C++ mac OS X El Capitan上的openmp，Valgrind错误(数据竞赛）

我在 mac 上用 opnemp 库C++测试一个简单的 helloword，通过终端使用以下两个命令：

/usr/local/bin/clang++-omp -fopenmp helloworld.cpp -o test
/usr/local/bin/valgrind  --tool=helgrind --log-file=a.log ./test

输出正确：

warning: no debug symbols in executable (-arch x86_64)
Hello World from thread = 0
Hello World from thread = 1
Hello World from thread = 3
Hello World from thread = 2
Number of threads = 4

但日志文件 （a.log） 包含："来自 231 个上下文的174986错误"（作为错误摘要）

下面是日志文件的一部分：

==643== ---Thread-Announcement------------------------------------------
==643== 
==643== Thread #1 is the program's root thread
==643== 
==643== ----------------------------------------------------------------
==643== 
==643== Possible data race during read of size 4 at 0x10057C118 by thread #3
==643== Locks held: none
==643==    at 0x10055D1F4: spin_lock (in /usr/lib/system/libsystem_platform.dylib)
==643==    by 0x10057092D: _pthread_start (in /usr/lib/system/libsystem_pthread.dylib)
==643==    by 0x10056E384: thread_start (in /usr/lib/system/libsystem_pthread.dylib)
==643== 
==643== This conflicts with a previous write of size 4 by thread #1
==643== Locks held: none
==643==    at 0x10055D200: spin_unlock (in /usr/lib/system/libsystem_platform.dylib)
==643==    by 0x1000434B0: __kmp_create_worker (in /usr/local/Cellar/libiomp/20150701/lib/libiomp5.dylib)
==643==    by 0x100031E3D: __kmp_allocate_thread (in /usr/local/Cellar/libiomp/20150701/lib/libiomp5.dylib)
==643==    by 0x10002E7A1: __kmp_allocate_team (in /usr/local/Cellar/libiomp/20150701/lib/libiomp5.dylib)
==643==    by 0x10002FA2D: __kmp_fork_call (in /usr/local/Cellar/libiomp/20150701/lib/libiomp5.dylib)
==643==    by 0x100027F0D: __kmpc_fork_call (in /usr/local/Cellar/libiomp/20150701/lib/libiomp5.dylib)
==643==    by 0x100000CE8: main (in ./test)
==643==  Address 0x10057c118 is in the Data segment of /usr/lib/system/libsystem_pthread.dylib

"helloworld"的代码是：

#include <stdio.h>
#include <libiomp/omp.h>
#include <stdlib.h>
int main (int argc, char *argv[]) {   
int nthreads=4, tid;
#pragma omp parallel num_threads(nthreads) private(tid)
 {
    //Obtain thread number
    tid = omp_get_thread_num();
    printf("Hello World from thread = %dn", tid);
    // Only master thread does this
    if (tid == 0)
    {
        printf("Number of threads = %dn", nthreads);
    }
 } 
return 0;
}

有没有人知道这些错误（数据竞赛）？我在这些线程之间没有共享数据。