multimap.clear() does not free its memory

[Toon Knapen <toon.knapen@xxxxxx>]

After filling up an std::multimap I print the memory consumed by my 
process (currently using mallinfo() which is probably not the best way 
to do it but anyway I verified in /proc and there the same numbers appear).

Next I clear() the multimap and I see that the consumed memory is still 
the same. Afterwards I create a new map of the same size but the memory 
consumption does not go up (as expected) but after the clear again the 
memory consumption does not go down either.

So I guess the memory handler is being smart and does lazy-delete the 
memory allocated when filling up the multimap. This is why the second 
multimap does not take 'extra' space.

However after deleting the second map and allocating 1M chars, the 
memory consumption goes up with 1M bytes and thus the memory that 
'should be' freed when deleting the multimap is not re-used by the 
character-array.

So finally I want to 'force' the memory used by the multimap to become 
_really_ free but how can I do that.

I'm using gcc 3.2.2 on linux 2.4.18 on an old pentium 3 but observe the 
same with gcc-3.4 and with gcc on an altix-sp3 machine.

Below is my test-program and the output generated on my machine

<begin program>

#include <map>
#include <malloc.h>
#include <iostream>

void report_mem()
{
  static struct mallinfo l_mallinfo ;
  l_mallinfo = mallinfo();
  std::cout << "mem: " <<  l_mallinfo.uordblks + l_mallinfo.hblkhd << 
std::endl ;
}

int main()
{

  report_mem() ;

  {
    std::multimap< int, int > map ;

    for(int i = 0 ; i < 100000 ; ++i ) {
      map.insert( std::pair< int, int >( 0, 0 ) ) ;
    }

    std::cout << "\nfirst map is filled" << std::endl ;
    report_mem() ;

    map.clear() ;

    std::cout << "\nfirst map is cleared" << std::endl ;
    report_mem() ;
  }

  {
    std::multimap< int, int > map ;

    for(int i = 0 ; i < 100000 ; ++i ) {
      map.insert( std::pair< int, int >( 0, 0 ) ) ;
    }

    std::cout << "\nsecond map is filled" << std::endl ;
    report_mem() ;

    map.clear() ;

    std::cout << "\nsecond map is cleared" << std::endl ;
    report_mem() ;
  }

  std::cout << "\nAll maps should be destroyed" << std::endl ;
  report_mem() ;

  new char[1000000]
  report_mem() ;

  return 0 ;
}

<end program>


<begin output>

mem: 0

first map is filled
mem: 2497184

first map is cleared
mem: 2497184

second map is filled
mem: 2497184

second map is cleared
mem: 2497184

All maps should be destroyed
mem: 2497184
mem: 3500704

<end output>

Thanks for any help,
Toon