下面的代码仅优先于第一个 child 修改共享变量。如何删除此错误？

Question

我想捕获所有 child 进程对 parent 进程对共享变量所做的更改。

问题来了。 parent 进程创建这样的结构：

struct data
{
        int pid;
        int n;
        char c;
};

它创建了 k 个进程，并且结构与它的所有 child 共享。变量 c 被初始化为 'n' 并且它作为一个标志。

Parent定期"wakes up"（来自"sleep"）在随机时间（不超过2秒）后检查c的值，如果c是'y' 然后打印 n 的值（连同相应的 pid）并使 c 再次成为 'n'。

另一方面，每隔 child Pi，1≤ i ≤ k，在随机时间（不超过 2 秒）后定期 "wakes up"（从 "sleep"） ) 检查 c 的值，如果 c 是 'n'，则将一些随机值分配给 n，连同其进程 ID 一起分配给 pid，打印该值及其自己的进程 ID，并使 c 为 'y'.

如果程序执行被用户终止（通过按 Ctrl-C），parent 及其所有 children 应该终止并且分配的内存应该被释放。

代码如下：

typedef void (*sighandler_t)(int);
int shmid;

struct data
{
        int pid;
        int n;
        char c;
};

void releaseSHM(int signum)
{
        int status;
        status = shmctl(shmid, IPC_RMID, NULL);
        if (status == 0)
                fprintf(stderr, "Remove shared memory with id = %d.\n", shmid);
        else if (status == -1)
                fprintf(stderr,"Cannot remove shared memory of id = %d.\n", shmid);
        else
                fprintf(stderr, "shmctl() returned wrong value while removing shared memory with id = %d.\n", shmid);



        status = kill(0, SIGKILL);
        exit(signum);
}

int main(int argc, char *argv[])
{
        int num,k=5,i, p1;
        struct data *s;

        sighandler_t shandler;

        shandler =  signal(SIGINT, releaseSHM);

        shmid =  shmget(IPC_PRIVATE, sizeof(struct data), IPC_CREAT | 0777);
        if(shmid==-1)
        {
                perror("shmget() failed");
                exit(1);
        }

        s=(struct data *)shmat(shmid, NULL, 0);
        s->c='n';

        for(i=0;i<k;i++)
        {
                p1=fork();
                if(p1==0)
                {
                        while(1)
                        {
                                //shandler =  signal(SIGINT, releaseSHM);
                                num=(rand()%3);
                                sleep(num);
                                if(s->c=='n')
                                {
                                                s->n=rand();
                                                s->pid=getpid();
                                                printf("Child with pid %d set value of n as %d \n",s->pid,s->n);
                                                s->c='y';
                                }
                                sleep(num);
                        }
                }
                else
                {
                        while(1)
                        {
                                num=(rand()%3);
                                sleep(num);
                                if(s->c=='y')
                                        printf("Parent reads value of n as %d set by child with process id %d \n",s->n, s->pid);
                                s->c='n';
                        }
                }
        }
        return 0;
}

输出为：

Child with pid 13883 set value of n as 846930886 
Parent reads value of n as 846930886 set by child with process id 13883 
Child with pid 13883 set value of n as 1957747793 
Parent reads value of n as 1957747793 set by child with process id 13883 
Child with pid 13883 set value of n as 719885386 
Parent reads value of n as 719885386 set by child with process id 13883 
Child with pid 13883 set value of n as 596516649 
Parent reads value of n as 596516649 set by child with process id 13883 
Child with pid 13883 set value of n as 1350490027 
Parent reads value of n as 1350490027 set by child with process id 13883 
^CRemove shared memory with id = 15368197.
Killed

很明显，它只捕获一个 child 进程所做的更改，不给其他 child 进程机会。如何消除这个错误？

如果需要任何其他信息，请在下方发表评论。

Answer 1

您打算让 parent 和 k children 争夺共享内存，但您只生成了一个 child 无论如何k.

的值

你这样写：

....
for (int i = 0; i < k; i++) {
  pid_t child = fork();
  if (child == 0) do_child_infinite_loop();
  else do_parent_infinite_loop(); // <- BUG: the for loop never resumes
}

你的意思是：

....
for (int i = 0; i < k; i++) {
  pid_t child = fork();
  if (child == 0) do_child_infinite_loop();
}
do_parent_infinite_loop();

修复此问题后，您可以继续进行其他调试：k children 会踩到彼此的更新，并且它们也共享相同的 pseudo-random数列.

下面的代码仅优先于第一个 child 修改共享变量。如何删除此错误？

The code below gives preference to only it's first child to modify a shared variable. How to remove this bug?

signals

ipc

process

shared-memory