近日,听说pthread_create会造成内存泄漏,觉得不可思议,因此对posix(nptl)的线程创建和销毁进行了分析。
分析结果:如果使用不当,确实会造成内存泄漏。
产生根源:pthread_create默认创建的线程是非detached的。
预防方式:要么创建detached的线程,要么线程线程的start_routine结束之前detached,要么join
分析过程如下:
1.查看pthread_create源代码,核心代码如下(nptl/pthread_create.c):
- int
- __pthread_create_2_1 (newthread, attr, start_routine, arg)
- pthread_t *newthread;
- const pthread_attr_t *attr;
- void *(*start_routine) (void *);
- void *arg;
- {
- STACK_VARIABLES;
- const struct pthread_attr *iattr = (struct pthread_attr *) attr;
- if (iattr == NULL)
- /* Is this the best idea? On NUMA machines this could mean
- accessing far-away memory. */
- iattr = &default_attr;
- struct pthread *pd = NULL;
- int err = ALLOCATE_STACK (iattr, &pd);//为tcb分配内存
- if (__builtin_expect (err != 0, 0))
- /* Something went wrong. Maybe a parameter of the attributes is
- invalid or we could not allocate memory. */
- return err;
- //……
- err = create_thread (pd, iattr, STACK_VARIABLES_ARGS);//正式创建线程
2.查看createthread.c(nptl/sysdeps/pthread/createthread.c)- static int
- create_thread (struct pthread *pd, const struct pthread_attr *attr,
- STACK_VARIABLES_PARMS)
- {
- #ifdef TLS_TCB_AT_TP
- assert (pd->header.tcb != NULL);
- #endif
- //……
- int res = do_clone (pd, attr, clone_flags, start_thread,
- STACK_VARIABLES_ARGS, 1);//clone一个进程
3.接着看start_thread(nptl/pthread_create.c)做了什么- static int
- start_thread (void *arg)
- {
- struct pthread *pd = (struct pthread *) arg;
- //……
- /* Run the code the user provided. */
- #ifdef CALL_THREAD_FCT
- THREAD_SETMEM (pd, result, CALL_THREAD_FCT (pd));
- #else
- THREAD_SETMEM (pd, result, pd->start_routine (pd->arg)); //正式启动线程的执行,并等待执行完成
- #endif
- //……
- if (IS_DETACHED (pd))
- /* Free the TCB. */
- __free_tcb (pd);//如果设置detached标志,则释放tcb占用的内容,否则直接返回
- else if (__builtin_expect (pd->cancelhandling & SETXID_BITMASK, 0))
- {
- /* Some other thread might call any of the setXid functions and expect
- us to reply. In this case wait until we did that. */
- do
- lll_futex_wait (&pd->setxid_futex, 0, LLL_PRIVATE);
- while (pd->cancelhandling & SETXID_BITMASK);
- /* Reset the value so that the stack can be reused. */
- pd->setxid_futex = 0;
- }
从上面的过程,我们可以看到,如果在创建线程的时候,如果没有设置detached标志,则tcb内存永远不会释放
接下来,我们看看pthread_detach(npth/pthread_detach.c)做了什么
- int
- pthread_detach (th)
- pthread_t th;
- {
- struct pthread *pd = (struct pthread *) th;
- /* Make sure the descriptor is valid. */
- if (INVALID_NOT_TERMINATED_TD_P (pd))
- /* Not a valid thread handle. */
- return ESRCH;
- int result = 0;
- /* Mark the thread as detached. */
- if (atomic_compare_and_exchange_bool_acq (&pd->joinid, pd, NULL))
- {
- /* There are two possibilities here. First, the thread might
- already be detached. In this case we return EINVAL.
- Otherwise there might already be a waiter. The standard does
- not mention what happens in this case. */
- if (IS_DETACHED (pd))
- result = EINVAL;
- }
- else
- /* Check whether the thread terminated meanwhile. In this case we
- will just free the TCB. */
- if ((pd->cancelhandling & EXITING_BITMASK) != 0)
- /* Note that the code in __free_tcb makes sure each thread
- control block is freed only once. */
- __free_tcb (pd);//经过一系列的容错判断,直接释放tcb占用的内存
- return result;
- }
最后,我们看一下pthread_join(nptl/pthread_join.c)做了什么- int
- pthread_join (threadid, thread_return)
- pthread_t threadid;
- void **thread_return;
- {
- struct pthread *pd = (struct pthread *) threadid;
- /* Make sure the descriptor is valid. */
- if (INVALID_NOT_TERMINATED_TD_P (pd))
- /* Not a valid thread handle. */
- return ESRCH;
- /* Is the thread joinable?. */
- if (IS_DETACHED (pd))
- /* We cannot wait for the thread. */
- return EINVAL;
- struct pthread *self = THREAD_SELF;
- int result = 0;
- /* During the wait we change to asynchronous cancellation. If we
- are canceled the thread we are waiting for must be marked as
- un-wait-ed for again. */
- pthread_cleanup_push (cleanup, &pd->joinid);
- /* Switch to asynchronous cancellation. */
- int oldtype = CANCEL_ASYNC ();
- if ((pd == self
- || (self->joinid == pd
- && (pd->cancelhandling
- & (CANCELING_BITMASK | CANCELED_BITMASK | EXITING_BITMASK
- | TERMINATED_BITMASK)) == 0))
- && !CANCEL_ENABLED_AND_CANCELED (self->cancelhandling))
- /* This is a deadlock situation. The threads are waiting for each
- other to finish. Note that this is a "may" error. To be 100%
- sure we catch this error we would have to lock the data
- structures but it is not necessary. In the unlikely case that
- two threads are really caught in this situation they will
- deadlock. It is the programmer's problem to figure this
- out. */
- result = EDEADLK;
- /* Wait for the thread to finish. If it is already locked something
- is wrong. There can only be one waiter. */
- else if (__builtin_expect (atomic_compare_and_exchange_bool_acq (&pd->joinid,
- self,
- NULL), 0))
- /* There is already somebody waiting for the thread. */
- result = EINVAL;
- else
- /* Wait for the child. */
- lll_wait_tid (pd->tid);
- /* Restore cancellation mode. */
- CANCEL_RESET (oldtype);
- /* Remove the handler. */
- pthread_cleanup_pop (0);
- if (__builtin_expect (result == 0, 1))
- {
- /* We mark the thread as terminated and as joined. */
- pd->tid = -1;
- /* Store the return value if the caller is interested. */
- if (thread_return != NULL)
- *thread_return = pd->result;//设置返回值
- /* Free the TCB. */
- __free_tcb (pd);/释放TCB占用内存
- }
- return result;
- }
综上,如果要保证创建线程之后,确保无内存泄漏,必须采用如下方法来规范pthread_create的使用:
方法一、创建detached的线程
- void run() {
- return;
- }
-
- int main(){
- pthread_t thread;
- pthread_attr_t attr;
- pthread_attr_init( &attr );
- pthread_attr_setdetachstate(&attr,1);
- pthread_create(&thread, &attr, run, 0);
-
- //......
- return 0;
- }
方法二、要么线程线程的start_routine结束之前detached
- void run() {
- pthread_detach(pthread_self());
- }
-
- int main(){
- pthread_t thread;
- pthread_create(&thread, NULL, run, 0);
-
- //......
- return 0;
- }
方法三、主线程使用pthread_join
- void run() {
- return;
- }
-
- int main(){
- pthread_t thread;
- pthread_create(&thread, NULL, run, 0);
-
- //......
- pthread_join(thread,NULL);
- return 0;
- }
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