Re: thread safety and std::map

Hi,
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
criterion..
first I cannot get the delete portion to work , what am i missing
here.
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
Digz

#include<map>
#include<iostream>
#include<boost/thread/thread.hpp>
#include<boost/thread/mutex.hpp>
#include<boost/bind.hpp>
#include<time.h>

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++);
           }
         else
           ++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));
 t.join();
 u.join();
}

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