我有一个boxIn[]类型的数组，它包含char个字符。R R G B G B O Y O O P R在共享内存中。共享内存中还有一个值为boxIn[]数组中char*s之一的p。
程序中定义了两个信号量，如下所示：
/* initialize semaphores */
if((sem_init(&sem, pshared, value)) == 1){ /* only 2(value) processes at the same time */
perror("Error initializing synch semaphore\n");
exit(1);
}
if((sem_init(&mutex, pshared, 1)) == 1){ /* only 1 processes at the same time */
perror("Error initializing synch semaphore\n");
exit(1);
}
我把孩子们从一个循环中分离出来：
/* fork child processes */
for(i=0; i<n ; i++){
pid = fork();
if(pid < 0){ /* check for error */
printf("Fork error.\n");
}
else if(pid == 0) break; /* child processes */
}
然后我让孩子们做一些处理。目前我正努力一步一步地达到我的目标。因此，我只更改一次
char的值。/******************************************************/
/****************** PARENT PROCESS ****************/
/******************************************************/
if(pid != 0){
while(wait()>0);
}
/******************************************************/
/****************** CHILD PROCESS *****************/
/******************************************************/
else{
while(*p != boxIn[i]); /* wait until it's my turn */
sem_wait(&sem);
printf("%c boxes are being painted.\n",boxIn[i]);
printf("Done painting.\n");
sleep(1);
sem_wait(&mutex);
if(*p=='R') *p='G';
sem_post(&mutex);
sem_post(&sem);
exit(1);
}
但是我得到了一个意外的输出：
我期望的是：
R boxes are being painted.
R boxes are being painted.
Done.
Done.
R boxes are being painted.
Done.
G boxes are being painted.
G boxes are being painted.
Done.
Done.
我得到的是：
R boxes are being painted.
Done painting.
R boxes are being painted.
Done painting.
R boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
varaquilex@computer ~/Dropbox/Courses/BLG312E - Computer Operating Systems/hw3 $ G boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
G boxes are being painted.
G boxes are being painted.
Done painting.
Done painting.
G boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
P boxes are being painted.
Done painting.
P boxes are being painted.
Done painting.
P boxes are being painted.
Done painting.
P boxes are being painted.
Done painting.
P boxes are being painted.
Done painting.
^C
问题是：尽管
boxIn[]数组中只有2个for框，但为什么要绘制2个以上的*p框，更重要的是，如何绘制G框，更不用说数组中只有一个G框，并且绘制了许多boxIn[]框？注：我已经试了很长时间了。更不用说在这之前我用了shift+c来停止很多程序。我重新启动终端，得到这个输出。更有趣的一点是，我试着把
P的值和正在绘制的框一起打印出来，我看到输出是混合的！我是说，有些行显示P，有些行显示P，即使i是G boxes(3) are being painted。这很尴尬，我重新启动了操作系统，从同一个程序得到了不同的输出。有人能解释一下为什么会发生这种情况，我怎样才能防止这种情况再次发生？ 最佳答案
为了使用进程共享信号量，它们必须在共享内存中。你的不是。所以在fork之后，每个进程都有自己的信号量副本。分配共享内存的最简单方法是：
sem_t *sem = mmap(0, sizeof(sem_t), PROT_READ|PROT_WRITE,
MAP_SHARED|MAP_ANONYMOUS, -1, 0);
然而，尽管匿名映射几乎是普遍可用的，但它并没有在posix中指定，因此在形式上“不可移植”。另一种可移植的方法是创建一个临时文件或posix共享内存对象。
或者（这里是推荐的方法），使用
sem_open而不是sem_init。关于c - fork 的过程，信号量，为什么要输出？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/16387348/