我将exit()
放入一个帖子中,但我的程序有时不会退出。
根据此link,exit()
不是async-signal-safe
。我想知道在一个线程中使用exit()
是否会导致未定义的行为。
答案 0 :(得分:1)
普通exit
(例如,与_exit
相对)需要执行所有通常的atexit
清理,输出 - 刷新等工作。 可以构建在某些情况下挂起的代码,但我不得不制作一个“明显的问题”来展示它。如果库例程(例如,内部stdio fflush
)试图在其他线程持有的退出线程中获取锁(例如,在stdio流上),则可能会获得类似的挂起甚至没有你自己的atexit
。由于你没有展示你的代码,我只是在猜测。
这是一个测试程序(有故意的,明显的问题),当被告知时,至少在FreeBSD上会挂起。 (格式化棘手,因为剪切和粘贴保持标签,但后来我必须编辑一些到空格......)
#include <pthread.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
pthread_mutex_t global_mtx;
void die(int error, const char *fmt, ...) {
va_list ap;
va_start(ap, fmt);
vfprintf(stderr, fmt, ap);
va_end(ap);
if (error)
fprintf(stderr, ": %s\n", strerror(error));
else
putc('\n', stderr);
fflush(stderr);
_exit(0);
}
enum behavior { NORMAL, EXIT_WO_HANG, EXIT_W_HANG };
struct behave {
enum behavior how;
pthread_mutex_t lock;
pthread_cond_t cond;
int th1_entered;
int th2_entered;
};
void hanger(void);
void *th1_main(void *);
void *th2_main(void *);
#define WR(x) (void)write(1, x, sizeof(x) - 1)
int main(int argc, char **argv) {
int error;
struct behave how;
pthread_t th1, th2;
error = pthread_mutex_init(&global_mtx, NULL);
if (error)
die(error, "pthread_mutex_init global_mtx");
error = pthread_mutex_init(&how.lock, NULL);
if (error)
die(error, "pthread_mutex_init how.lock");
error = pthread_cond_init(&how.cond, NULL);
if (error)
die(error, "pthread_cond_init how.cond");
how.how = NORMAL;
how.th1_entered = 0;
how.th2_entered = 0;
if (argc > 1) {
if (strcmp(argv[1], "exit") == 0)
how.how = EXIT_WO_HANG;
else if (strcmp(argv[1], "hang") == 0)
how.how = EXIT_W_HANG;
else if (strcmp(argv[1], "normal") != 0)
die(0, "usage: example [normal|exit|hang]");
}
atexit(hanger);
error = pthread_create(&th1, NULL, th1_main, &how);
if (error)
die(error, "pthread_create th1");
error = pthread_create(&th2, NULL, th2_main, &how);
if (error)
die(error, "pthread_create th2");
/* now wait for threads */
error = pthread_join(th1, NULL);
error = pthread_join(th2, NULL);
printf("joined, normal exit\n");
return 0;
}
void *th1_main(void *arg) {
struct behave *how = arg;
WR("thread 1 start\n");
(void) pthread_mutex_lock(&global_mtx);
(void) pthread_mutex_lock(&how->lock);
how->th1_entered = 1;
pthread_cond_signal(&how->cond);
while (how->th2_entered == 0)
(void) pthread_cond_wait(&how->cond, &how->lock);
WR("thread 1 sees thread 2 started\n");
(void) pthread_mutex_unlock(&how->lock);
if (how->how == EXIT_W_HANG)
WR("thread 1 not unlocking\n");
else
(void) pthread_mutex_unlock(&global_mtx);
return NULL;
}
void *th2_main(void *arg) {
struct behave *how = arg;
WR("thread 2 start\n");
(void) pthread_mutex_lock(&how->lock);
how->th2_entered = 1;
pthread_cond_signal(&how->cond);
while (how->th1_entered == 0)
(void) pthread_cond_wait(&how->cond, &how->lock);
WR("thread 2 sees thread 1 started\n");
(void) pthread_mutex_unlock(&how->lock);
if (how->how != NORMAL) {
WR("thread 2 exit()\n");
exit(1);
}
return NULL;
}
void hanger(void) {
/* this is what will cause us to hang, in the one case */
WR("hanger start\n");
pthread_mutex_lock(&global_mtx);
WR("hanger got global mutex\n");
pthread_mutex_unlock(&global_mtx);
WR("hanger finish\n");
}