title: multi-thread access MySQL error
body: <p>I have written a simple multi-threaded C program  to access MySQL,it works fine except <strong>when 
  i add usleep() or sleep() function in each thread function.</strong>
  i created two pthreads in the main method,</p>
  
  <pre><code>int main(){
          mysql_library_init(0,NULL,NULL);
          printf("Hello world!\n");
          init_pool(&amp;p,100);
          pthread_t producer;
          pthread_t consumer_1;
          pthread_t consumer_2;
          pthread_create(&amp;producer,NULL,produce_fun,NULL);
          pthread_create(&amp;consumer_1,NULL,consume_fun,NULL);
          pthread_create(&amp;consumer_2,NULL,consume_fun,NULL);
          mysql_library_end();
  }
  
  
  
     void * produce_fun(void *arg){
      pthread_detach(pthread_self());
      //procedure
      while(1){
          usleep(500000);
          printf("producer...\n");
          produce(&amp;p,cnt++);
      }
      pthread_exit(NULL);
  }
  
  void * consume_fun(void *arg){
      pthread_detach(pthread_self());
      MYSQL db;
      MYSQL *ptr_db=mysql_init(&amp;db);
      mysql_real_connect();
  
      //procedure
      while(1){
          usleep(1000000);
          printf("consumer...");
          int item=consume(&amp;p);
          addRecord_d(ptr_db,"test",item);
      }
      mysql_thread_end();
      pthread_exit(NULL);
  }
  
  void addRecord_d(MYSQL *ptr_db,const char *t_name,int item){
      char query_buffer[100];
      sprintf(query_buffer,"insert into %s values(0,%d)",t_name,item);
  //pthread_mutex_lock(&amp;db_t_lock);
      int ret=mysql_query(ptr_db,query_buffer);
      if(ret){
          fprintf(stderr,"%s%s\n","cannot add record to ",t_name);
          return;
      }
  
      unsigned long long update_id=mysql_insert_id(ptr_db);
  //    pthread_mutex_unlock(&amp;db_t_lock);
      printf("add record (%llu,%d) ok.",update_id,item);
  }
  </code></pre>
  
  <p><strong>the program output errors like:</strong></p>
  
  <pre><code>[Thread debugging using libthread_db enabled]
  [New Thread 0xb7ae3b70 (LWP 7712)]
  Hello world!
  [New Thread 0xb72d6b70 (LWP 7713)]
  [New Thread 0xb6ad5b70 (LWP 7714)]
  [New Thread 0xb62d4b70 (LWP 7715)]
  [Thread 0xb7ae3b70 (LWP 7712) exited]
  producer...
  producer...
  consumer...consumer...add record (31441,0) ok.add record (31442,1) ok.producer...
  producer...
  consumer...consumer...add record (31443,2) ok.add record (31444,3) ok.producer...
  producer...
  consumer...consumer...add record (31445,4) ok.add record (31446,5) ok.producer...
  producer...
  consumer...consumer...add record (31447,6) ok.add record (31448,7) ok.producer...
  Error in my_thread_global_end(): 2 threads didn't exit
  [Thread 0xb72d6b70 (LWP 7713) exited]
  [Thread 0xb6ad5b70 (LWP 7714) exited]
  [Thread 0xb62d4b70 (LWP 7715) exited]
  
  Program exited normally.
  </code></pre>
  
  <p><strong>and when i add pthread_mutex_lock in function addRecord_d,the error still exists.
  So what exactly the problem is?</strong></p>