Linux wait返回及timer_create问题解决
libowenhit 2018-08-11
前言
前段时间查一个问题，发现应用层在使用wait函数时，在没有等到信号的情况下，wait函数返回了，并且返回值为0，没有超时及异常提示，不符合常理，跟进后发现，虽然c库代码编写不够严谨，但根源是应用层代码对timer_create的不当使用，引入了隐患。在这做一个分析，作为以后分析同类问题的参考。
一、 wait函数不合理返回问题
如下面代码，在postAndWait函数中，先把task queue进处理队列，然后调用wait等待task处理完成发送信号，接着在run函数中运行task及发送信号，当wait函数收到信号后，正常返回，这为正常的运行流程。但发现有时出现了在run中，task还没运行，也没有发送信号，wait函数就已经返回，并且返回值为0（success）。
frameworks\base\libs\hwui\renderthread\ RenderProxy.cpp
void* RenderProxy::postAndWait(MethodInvokeRenderTask* task) {
void* retval;
task->setReturnPtr(&retval);
SignalingRenderTask syncTask(task, &mSyncMutex, &mSyncCondition);
AutoMutex _lock(mSyncMutex);
mRenderThread.queue(&syncTask); // queue task
mSyncCondition.wait(mSyncMutex); // 等待task运行完成发送信号
return retval;
// 若在task还没运行，wait就返回，task被释放，task运行线程不知道task被释放，一到task运行就出问题
}
frameworks\base\libs\hwui\renderthread\ RenderTask.cpp
void SignalingRenderTask::run() {
mTask->run(); // task的运行
mLock->lock();
mSignal->signal(); // 发送信号给wait
mLock->unlock();
}
二、wait不合理返回分析
跟进内核代码发现，当wait函数在等待时，wait所在的线程被挂起，正常情况下，当task的运行线程给wait所在的线程发送信号后，wait所在的线程被设置为可运行状态，等待系统调度运行并正常返回，wait函数调用路径及返回如下，调用路径如绿色标示的，返回点如紫色标示。（发送信号流程的代码位置与wait流程代码处于相同文件中，可自行跟踪）
system\core\include\utils\ Condition.h
inline status_t Condition::wait(Mutex& mutex) {
return -pthread_cond_wait(&mCond, &mutex.mMutex);
}
bionic\libc\bionic\ Pthread_cond.cpp
int pthread_cond_wait(pthread_cond_t* cond, pthread_mutex_t* mutex) {
return __pthread_cond_timedwait(cond, mutex, NULL, COND_GET_CLOCK(cond->value));
}
bionic\libc\bionic\ Pthread_cond.cpp
__LIBC_HIDDEN__
int __pthread_cond_timedwait(pthread_cond_t* cond, pthread_mutex_t* mutex, const timespec* abstime, clockid_t clock) {
timespec ts;
timespec* tsp;
if (abstime != NULL) { // 没有设置超时时间，不走这里
if (__timespec_from_absolute(&ts, abstime, clock) < 0) {
return ETIMEDOUT;
}
tsp = &ts;
} else {
tsp = NULL;
}
return __pthread_cond_timedwait_relative(cond, mutex, tsp);
}
bionic\libc\bionic\ Pthread_cond.cpp
__LIBC_HIDDEN__
int __pthread_cond_timedwait_relative(pthread_cond_t* cond, pthread_mutex_t* mutex, const timespec* reltime) {
int old_value = cond->value;
pthread_mutex_unlock(mutex);
int status = __futex_wait_ex(&cond->value, COND_IS_SHARED(cond->value), old_value, reltime);
pthread_mutex_lock(mutex);
if (status == -ETIMEDOUT) {
return ETIMEDOUT;
}
return 0;
}
bionic\libc\private\ Bionic_futex.h
static inline int __futex_wait_ex(volatile void* ftx, bool shared, int value, const struct timespec* timeout) {
return __futex(ftx, shared ? FUTEX_WAIT : FUTEX_WAIT_PRIVATE, value, timeout);
}
bionic\libc\private\ Bionic_futex.h
static inline __always_inline int __futex(volatile void* ftx, int op, int value, const struct timespec* timeout) {
// Our generated syscall assembler sets errno, but our callers (pthread functions) don't want to.
