你想使用信号还是线程?
首先,设置信号处理程序或准备合适的线程函数;有关详细信息,请参阅man 7 sigevent。
接下来,创建一个合适的计时器,使用timer_create()
. 有关详细信息,请参见man 2 timer_create。
根据您在计时器触发时执行的操作,您可能希望将计时器设置为单次,或者之后以较短的间隔重复。您timer_settime()
既可以使用计时器,也可以使用解除计时器;有关详细信息,请参见man 2 timer_settime。
在实际应用中,您通常需要复用定时器。即使一个进程可以创建多个计时器,它们也是有限的资源。特别是超时计时器——设置标志和/或向特定线程发送信号是微不足道的——应该使用单个计时器,该计时器在下一次超时时触发,设置相关的超时标志,并可选择发送信号(使用空体处理程序)到所需的线程以确保它被中断。(对于单线程进程,原始信号传递将中断阻塞 I/O 调用。)考虑一个服务器,响应某个请求:请求本身可能有大约一分钟左右的超时,同时处理请求可能需要连接超时、I/O 超时等。
现在,最初的问题很有趣,因为定时器在有效使用时非常强大。但是,示例程序基本上是无稽之谈。你为什么不创建一个设置一个或多个计时器的程序,例如每个输出一些东西到标准输出?请记住使用write()
et al fromunistd.h
因为它们是异步信号安全的,而printf()
et cetera fromstdio.h
则不是。(如果您的信号处理程序使用非异步信号安全函数,则结果是未定义的。它通常可以工作,但根本不能保证;它可能和工作一样崩溃。测试不会说明,因为它是未定义的。)
编辑添加:这是多路复用超时的简单示例。
(在法律允许的范围内,我将下面显示的代码片段的所有版权和相关权利和邻接权奉献给全球公共领域;请参阅CC0 公共领域奉献。换句话说,您可以随意以任何方式使用下面的代码希望,只是不要怪我有任何问题。)
我使用了老式的 GCC atomic built-ins,所以它应该是线程安全的。通过一些添加,它也应该适用于多线程代码。(例如,您不能使用互斥锁,因为pthread_mutex_lock()
它不是异步信号安全的。原子操作超时状态应该可以工作,尽管如果您在触发时禁用超时,可能会留下一些竞争。)
#define _POSIX_C_SOURCE 200809L
#include <unistd.h>
#include <signal.h>
#include <time.h>
#include <errno.h>
#define TIMEOUTS 16
#define TIMEOUT_SIGNAL (SIGRTMIN+0)
#define TIMEOUT_USED 1
#define TIMEOUT_ARMED 2
#define TIMEOUT_PASSED 4
static timer_t timeout_timer;
static volatile sig_atomic_t timeout_state[TIMEOUTS] = { 0 };
static struct timespec timeout_time[TIMEOUTS];
/* Return the number of seconds between before and after, (after - before).
* This must be async-signal safe, so it cannot use difftime().
*/
static inline double timespec_diff(const struct timespec after, const struct timespec before)
{
return (double)(after.tv_sec - before.tv_sec)
+ (double)(after.tv_nsec - before.tv_nsec) / 1000000000.0;
}
/* Add positive seconds to a timespec, nothing if seconds is negative.
* This must be async-signal safe.
*/
static inline void timespec_add(struct timespec *const to, const double seconds)
{
if (to && seconds > 0.0) {
long s = (long)seconds;
long ns = (long)(0.5 + 1000000000.0 * (seconds - (double)s));
/* Adjust for rounding errors. */
if (ns < 0L)
ns = 0L;
else
if (ns > 999999999L)
ns = 999999999L;
to->tv_sec += (time_t)s;
to->tv_nsec += ns;
if (to->tv_nsec >= 1000000000L) {
to->tv_nsec -= 1000000000L;
to->tv_sec++;
}
}
}
/* Set the timespec to the specified number of seconds, or zero if negative seconds.
*/
static inline void timespec_set(struct timespec *const to, const double seconds)
{
if (to) {
if (seconds > 0.0) {
const long s = (long)seconds;
long ns = (long)(0.5 + 1000000000.0 * (seconds - (double)s));
if (ns < 0L)
ns = 0L;
else
if (ns > 999999999L)
ns = 999999999L;
to->tv_sec = (time_t)s;
to->tv_nsec = ns;
} else {
to->tv_sec = (time_t)0;
to->tv_nsec = 0L;
}
}
}
/* Return nonzero if the timeout has occurred.
