Re: thread safety and std::map

"digz" <>
5 Mar 2007 19:37:59 -0800
On Mar 4, 1:43 pm, "Jim Langston" <> wrote:

"digz" <> wrote in message

I am trying to write a program which has two threads one of them write
to a map , and the other one deletes entries based on a certain
first I cannot get the delete portion to work , what am i missing
also is it possible/correct that the removeKeyValue function acquire
the mutex lock only during the call to map.erase(), and not the during
the whole iteration process as i have done here( i logically felt it
was safer to lock up the whole iteration ) but that can take up
massive amounts of time when the addKeyValue function would wait for
the lock to be released...

Thanks in advance


boost::mutex _mutex;
typedef boost::mutex::scoped_lock Lock;
typedef std::map <int, time_t>::iterator mapIter;
std::map <int, time_t> timerMap;

void addKeyValue (std::map < int, time_t > &map) {
 int i = 0;
 time_t t;
 while (true) {
       Lock guard (_mutex);
       map[++i] = time (&t);
       std::cout << "added: " << i << ", " << t << std::endl;
     } //end lock
     sleep (1);
   } //end while

void removeKeyValue (std::map < int, time_t > &map) {
 time_t t;
 while (true) {
     Lock guard1 (_mutex);
     mapIter end = map.end ();
     for (mapIter beg = map.begin (); beg != end;) {
         if (time (&t) - (*beg).second > 3) {//anything older than 3
seconds needs attention
             std::cout << "removing: " << (*beg).first << ", " <<
(*beg).second << std::endl;
             map.erase (beg++);
       } //end for
     } //end lock scope
   } //end while true

int main ()
 boost::thread t (boost::bind (addKeyValue, timerMap));
 boost::thread u (boost::bind (removeKeyValue, timerMap));


comp.programming.threads may be a better newsgroup for this question.

You say, "I cannot get the delete portion to work" how is it not working?
Is it not deleting the entries or locking up or... ?

Will post to the correct list ,however to be clear
It is not deleting the entries , when i try gdb debug , the begin !=
end test always fails
and the deleting thread is never able to enter the iteration loop,
In normal execution , i only see stuff getting added (the cout
but none of getting deleted..

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