int saved_errno = errno;
int result = syscall(__NR_futex, ftx, op, value, timeout);
if (__predict_false(result == -1)) {
result = -errno;
errno = saved_errno;
}
return result;
}
kernel\kernel\ Futex.c
SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
struct timespec __user *, utime, u32 __user *, uaddr2,
u32, val3)
{
struct timespec ts;
ktime_t t, *tp = NULL;
u32 val2 = 0;
int cmd = op & FUTEX_CMD_MASK;
if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
cmd == FUTEX_WAIT_BITSET ||
cmd == FUTEX_WAIT_REQUEUE_PI)) {
if (copy_from_user(&ts, utime, sizeof(ts)) != 0)
return -EFAULT;
if (!timespec_valid(&ts))
return -EINVAL;
t = timespec_to_ktime(ts);
if (cmd == FUTEX_WAIT)
t = ktime_add_safe(ktime_get(), t);
tp = &t;
}
/*
* requeue parameter in 'utime' if cmd == FUTEX_*_REQUEUE_*.
* number of waiters to wake in 'utime' if cmd == FUTEX_WAKE_OP.
*/
if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE ||
cmd == FUTEX_CMP_REQUEUE_PI || cmd == FUTEX_WAKE_OP)
val2 = (u32) (unsigned long) utime;
return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
}
kernel\kernel\ Futex.c
long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
u32 __user *uaddr2, u32 val2, u32 val3)
{
int cmd = op & FUTEX_CMD_MASK;
unsigned int flags = 0;
if (!(op & FUTEX_PRIVATE_FLAG))
flags |= FLAGS_SHARED;
if (op & FUTEX_CLOCK_REALTIME) {
flags |= FLAGS_CLOCKRT;
if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI)
return -ENOSYS;
}
switch (cmd) {
case FUTEX_LOCK_PI:
case FUTEX_UNLOCK_PI:
case FUTEX_TRYLOCK_PI:
case FUTEX_WAIT_REQUEUE_PI:
case FUTEX_CMP_REQUEUE_PI:
if (!futex_cmpxchg_enabled)
return -ENOSYS;
}
switch (cmd) {
case FUTEX_WAIT:
val3 = FUTEX_BITSET_MATCH_ANY;
case FUTEX_WAIT_BITSET:
return futex_wait(uaddr, flags, val, timeout, val3);
case FUTEX_WAKE:
val3 = FUTEX_BITSET_MATCH_ANY;
case FUTEX_WAKE_BITSET:
return futex_wake(uaddr, flags, val, val3);
case FUTEX_REQUEUE:
return futex_requeue(uaddr, flags, uaddr2, val, val2, NULL, 0);
case FUTEX_CMP_REQUEUE:
return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 0);
case FUTEX_WAKE_OP:
return futex_wake_op(uaddr, flags, uaddr2, val, val2, val3);
case FUTEX_LOCK_PI:
return futex_lock_pi(uaddr, flags, val, timeout, 0);
case FUTEX_UNLOCK_PI:
return futex_unlock_pi(uaddr, flags);
case FUTEX_TRYLOCK_PI:
return futex_lock_pi(uaddr, flags, 0, timeout, 1);
case FUTEX_WAIT_REQUEUE_PI:
val3 = FUTEX_BITSET_MATCH_ANY;
return futex_wait_requeue_pi(uaddr, flags, val, timeout, val3,
uaddr2);
case FUTEX_CMP_REQUEUE_PI:
return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 1);
}
return -ENOSYS;
}
kernel\kernel\ Futex.c
static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
ktime_t *abs_time, u32 bitset)
{
struct hrtimer_sleeper timeout, *to = NULL;
struct restart_block *restart;
struct futex_hash_bucket *hb;
struct futex_q q = futex_q_init;
int ret;
if (!bitset)
return -EINVAL;
q.bitset = bitset;
if (abs_time) {
to = &timeout;
hrtimer_init_on_stack(&to->timer, (flags & FLAGS_CLOCKRT) ?
CLOCK_REALTIME : CLOCK_MONOTONIC,
HRTIMER_MODE_ABS);
hrtimer_init_sleeper(to, current);
hrtimer_set_expires_range_ns(&to->timer, *abs_time,
current->timer_slack_ns);
}
retry:
/*
* Prepare to wait on uaddr. On success, holds hb lock and increments
* q.key refs.
*/
ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
if (ret)
goto out;
/* queue_me and wait for wakeup, timeout, or a signal. */
futex_wait_queue_me(hb, &q, to);
/* If we were woken (and unqueued), we succeeded, whatever. */
ret = 0;
/* unqueue_me() drops q.key ref */
if (!unqueue_me(&q)) {
/* unqueue_me返回值情况 */
/* 1 - if the futex_q was still queued (and we removed unqueued it); */
/* 0 - if the futex_q was already removed by the waking thread(发送信号唤醒的情况) */
goto out; // 正常等到信号后返回走这里
}
ret = -ETIMEDOUT;
if (to && !to->task) {
goto out;
}
/*
* We expect signal_pending(current), but we might be the
* victim of a spurious wakeup as well.