*/
static inline int timeout_passed(const int timeout)
{
if (timeout >= 0 && timeout < TIMEOUTS) {
const int state = __sync_or_and_fetch(&timeout_state[timeout], 0);
/* Refers to an unused timeout? */
if (!(state & TIMEOUT_USED))
return -1;
/* Not armed? */
if (!(state & TIMEOUT_ARMED))
return -1;
/* Return 1 if timeout passed, 0 otherwise. */
return (state & TIMEOUT_PASSED) ? 1 : 0;
} else {
/* Invalid timeout number. */
return -1;
}
}
/* Release the timeout.
* Returns 0 if the timeout had not fired yet, 1 if it had.
*/
static inline int timeout_unset(const int timeout)
{
if (timeout >= 0 && timeout < TIMEOUTS) {
/* Obtain the current timeout state to 'state',
* then clear all but the TIMEOUT_PASSED flag
* for the specified timeout.
* Thanks to Bylos for catching this bug. */
const int state = __sync_fetch_and_and(&timeout_state[timeout], TIMEOUT_PASSED);
/* Invalid timeout? */
if (!(state & TIMEOUT_USED))
return -1;
/* Not armed? */
if (!(state & TIMEOUT_ARMED))
return -1;
/* Return 1 if passed, 0 otherwise. */
return (state & TIMEOUT_PASSED) ? 1 : 0;
} else {
/* Invalid timeout number. */
return -1;
}
}
int timeout_set(const double seconds)
{
struct timespec now, then;
struct itimerspec when;
double next;
int timeout, i;
/* Timeout must be in the future. */
if (seconds <= 0.0)
return -1;
/* Get current time, */
if (clock_gettime(CLOCK_REALTIME, &now))
return -1;
/* and calculate when the timeout should fire. */
then = now;
timespec_add(&then, seconds);
/* Find an unused timeout. */
for (timeout = 0; timeout < TIMEOUTS; timeout++)
if (!(__sync_fetch_and_or(&timeout_state[timeout], TIMEOUT_USED) & TIMEOUT_USED))
break;
/* No unused timeouts? */
if (timeout >= TIMEOUTS)
return -1;
/* Clear all but TIMEOUT_USED from the state, */
__sync_and_and_fetch(&timeout_state[timeout], TIMEOUT_USED);
/* update the timeout details, */
timeout_time[timeout] = then;
/* and mark the timeout armable. */
__sync_or_and_fetch(&timeout_state[timeout], TIMEOUT_ARMED);
/* How long till the next timeout? */
next = seconds;
for (i = 0; i < TIMEOUTS; i++)
if ((__sync_fetch_and_or(&timeout_state[i], 0) & (TIMEOUT_USED | TIMEOUT_ARMED | TIMEOUT_PASSED)) == (TIMEOUT_USED | TIMEOUT_ARMED)) {
const double secs = timespec_diff(timeout_time[i], now);
if (secs >= 0.0 && secs < next)
next = secs;
}
/* Calculate duration when to fire the timeout next, */
timespec_set(&when.it_value, next);
when.it_interval.tv_sec = 0;
when.it_interval.tv_nsec = 0L;
/* and arm the timer. */
if (timer_settime(timeout_timer, 0, &when, NULL)) {
/* Failed. */
__sync_and_and_fetch(&timeout_state[timeout], 0);
return -1;
}
/* Return the timeout number. */
return timeout;
}
static void timeout_signal_handler(int signum __attribute__((unused)), siginfo_t *info, void *context __attribute__((unused)))
{
struct timespec now;
struct itimerspec when;
int saved_errno, i;
double next;
/* Not a timer signal? */
if (!info || info->si_code != SI_TIMER)
return;
/* Save errno; some of the functions used may modify errno. */
saved_errno = errno;
if (clock_gettime(CLOCK_REALTIME, &now)) {
errno = saved_errno;
return;
}
/* Assume no next timeout. */
next = -1.0;
/* Check all timeouts that are used and armed, but not passed yet. */
for (i = 0; i < TIMEOUTS; i++)
if ((__sync_or_and_fetch(&timeout_state[i], 0) & (TIMEOUT_USED | TIMEOUT_ARMED | TIMEOUT_PASSED)) == (TIMEOUT_USED | TIMEOUT_ARMED)) {
const double seconds = timespec_diff(timeout_time[i], now);
if (seconds <= 0.0) {
/* timeout [i] fires! */
__sync_or_and_fetch(&timeout_state[i], TIMEOUT_PASSED);
} else
if (next <= 0.0 || seconds < next) {
/* This is the soonest timeout in the future. */
next = seconds;
}
}
/* Note: timespec_set() will set the time to zero if next <= 0.0,
* which in turn will disarm the timer.
* The timer is one-shot; it_interval == 0.
