Memory leaks using shared_mutex

Question

The following code leads into an increasing usage of memory:

#include <shared_mutex>

class foo
{
public:
   void bar()
   {
      std::unique_lock lock(m_mtx);
   }
   std::shared_mutex m_mtx;
};

int main()
{
   while (1)
   {
      foo obj;
      obj.bar();
   }
}

The following do not: (only changes the mutex type)

#include <mutex>

class foo
{
public:
   void bar()
   {
      std::unique_lock lock(m_mtx);
   }
   std::mutex m_mtx;
};

int main()
{
   while (1)
   {
      foo obj;
      obj.bar();
   }
}

I am using Windows 7 and using the task manager to track the memory consumption of my program.

I compile with mingw and this simple command line to compile:

g++.exe -std=c++17 -o mytest main.cpp

What i am doing wrong with the usage of shared_mutex ?

Answer 1

Found it! Quite an old post, don't known if it's still relevant for you.

The problem seems to be using g++, mingw64 and the std::shared_mutex (at least version 12-posix).

In fact, there is a memory leak in the C++ library using the standard header file, <shared_mutex>. The pthread_rwlock_destroy is not called when PTHREAD_RWLOCK_INITIALIZER is used to initialize the mutex. However, pthread_rwlock_destroy is called as expected when using the pthread_rwlock_init version.

I found a way around and a fix. The easy way is to "disable" the constant initializer PTHREAD_RWLOCK_INITIALIZER from your source code; this forces the C++ library to call the pthread_rwlock_init function, then the pthread_rwlock_destroy function. To do that, just insert an #undef between #include <pthread> and #include <mutex>:

#include <thread>
#undef PTHREAD_RWLOCK_INITIALIZER
#include <mutex>
#include <shared_mutex>

The proper way is to patch the <shared_mutex> header file of the C++ standard library and call the pthread_rwlock_destroy function in the shared_mutex destructor, even if the constant initializer PTHREAD_RWLOCK_INITIALIZER is used.

If I find the way to and the time, I will submit a regular patch for the standard <shared_mutex> header file.