*/
if (!signal_pending(current)) {
trace_printk("retry\n");
goto retry;
}
ret = -ERESTARTSYS;
if (!abs_time) {
goto out;
}
restart = ¤t_thread_info()->restart_block;
restart->fn = futex_wait_restart;
restart->futex.uaddr = uaddr;
restart->futex.val = val;
restart->futex.time = abs_time->tv64;
restart->futex.bitset = bitset;
restart->futex.flags = flags | FLAGS_HAS_TIMEOUT;
ret = -ERESTART_RESTARTBLOCK;
out:
if (to) {
hrtimer_cancel(&to->timer);
destroy_hrtimer_on_stack(&to->timer);
}
return ret; // 正常返回值为0
}
kernel\kernel\ Futex.c
static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
struct hrtimer_sleeper *timeout)
{
/*
* The task state is guaranteed to be set before another task can
* wake it. set_current_state() is implemented using set_mb() and
* queue_me() calls spin_unlock() upon completion, both serializing
* access to the hash list and forcing another memory barrier.
*/
set_current_state(TASK_INTERRUPTIBLE);
queue_me(q, hb);
/* Arm the timer */
if (timeout) {
hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
if (!hrtimer_active(&timeout->timer))
timeout->task = NULL;
}
/*
* If we have been removed from the hash list, then another task
* has tried to wake us, and we can skip the call to schedule().
*/
if (likely(!plist_node_empty(&q->list))) {
/*
* If the timer has already expired, current will already be
* flagged for rescheduling. Only call schedule if there
* is no timeout, or if it has yet to expire.
*/
if (!timeout || timeout->task) {
freezable_schedule();
}
}
__set_current_state(TASK_RUNNING);
}
如果该等待线程使用timer_create创建了定时器，并且创建的定时器超时是给当前线程发送信号（timer_create的创建在第4节分析），当定时器超时后，就会把当前线程设置为可运行的状态，等待系统调度运行。若这时该线程刚好调用了wait在等待信号，由于该线程已经被设置为可运行状态，当调度到该线程运行时，futex_wait_queue_me函数的freezable_schedule()就会返回，这时futex_wait返回流程与返回值都与正常接收到信号时返回的不一样，如下面代码标示：
kernel\kernel\ Futex.c
static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
ktime_t *abs_time, u32 bitset)
{
struct hrtimer_sleeper timeout, *to = NULL;
struct restart_block *restart;
struct futex_hash_bucket *hb;
struct futex_q q = futex_q_init;
int ret;
if (!bitset)
return -EINVAL;
q.bitset = bitset;
if (abs_time) {
to = &timeout;
hrtimer_init_on_stack(&to->timer, (flags & FLAGS_CLOCKRT) ?
CLOCK_REALTIME : CLOCK_MONOTONIC,
HRTIMER_MODE_ABS);
hrtimer_init_sleeper(to, current);
hrtimer_set_expires_range_ns(&to->timer, *abs_time,
current->timer_slack_ns);
}
retry:
/*
* Prepare to wait on uaddr. On success, holds hb lock and increments
* q.key refs.
*/
ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
if (ret)
goto out;
/* queue_me and wait for wakeup, timeout, or a signal. */
futex_wait_queue_me(hb, &q, to);
/* If we were woken (and unqueued), we succeeded, whatever. */
ret = 0;
/* unqueue_me() drops q.key ref */
if (!unqueue_me(&q)) {
/* unqueue_me返回值情况 */
/* 1 - if the futex_q was still queued (and we removed unqueued it); */
/* 0 - if the futex_q was already removed by the waking thread(发送信号唤醒的情况) */
goto out; // 不是等待的信号唤醒，futex_q was still queued，unqueue_me返回1，流程不走这
}
ret = -ETIMEDOUT;
if (to && !to->task) {
goto out; //没有设置超时返回，没走这
}
/*
* We expect signal_pending(current), but we might be the
* victim of a spurious wakeup as well.
