POSIX线程-使用条件变量MEMORY LEAK同步已分离的线程

Question

Hello I'm trying to synchronize detached threads using conditional variable, but I found a bug that sometimes causes memory leak (depends on scheduler mood). I think the code is self explanatory. I would appreciate any advice.

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <unistd.h>
#include <pthread.h>

using namespace std;

struct TThrArg
{
    pthread_t m_ID;
    bool      m_IsRunning;
};

TThrArg         g_Threads[64];
int             g_Counter;
pthread_mutex_t g_Mtx;
pthread_cond_t  g_Cond;

void * thrFunc ( void * arg )
{
    TThrArg * data = (TThrArg *) arg;

    // do some stuff
    // -----------------------------------
    // for ( int i = 0; i < 5000; ++i )
    //  for ( int j = 0; j < 5000; ++j )
    //      int x = 0;
    // printf("Thread: %lu running...\n", data->m_ID);
    // -----------------------------------

    pthread_mutex_lock(&g_Mtx);
    memset(data, 0, sizeof(TThrArg));
    --g_Counter;
    pthread_cond_signal(&g_Cond);
    pthread_mutex_unlock(&g_Mtx);
    sleep(1); // --> this spot causes that main may end before return NULL so resources will not be freed 
    return NULL;
}

void createThread ( void )
{
    pthread_mutex_lock(&g_Mtx);
    for ( int i = 0; i < 64; ++i )
    {
        if ( g_Threads[i].m_IsRunning == 0 )
        {
            g_Threads[i].m_IsRunning = 1;
            ++g_Counter;

            pthread_attr_t attr;
            pthread_attr_init(&attr);
            pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
            pthread_create(&g_Threads[i].m_ID, &attr, thrFunc, &g_Threads[i]);
            pthread_attr_destroy(&attr);

            break;
        }
    }
    pthread_mutex_unlock(&g_Mtx);
}

int main ( int argc, char * argv[] )
{
    pthread_mutex_init(&g_Mtx, NULL);
    pthread_cond_init(&g_Cond, NULL);
    g_Counter = 0;

    for ( int i = 0; i < 64; ++i )
        createThread();

    pthread_mutex_lock(&g_Mtx);
    while ( g_Counter != 0 )
    {
        pthread_cond_wait(&g_Cond, &g_Mtx);
    }
    pthread_mutex_unlock(&g_Mtx);

    pthread_mutex_destroy(&g_Mtx);
    pthread_cond_destroy(&g_Cond);

    return 0;
}

Answer 1

The leak you see is because the terminating thread decrements the mutex-protected thread counter, and pauses for a second before the thread actually terminates.

The main execution thread will immediately see that the thread counter reached 0, and terminate before the actual detached threads have exited. Each running thread, even a detached thread, consumes and allocates a little bit of internal memory, which does not get released until the thread actually terminates. This is the leak you see, from execution threads that did not terminate before the main execution thread stopped.

This is not the kind of a leak that you need to worry about. It is rather annoying, and makes debugging difficult, true.

In the past, I took one approach in a framework class library that I wrote some time ago. I did not use detached threads at all, but all threads were joinable threads. The framework started one singleton background thread whose only job was to join() the terminated threads. Then, each thread started by the framework will queue up its own thread id for the singleton background thread, just before each thread terminates.

The net effect was equivalent to detached threads. I could start each thread and not worry about joining to it. It's going to be the background thread's job. The framework would signal the background thread to terminate itself, and join it, before exiting. So, if all goes well, there will not be any reported memory leaks that can be accounted to thread support.