Pointer variable assignment has copied partial memory when using multithread - c++

Question

I'm trying to make multithread c++ program.
I've been testing my program for long time, and sometimes got SIGSEGV (segmentation fault).
And I found a pointer variable copied partial memory only, not all.
Here is parts of my code.
Class definition
class Message
{
public:
Message() {}
virtual ~Message() {}
virtual void execute() = 0; //do something...
}
class Job
{
public:
Job();
~Job();
void post(Message *message);
void execute(Message *message);
private:
Message *_queue[1024] = {nullptr,};
volatile int _head = 0;
int _tail = 0;
};
And 4 of threads were running and call method of same Job class instance.
Thread 1 call this method.
void Job::execute(Message *message)
{
if (message != nullptr)
{
int index = __sync_fetch_and_add(&_head, 1) % 1024;
_queue[index] = message;
}
while (_tail != _head)
{
int index = _tail % 1024;
Message *workingMessage = _queue[index];
while (workingMessage == nullptr)
{
sched_yield();
workingMessage = _queue[index];
}
workingMessage->execute();
_queue[index] = nullptr;
++_tail; //this is changed on Thread 1 only!
}
}
Thread 2~4 call this method.
void Job::post(Message *message)
{
int index = __sync_fetch_and_add(&_head, 1) % 1024;
_queue[index] = message;
}
After several minutes, I've got SIGSEGV crash on this part of Thread 1.
workingMessage->execute(); // Crash!
On debugger, workingMessage had only some part of _queue[Index] value.
For example,
_queue[index] : 0x7fffdc09abdb
workingMessage : 0xffdc09abdb
or
_queue[index] : 0x7fffe8048e35
workingMessage : 0x7fffe8040000
like these cases.
I've tried to use std::atomic< Message *> for _queue and workingMessage, same crashes were happened.
What's wrong? I've compiled and tested this on CentOS 7, gcc 4.8.5.