Re: [Boost-users] valgrind reports memory still reachable when using shared_lock and shared_mutex

Subject: Re: [Boost-users] valgrind reports memory still reachable when using shared_lock and shared_mutex
From: Rajeev Rao (rbsrao79_at_[hidden])
Date: 2009-09-02 23:14:01

I found similar problems when getting an exclusive lock via unique_lock as well. I wrote this test code to reproduce. 
I get the still-reachable problem if I use either the read() or the write() call (shared_lock or unique_lock). 
Multiple calls to these functions  did not increase the number of reachable bytes in the valgrind report. 
I understand my previous use of adopt_lock was undefined (hence i've not used it here). Is my use as described by the code correct now ? If so, is this a bug ?
<code>// boost-1.39.0#include <iostream>#include "boost/thread/locks.hpp"#include "boost/thread/shared_mutex.hpp"

using namespace std ;
boost::shared_mutex & get_mutex() {    static boost::shared_mutex global_lock;    return global_lock;}
void write() {    boost::unique_lock<boost::shared_mutex> write_lock(get_mutex()) ;    std::cout << "write" << std::endl ;}
void read() {    boost::shared_lock<boost::shared_mutex> read_lock(get_mutex()) ;    std::cout << "read" << std::endl ;}
int main(int argc,char **argv) {    write() ; // LINE A    read() ;  // LINE B    std::cout << "done" << std::endl ;    return 0 ;}</code>

--- On Thu, 9/3/09, Steven Watanabe <watanabesj_at_[hidden]> wrote:

From: Steven Watanabe <watanabesj_at_[hidden]>
Subject: Re: [Boost-users] valgrind reports memory still reachable when using shared_lock and shared_mutex
To: boost-users_at_[hidden]
Date: Thursday, September 3, 2009, 7:01 AM

AMDG

Rajeev Rao wrote:
>  When using shared_mutex class with shared_lock. I see the following behavior
>
>     boost::shared_lock<boost::shared_mutex> lock(m_mutex) ;
>
> When running valgrind
>
> ==32255== LEAK SUMMARY:
> ==32255==    definitely lost: 0 bytes in 0 blocks.
> ==32255==      possibly lost: 0 bytes in 0 blocks.
> ==32255==    still reachable: 8 bytes in 1 blocks.
> ==32255==         suppressed: 0 bytes in 0 blocks.
>
> However, if I change the code
> to     boost::shared_lock<boost::shared_mutex> lock(m_mutex,boost::adopt_lock) ;
> then the valgrind report goes away.

This is undefined behavior.  From the documentation

shared_lock(Lockable &  m,boost::adopt_lock_t)
Precondition:
    The current thread owns an exclusive lock on m.

> I understand that there are valid cases to expect reachable memory at exit Should this be expected in this case ?
>   

Yes.  This is not expected, and there is no obvious way to change it.

In Christ,
Steven Watanabe

P.S. Please start a new thread instead of replying to an unrelated 
existing thread.

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