*/
if (!signal_pending(current)) {
goto retry; // 是该线程定时器唤醒，不走这
}
ret = -ERESTARTSYS;
if (!abs_time) {
goto out; // 最后流程到这里，则ret = -ERESTARTSYS
}
restart = ¤t_thread_info()->restart_block;
restart->fn = futex_wait_restart;
restart->futex.uaddr = uaddr;
restart->futex.val = val;
restart->futex.time = abs_time->tv64;
restart->futex.bitset = bitset;
restart->futex.flags = flags | FLAGS_HAS_TIMEOUT;
ret = -ERESTART_RESTARTBLOCK;
out:
if (to) {
hrtimer_cancel(&to->timer);
destroy_hrtimer_on_stack(&to->timer);
}
return ret; // 返回值为-ERESTARTSYS（-512）
}
从上面代码可以看出，futex_wait的返回值并不为0，但到了应用层得到的返回值就变成0了，分析后发现是c库的代码不严谨导致的，如下面代码：
bionic\libc\bionic\ Pthread_cond.cpp
__LIBC_HIDDEN__
int __pthread_cond_timedwait_relative(pthread_cond_t* cond, pthread_mutex_t* mutex, const timespec* reltime) {
int old_value = cond->value;
pthread_mutex_unlock(mutex);
int status = __futex_wait_ex(&cond->value, COND_IS_SHARED(cond->value), old_value, reltime);
pthread_mutex_lock(mutex);
if (status == -ETIMEDOUT) {
return ETIMEDOUT; // 只有超时返回时，才返回非0值，其它情况都是返回0
}
return 0;
}
这样就导致了上层无法识别出除了超时之外的其它情况返回。
三、定时器超时唤醒线程的流程
定时器超时唤醒线程与发送信号唤醒线程流程不同，下面代码分析定时器唤醒时走的流程，调用路径如绿色标示(有关linux定时器的知识，可以搜索“linux定时器的实现”，可以找到很多介绍)。
kernel\kernel\Posix-timers.c
int posix_timer_event(struct k_itimer *timr, int si_private) // posix定时器超时后调用到这里
{
struct task_struct *task;
int shared, ret = -1;
/*
* FIXME: if ->sigq is queued we can race with
* dequeue_signal()->do_schedule_next_timer().
*
* If dequeue_signal() sees the "right" value of
* si_sys_private it calls do_schedule_next_timer().
* We re-queue ->sigq and drop ->it_lock().
* do_schedule_next_timer() locks the timer
* and re-schedules it while ->sigq is pending.
* Not really bad, but not that we want.
*/
timr->sigq->info.si_sys_private = si_private;
rcu_read_lock();
task = pid_task(timr->it_pid, PIDTYPE_PID);
if (task) {
shared = !(timr->it_sigev_notify & SIGEV_THREAD_ID);
ret = send_sigqueue(timr->sigq, task, shared);
}
rcu_read_unlock();
/* If we failed to send the signal the timer stops. */
return ret > 0;
}
kernel\kernel\Signal.c
int send_sigqueue(struct sigqueue *q, struct task_struct *t, int group)
{
int sig = q->info.si_signo;
int sival = q->info.si_value.sival_int;
struct sigpending *pending;
unsigned long flags;
int ret, result;
BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
ret = -1;
if (!likely(lock_task_sighand(t, &flags)))
goto ret;
ret = 1; /* the signal is ignored */
result = TRACE_SIGNAL_IGNORED;
if (!prepare_signal(sig, t, false))
goto out;
ret = 0;
if (unlikely(!list_empty(&q->list))) {
/*
* If an SI_TIMER entry is already queue just increment
* the overrun count.
*/
BUG_ON(q->info.si_code != SI_TIMER);
q->info.si_overrun++;
result = TRACE_SIGNAL_ALREADY_PENDING;
goto out;
}
q->info.si_overrun = 0;
signalfd_notify(t, sig);
pending = group ? &t->signal->shared_pending : &t->pending;
list_add_tail(&q->list, &pending->list);
sigaddset(&pending->signal, sig);
complete_signal(sig, t, group);
result = TRACE_SIGNAL_DELIVERED;
out:
trace_signal_generate(sig, &q->info, t, group, result);
unlock_task_sighand(t, &flags);
ret:
return ret;
}
kernel\kernel\Signal.c
static void complete_signal(int sig, struct task_struct *p, int group)
{
struct signal_struct *signal = p->signal;
struct task_struct *t;
/*
* Now find a thread we can wake up to take the signal off the queue.
*
* If the main thread wants the signal, it gets first crack.
* Probably the least surprising to the average bear.
