaryThreadTlvVal);
    CmtDiscardThreadLocalVar (tlvHandle);
    return 0;
}
int CVICALLBACK ThreadFunction (void *functionData)
{
    int *tlvPtr;

    CmtGetThreadLocalVar (tlvHandle, &tlvPtr);
    (*tlvPtr)++;
    gSecondaryThreadTlvVal = *tlvPtr;
    return 0;
}

the only advantage that I see is that also the main thread can have it's value.

maybe it's because of the limit as explained here?

"Although the Windows API provides a mechanism for creating and accessing thread-local variables, this mechanism limits the number of thread-local variables you can have in each process. The LabWindows/CVI Utility Library thread-local variable functions do not have this limitation. The following code demonstrates how to create and access a thread-local variable that holds an integer."

How much is the Windows API limit?

Thanks

[gdargaud · Accepted solution]

Hi Holly,

I'll start by saying there are 2 other more standard ways to define thread-local variables:

static __thread char TimeStr[80]="";

and

 thread_local static char TimeStr[80]="";

Depending on you C99/C11 compiler.

Yes, declaring a normal variable inside a function will make it local to each thread, but what if you need it static ? For instance to remember a value between calls, or to return a string ? You can do this very simply with thread local variables without complex semaphore logic. The drawback is that you cannot share the values between the threads, but then you use a normal static for that.

[holly7787]

Actually you hit the only problem that i have, and I didn't think that I can solve with this.

I have a function that i want to return a string, i have to create always a char array variable and pass it as a parameter to get it returned.

[gdargaud]

Si, e facile fare cosi:

char* SomeFunc(int arg) {
  static __thread char TimeStr[80]="";
  ...
  return TimeStr;
}

It works well for many things... but you can't use it for this, it'll fail:

printf("%s %s", SomeFunc(1), SomeFunc(10));