【发布时间】：2020-09-28 20:17:43
【问题描述】：
我正在编写一个非常简单的程序来演示我从 C++ 移植回 C 的 Pthreads 实现。
我创建了两个锁步线程并给他们两个工作
每一步增加一次 a1
一个递减a2每一步
在同步阶段（当互斥锁为 t1 和 t2 都锁定时）我比较 a1 和 a2 以查看我们是否应该停止步进。
我想知道我是不是发疯了，因为不仅变量在步进和锁定后并不总是变化，而且它们有时以不同的速率变化即使在锁定之后线程仍在运行。
编辑：是的，我确实对此进行了研究。是的，C++ 实现有效。是的，C++ 实现与这个几乎相同，但我必须在 c 中转换 PTHREAD_MUTEX_INITIALIZER 和 PTHREAD_COND_INITIALIZER 并将其作为第一个参数传递给每个函数。我花了一段时间试图调试这个（没有淘汰 gdb）但无济于事。
#ifndef LOCKSTEPTHREAD_H
#define LOCKSTEPTHREAD_H
#include <pthread.h>
#include <stdio.h>
typedef struct {
pthread_mutex_t myMutex;
pthread_cond_t myCond;
pthread_t myThread;
int isThreadLive;
int shouldKillThread;
void (*execute)();
} lsthread;
void init_lsthread(lsthread* t);
void start_lsthread(lsthread* t);
void kill_lsthread(lsthread* t);
void kill_lsthread_islocked(lsthread* t);
void lock(lsthread* t);
void step(lsthread* t);
void* lsthread_func(void* me_void);
#ifdef LOCKSTEPTHREAD_IMPL
//function declarations
void init_lsthread(lsthread* t){
//pthread_mutex_init(&(t->myMutex), NULL);
//pthread_cond_init(&(t->myCond), NULL);
t->myMutex = (pthread_mutex_t)PTHREAD_MUTEX_INITIALIZER;
t->myCond = (pthread_cond_t)PTHREAD_COND_INITIALIZER;
t->isThreadLive = 0;
t->shouldKillThread = 0;
t->execute = NULL;
}
void destroy_lsthread(lsthread* t){
pthread_mutex_destroy(&t->myMutex);
pthread_cond_destroy(&t->myCond);
}
void kill_lsthread_islocked(lsthread* t){
if(!t->isThreadLive)return;
//lock(t);
t->shouldKillThread = 1;
step(t);
pthread_join(t->myThread,NULL);
t->isThreadLive = 0;
t->shouldKillThread = 0;
}
void kill_lsthread(lsthread* t){
if(!t->isThreadLive)return;
lock(t);
t->shouldKillThread = 1;
step(t);
pthread_join(t->myThread,NULL);
t->isThreadLive = 0;
t->shouldKillThread = 0;
}
void lock(lsthread* t){
if(pthread_mutex_lock(&t->myMutex))
puts("\nError locking mutex.");
}
void step(lsthread* t){
if(pthread_cond_signal(&(t->myCond)))
puts("\nError signalling condition variable");
if(pthread_mutex_unlock(&(t->myMutex)))
puts("\nError unlocking mutex");
}
void* lsthread_func(void* me_void){
lsthread* me = (lsthread*) me_void;
int ret;
if (!me)pthread_exit(NULL);
if(!me->execute)pthread_exit(NULL);
while (!(me->shouldKillThread)) {
ret = pthread_cond_wait(&(me->myCond), &(me->myMutex));
if(ret)pthread_exit(NULL);
if (!(me->shouldKillThread) && me->execute)
me->execute();
}
pthread_exit(NULL);
}
void start_lsthread(lsthread* t){
if(t->isThreadLive)return;
t->isThreadLive = 1;
t->shouldKillThread = 0;
pthread_create(
&t->myThread,
NULL,
lsthread_func,
(void*)t
);
}
#endif
#endif
这是我的驱动程序：
#define LOCKSTEPTHREAD_IMPL
#include "include/lockstepthread.h"
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
unsigned char a1, a2;
void JobThread1(){
unsigned char copy = a1;
copy++;
a1 = copy;
}
void JobThread2(){
unsigned char copy = a2;
copy--;
a2 = copy;
}
int main(){
char inputline[2048];
inputline[2047] = '\0';
lsthread t1, t2;
init_lsthread(&t1);
init_lsthread(&t2);
t1.execute = JobThread1;
t2.execute = JobThread2;
printf(
"\nThis program demonstrates threading by having"
"\nTwo threads \"walk\" toward each other using unsigned chars."
"\nunsigned Integer overflow guarantees the two will converge."
);
printf("\nEnter a number for thread 1 to process: ");
fgets(inputline, 2047,stdin);
a1 = (unsigned char)atoi(inputline);
printf("\nEnter a number for thread 2 to process: ");
fgets(inputline, 2047,stdin);
a2 = (unsigned char)atoi(inputline);
start_lsthread(&t1);
start_lsthread(&t2);
unsigned int i = 0;
lock(&t1);
lock(&t2);
do{
printf("\n%u: a1 = %d, a2 = %d",i++,(int)a1,(int)a2);
fflush(stdout);
step(&t1);
step(&t2);
lock(&t1);
lock(&t2);
}while(a1 < a2);
kill_lsthread_islocked(&t1);
kill_lsthread_islocked(&t2);
destroy_lsthread(&t1);
destroy_lsthread(&t2);
return 0;
}
示例程序用法：
Enter a number for thread 1 to process: 5
Enter a number for thread 2 to process: 10
0: a1 = 5, a2 = 10
1: a1 = 5, a2 = 10
2: a1 = 5, a2 = 10
3: a1 = 5, a2 = 10
4: a1 = 5, a2 = 10
5: a1 = 5, a2 = 10
6: a1 = 6, a2 = 9
7: a1 = 6, a2 = 9
8: a1 = 7, a2 = 9
9: a1 = 7, a2 = 9
10: a1 = 7, a2 = 9
11: a1 = 7, a2 = 9
12: a1 = 8, a2 = 9
那么，交易是什么？
【问题讨论】：
-
您应该阅读
pthread_cond_wait()和pthread_cond_signal()的联机帮助页。所谓阅读，我的意思是仔细阅读，就好像你想真正理解它们一样。 -
我已经读了好几遍了。是的，当我编写 C++ 实现时，我花了一段时间盯着手册页。我又盯着它看。当它被调用时，互斥锁被调用线程释放，因此它可以被另一个线程锁定。当它返回时，互斥锁被调用线程锁定。 pthread_cond_signal 允许 pthread_cond_wait 继续进行下一次迭代。我花了很长时间玩它。
-
阅读理解似乎有点弱。 POSIX documentation "当使用条件变量时，总是有一个布尔谓词涉及与每个条件等待关联的共享变量，如果线程应该继续，则该谓词为真。可能会发生来自 pthread_cond_wait() 或 pthread_cond_timedwait() 函数的虚假唤醒。"
-
我犯了一个明显的错误吗？你能帮我而不是给我谜语吗？我不明白关于虚假唤醒的一点。是这样吗？
-
游戏示例正是您使用
pthread_barrier_wait()让线程在帧结束时同步在一起的地方。