"printf" appears to be non-deterministic in Qt? - c++

Question

I know "printf" is standard-c and should be deterministic. But when run in Qt I see a more non-deterministic response(clock cycles). Could this be due to Qt adding some "pork" to its response?
I have multiple threads that make call to function that uses a mutex. When one thread enters it set a switch so the others can't until it is done. Things appeared to work ok for acouple seconds and then threads appeared to be killed off from 10 to 1 thread. So I tried adding a delay: (k=k+1: no help), then (looping k=k+1: no help), (usleep works), and so does (printf) work at creating a random delay and allowing all threads to continue running.
void CCB::Write(int iThread)
{
static bool bUse = false;
bool bDone = false;
char cStr[20];
int posWrite;// = *m_posWrite; // issue of posWrite be altered with next extrance
long k = 0;
long m = 0;
m_threadCount++;
while(bDone == false){
if(bUse == false){
bUse = true;
posWrite = *m_posWrite;
memcpy(m_cmMessageCB + posWrite, &m_cmMessageWrite, sizeof(typeCanMessage));
memset(cStr, '\0', 20);
memcpy(cStr, (m_cmMessageCB + posWrite)->cMessage, 11); //fails: every 20
*m_posWrite = *m_posWrite + 1;
if(*m_posWrite == m_iNBufferLength)
*m_posWrite = 0;
bDone = true;
bUse = false;
}else if(bUse == true){
//why are threads being killed ?
// printf("T%d_%d ", iThread, m_threadCount);//non-deterministic value ?
usleep(1);//non-deterministic value
//k++;//delay of a couple clock cycles was not enough
/*
for(k = 0; k < iThread * 100; k++){//deterministic and fails to resolve thread problem
m++;
}
*/
}
}
}