*/
timespec_set(&when.it_value, next);
when.it_interval.tv_sec = 0;
when.it_interval.tv_nsec = 0L;
timer_settime(timeout_timer, 0, &when, NULL);
/* Restore errno. */
errno = saved_errno;
}
int timeout_init(void)
{
struct sigaction act;
struct sigevent evt;
struct itimerspec arm;
/* Install timeout_signal_handler. */
sigemptyset(&act.sa_mask);
act.sa_sigaction = timeout_signal_handler;
act.sa_flags = SA_SIGINFO;
if (sigaction(TIMEOUT_SIGNAL, &act, NULL))
return errno;
/* Create a timer that will signal to timeout_signal_handler. */
evt.sigev_notify = SIGEV_SIGNAL;
evt.sigev_signo = TIMEOUT_SIGNAL;
evt.sigev_value.sival_ptr = NULL;
if (timer_create(CLOCK_REALTIME, &evt, &timeout_timer))
return errno;
/* Disarm the timeout timer (for now). */
arm.it_value.tv_sec = 0;
arm.it_value.tv_nsec = 0L;
arm.it_interval.tv_sec = 0;
arm.it_interval.tv_nsec = 0L;
if (timer_settime(timeout_timer, 0, &arm, NULL))
return errno;
return 0;
}
int timeout_done(void)
{
struct sigaction act;
struct itimerspec arm;
int errors = 0;
/* Ignore the timeout signals. */
sigemptyset(&act.sa_mask);
act.sa_handler = SIG_IGN;
if (sigaction(TIMEOUT_SIGNAL, &act, NULL))
if (!errors) errors = errno;
/* Disarm any current timeouts. */
arm.it_value.tv_sec = 0;
arm.it_value.tv_nsec = 0L;
arm.it_interval.tv_sec = 0;
arm.it_interval.tv_nsec = 0;
if (timer_settime(timeout_timer, 0, &arm, NULL))
if (!errors) errors = errno;
/* Destroy the timer itself. */
if (timer_delete(timeout_timer))
if (!errors) errors = errno;
/* If any errors occurred, set errno. */
if (errors)
errno = errors;
/* Return 0 if success, errno otherwise. */
return errors;
}
编译时记得包含rt
库,即使用gcc -W -Wall *source*.c -lrt -o *binary*
编译。
这个想法是主程序首先调用timeout_init()
以安装所有必要的处理程序等,并且可能会timeout_done()
在退出之前调用以取消它(或在fork()
ing 之后的子进程中)。
要设置超时,请调用timeout_set(seconds)
. 返回值是一个超时描述符。目前只有一个标志可以使用 来检查timeout_passed()
,但超时信号的传递也会中断任何阻塞的 I/O 调用。因此,您可以预期超时会中断任何阻塞的 I/O 调用。
如果您想做的不仅仅是在超时时设置一个标志,您不能在信号处理程序中执行此操作;请记住,在信号处理程序中,您仅限于异步信号安全功能。最简单的解决方法是使用带有无限循环的单独线程sigwaitinfo()
,并TIMEOUT_SIGNAL
在所有其他线程中阻塞信号。这样可以保证专用线程捕获信号,但同时不限于异步信号安全功能。例如,它可以做更多的工作,甚至可以使用pthread_kill()
. (只要该信号有一个处理程序,即使是一个空主体,它的传递也会中断该线程中的任何阻塞 I/O 调用。)
这是一个使用超时的简单示例main()
。这很愚蠢,并且依赖于fgets()
不重试(当被信号中断时),但它似乎有效。
#include <string.h>
#include <stdio.h>
int main(void)
{
char buffer[1024], *line;
int t1, t2, warned1;
if (timeout_init()) {
fprintf(stderr, "timeout_init(): %s.\n", strerror(errno));
return 1;
}
printf("You have five seconds to type something.\n");
t1 = timeout_set(2.5); warned1 = 0;
t2 = timeout_set(5.0);
line = NULL;
while (1) {
if (timeout_passed(t1)) {
/* Print only the first time we notice. */
if (!warned1++)
printf("\nTwo and a half seconds left, buddy.\n");
}
if (timeout_passed(t2)) {
printf("\nAw, just forget it, then.\n");
break;
}
line = fgets(buffer, sizeof buffer, stdin);
if (line) {
printf("\nOk, you typed: %s\n", line);
break;
}
}
/* The two timeouts are no longer needed. */
timeout_unset(t1);
timeout_unset(t2);
/* Note: 'line' is non-NULL if the user did type a line. */
if (timeout_done()) {
fprintf(stderr, "timeout_done(): %s.\n", strerror(errno));
return 1;
}
return 0;
}