Re: thread safety and std::map

From:
"digz" <Digvijoy.C@gmail.com>
Newsgroups:
comp.lang.c++
Date:
5 Mar 2007 19:37:59 -0800
Message-ID:
<1173152278.894158.215650@64g2000cwx.googlegroups.com>
On Mar 4, 1:43 pm, "Jim Langston" <tazmas...@rocketmail.com> wrote:

"digz" <Digvijo...@gmail.com> wrote in message

news:1173031748.463232.207410@j27g2000cwj.googlegroups.com...

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();
}

----------------


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... ?


Thanks
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
messages)
but none of getting deleted..

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