*/
if (wants_signal(sig, p))
t = p;
else if (!group || thread_group_empty(p))
/*
* There is just one thread and it does not need to be woken.
* It will dequeue unblocked signals before it runs again.
*/
return;
else {
/*
* Otherwise try to find a suitable thread.
*/
t = signal->curr_target;
while (!wants_signal(sig, t)) {
t = next_thread(t);
if (t == signal->curr_target)
/*
* No thread needs to be woken.
* Any eligible threads will see
* the signal in the queue soon.
*/
return;
}
signal->curr_target = t;
}
/*
* Found a killable thread. If the signal will be fatal,
* then start taking the whole group down immediately.
*/
if (sig_fatal(p, sig) &&
!(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) &&
!sigismember(&t->real_blocked, sig) &&
(sig == SIGKILL || !t->ptrace)) {
/*
* This signal will be fatal to the whole group.
*/
if (!sig_kernel_coredump(sig)) {
/*
* Start a group exit and wake everybody up.
* This way we don't have other threads
* running and doing things after a slower
* thread has the fatal signal pending.
*/
signal->flags = SIGNAL_GROUP_EXIT;
signal->group_exit_code = sig;
signal->group_stop_count = 0;
t = p;
do {
task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
sigaddset(&t->pending.signal, SIGKILL);
signal_wake_up(t, 1);
} while_each_thread(p, t);
return;
}
}
/*
* The signal is already in the shared-pending queue.
* Tell the chosen thread to wake up and dequeue it.
*/
signal_wake_up(t, sig == SIGKILL);
return;
}
kernel\include\linux\Sched.h
static inline void signal_wake_up(struct task_struct *t, bool resume)
{
signal_wake_up_state(t, resume ? TASK_WAKEKILL : 0);
}
kernel\kernel\Signal.c
void signal_wake_up_state(struct task_struct *t, unsigned int state)
{
set_tsk_thread_flag(t, TIF_SIGPENDING);
/*
* TASK_WAKEKILL also means wake it up in the stopped/traced/killable
* case. We don't check t->state here because there is a race with it
* executing another processor and just now entering stopped state.
* By using wake_up_state, we ensure the process will wake up and
* handle its death signal.
*/
if (!wake_up_state(t, state | TASK_INTERRUPTIBLE))
kick_process(t);
}
kernel\kernel\sched\Core.c
int wake_up_state(struct task_struct *p, unsigned int state)
{
return try_to_wake_up(p, state, 0);
}
kernel\kernel\sched\Core.c
static int
try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
{
unsigned long flags;
int cpu, success = 0;
/*
* If we are going to wake up a thread waiting for CONDITION we
* need to ensure that CONDITION=1 done by the caller can not be
* reordered with p->state check below. This pairs with mb() in
* set_current_state() the waiting thread does.
*/
smp_mb__before_spinlock();
raw_spin_lock_irqsave(&p->pi_lock, flags);
if (!(p->state & state))
goto out;
success = 1; /* we're going to change ->state */
cpu = task_cpu(p);
if (p->on_rq && ttwu_remote(p, wake_flags))
goto stat;
#ifdef CONFIG_SMP
/*
* If the owning (remote) cpu is still in the middle of schedule() with
* this task as prev, wait until its done referencing the task.
*/
while (p->on_cpu)
cpu_relax();
/*
* Pairs with the smp_wmb() in finish_lock_switch().
