To print data to a file

I have overloaded << operator such that it write to file and also on the console. I have created 8 threads to the same function, and I want to output

 hello
  hi

If I run this thread routine in an infinite loop th开发者_运维百科e o/p in the file is

hello

hi

hello

hi

hello

hi

ello

i

hello

hi

hello

hi

llo

i

hello

hi

This has no pattern. What is the problem? I am able to print it properly on console but not into a file.

Are you locking a mutex when you are sending data to a stream? The c++ stream classes are not thread-safe.

Create a queue and write to it. Dequeue queue in desired granularity - one line for example (I guess that that's what you want from your example).

Something like:

CRITICAL_SECTION cs;
list<string> _queue;
void Write(string line) 
{
    EnterCriticalSection(&cs);
    _queue.push_back(line); 
    LeaveCriticalSection(&cs);
}
ThreadProc()
{
    while (!_shouldStop)
    {
        Dequeue();
        Sleep(100);
    }
}
void Dequeue()
{
    EnterCriticalSection(&cs);
    if (!_queueIsEmpty()) 
    {
         string line=_queue.front(); 
         _queue.pop_front();
         stream << line;
    }
    LeaveCriticalSection(&cs);
}

This is not C code - this is just a example, there are lot's of other things to have in mind, just to name few of them:

• your granularity might vary, I used line here
• you might want to use dequeue<> or some similar FIFO structure
• you might want to WaitForSingleObject() instead of Sleep()-ing to have better control for cases when you want to stop
• create singleton object for the stuff and use it where you need it (I guess that you are creating some kind of logging facility?)

Actually you have a problem of synchronization between thread.

For example with 2 thread: (cout of a string is representing by "print" character).

thread1  thread2
print h
print e 
print l
         print h
print l
print o
         print e

print \n
         print l
         print l
         print o
         print \n

the result will be:

helhloe
llo

you need to make sure that your function is not used by two thread at the same time.

function_cout_hello()
  lock()
  file << ...
  unlock()

like that, two different thread can not use the function at the same time.