WIN32 Threads

Question 1

WaitFor{Single|Multiple}Object(s)

Answer 1

Single:

• hObject
• dwTimeOut: if zero, returns immediately after the state of the object has been tested.

Multiple:

• nCount: cannot exceed the maximum (usally 64)
• lpHandles: array of handles of objects to wait for
• fWaitAll: if true, waits for all the objects in the array (at most 64), otherwise asa one gets released
• dwTimeOut

Possible return values:

• WAIT_OBJECT_0: for WFSO or WFMO when fWaitAll is true.
• WAIT_OBJECT_0 + n (where 0 <= n <= nCount): thanks to this it is possible to determine the index of the object that just terminated in the array passed.
• WAIT_TIMEOUT: timeout elapsed before the object was released
• WAIT_FAILED: the call to wfso or wfmo failed
• WAIT_ABANDONED_0: used for mutex handles

Question 2

Waiting on more than 64 objects

Answer 2

Wait for all the first group, then go to the next

for(i = 0; i < N; i+=MAXIMUM_WAIT_OBJECTS)

WaitForMultipleObjects(min(MAXMIMUM_WAIT_OBJECTS, N-i), &threadH[i], TRUE, INFINITE);

Sliding window waiting

while(N > 0) {

return_val = WaitForMultipleObjects(min(MAXIMUM_WAIT_OBJECTS, N), threadH, FALSE, INFINITE);

index = (int) return_val - (int) WAIT_OBJECT_0;

CloseHandle(threadH[index]);

…

threadH[index] = threadH[N-1];

… Free threadData …

threadData[index] = threadData[N-1];

N–;

}

Question 3

CreateThread

Answer 3

include

CreateThread:

• lpsa
• dwStackSize: byte size for new thread stack (0 for default)
• lpStartAddr: pointer to function
• lpThreadParm: pointer passed as thread argument
• dwCreationFlags
  
  • if 0 -> immediately run
  • if CREATE_SUSPENDED the new thread will need to be resumed (ResumeThread) to run)
• lpThreadId points to DWORD that received threadId

Question 4

Thread identifiers

Answer 4

Terminated threads will exists until their handle will be closed.

Other threads can retrieve exitcodes of terminated threads.

Threads can retrieve informations about others.

Question 5

Resume and suspend

Answer 5

• Resume
• Suspend

Threads can increment or decrement suspend count of another thread.

Both resume and suspend return the previous suspend count;

This techiniques are not safe for general synch. but useful to avoid some race conditions.

Question 6

Thread priority

Answer 6

• It is possible for a thread to change or to determine a thread’s priority, either from the same process or from another.
• Thread priorities are relative to the process base priority.
• Avoid real time properties for user processes, be carefult with high priorities, ensure fairness!