*/
smp_rmb();
p->sched_contributes_to_load = !!task_contributes_to_load(p);
p->state = TASK_WAKING;
if (p->sched_class->task_waking)
p->sched_class->task_waking(p);
cpu = select_task_rq(p, SD_BALANCE_WAKE, wake_flags);
if (task_cpu(p) != cpu) {
wake_flags |= WF_MIGRATED;
set_task_cpu(p, cpu);
}
#endif /* CONFIG_SMP */
ttwu_queue(p, cpu); /* run ttwu_do_activate->ttwu_do_wakeup */
stat:
ttwu_stat(p, cpu, wake_flags);
out:
raw_spin_unlock_irqrestore(&p->pi_lock, flags);
return success;
}
kernel\kernel\sched\Core.c
static void ttwu_queue(struct task_struct *p, int cpu)
{
struct rq *rq = cpu_rq(cpu);
#if defined(CONFIG_SMP)
if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) {
sched_clock_cpu(cpu); /* sync clocks x-cpu */
ttwu_queue_remote(p, cpu);
return;
}
#endif
raw_spin_lock(&rq->lock);
ttwu_do_activate(rq, p, 0);
raw_spin_unlock(&rq->lock);
}
kernel\kernel\sched\Core.c
static void
ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags)
{
#ifdef CONFIG_SMP
if (p->sched_contributes_to_load)
rq->nr_uninterruptible--;
#endif
ttwu_activate(rq, p, ENQUEUE_WAKEUP | ENQUEUE_WAKING);
ttwu_do_wakeup(rq, p, wake_flags);
}
kernel\kernel\sched\Core.c
static void
ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
{
check_preempt_curr(rq, p, wake_flags);
trace_sched_wakeup(p, true);
p->state = TASK_RUNNING; // 设置为可运行状态，后面任务被调度了就可以运行
#ifdef CONFIG_SMP
if (p->sched_class->task_woken)
p->sched_class->task_woken(rq, p);
if (rq->idle_stamp) {
u64 delta = rq->clock - rq->idle_stamp;
u64 max = 2*sysctl_sched_migration_cost;
if (delta > max)
rq->avg_idle = max;
else
update_avg(&rq->avg_idle, delta);
rq->idle_stamp = 0;
}
#endif
}
四、timer_create创建及参数说明
从前面的分析看，看起来只要在一个线程内创建了定时器，并且使用wait等待，就会存在问题，其实并不是这样，从timer_create创建时的参数及做实际情况发现，只有在timer_create使用不当时，才会存在该问题。timer_create函数如下（timer_create的实现在bionic\libc\bionic\ Posix_timers.cpp）：
int timer_create(clockid_t clock_id, sigevent* evp, timer_t* timer_id)；
这里我们只关心第二个参数，第二个参数 struct sigevent 用来设置定时器到时时的通知方式。该数据结构如下：
struct sigevent {
int sigev_notify; /* Notification method */
int sigev_signo; /* Notification signal */
union sigval sigev_value; /* Data passed with notification */
void (*sigev_notify_function) (union sigval); /* Function used for thread notification (SIGEV_THREAD) */
void *sigev_notify_attributes; /* Attributes for notification thread (SIGEV_THREAD) */
pid_t sigev_notify_thread_id; /* ID of thread to signal (SIGEV_THREAD_ID) */
};
其中sigev_notify 表示通知方式，有如下几种：
通知方式 描述
SIGEV_NONE 定时器到期时不产生通知。。。
SIGEV_SIGNAL 定时器到期时将给进程投递一个信号，sigev_signo 可以用来指定使用什么信号。
SIGEV_THREAD 定时器到期时将启动新的线程进行需要的处理
SIGEV_THREAD_ID（仅针对 Linux) 定时器到期时将向指定线程发送信号。
■如果采用 SIGEV_NONE 方式，使用者必须调用timer_gettime 函数主动读取定时器已经走过的时间。类似轮询。
■如果采用 SIGEV_SIGNAL 方式，使用者可以选择使用什么信号，用 sigev_signo 表示信号值，比如 SIG_ALARM。
■如果使用 SIGEV_THREAD 方式，timer_create时会专门创建一个线程用于调用超时处理函数。需要设置 sigev_notify_function为超时调用函数入口；sigev_value 保存了传入 sigev_notify_function 的参数。sigev_notify_attributes 如果非空，则应该是一个指向 pthread_attr_t 的指针，用来设置线程的属性（比如 stack 大小,detach 状态等）。
■SIGEV_THREAD_ID 通常和 SIGEV_SIGNAL 联合使用，这样当 Timer 到期时，系统会向由 sigev_notify_thread_id 指定的线程发送信号，否则可能进程中的任意线程都可能收到该信号。这个选项是 Linux 对 POSIX 标准的扩展，目前主要是 GLibc 在实现 SIGEV_THREAD 的时候使用到，应用程序很少会需要用到这种模式。
从实际的情况看，当sigev_notify设置为SIGEV_SIGNAL时，当定时器超时就会唤醒调用timer_create创建定时器的线程，若该线程刚好在wait，就会出现前面分析的wait返回的情况。如果sigev_notify设置为SIGEV_THREAD，则在定时器超时后，只会唤醒专门创建的定时器处理函数线程，而不会唤醒调用timer_create创建定时器的线程，就不会存在前面分析的wait返回的情况。
五、总结
从分析看，虽然c库代码处理不够严谨，但问题的根源还是timer_create使用不当引起的。在使用timer_create时，不建议把sigev_notify设置为SIGEV_SIGNAL，除非能明确该线程只是进行定时器超时的处理。
