如何让守护进程与 C 中的子进程通信

Question

我正在编写一个程序，其中父进程（后来成为守护进程）创建一个子进程，然后该子进程进一步创建多个线程，这些线程同时运行以处理查询。这需要线程通过子进程与守护进程通信，然后守护进程通过子进程将控制消息发送回线程。我为此编写了以下代码。

typedef struct shared SHAREDM;
struct shared{
     char tmsg[500];
     char pmsg[500];
} shrd_tc,*shrd_pc;

int main(){
    initialize();
    int pid;
    int shmID;
    int status;
    shmID = shmget(IPC_PRIVATE, sizeof(SHAREDM), IPC_CREAT | 0666);
    if (shmID < 0) {
          printf("*** shmget error (server) ***\n");
          exit(1);
    }
    shrd_pc = (SHAREDM*) shmat(shmID, NULL, 0);  //shared memory b/w parent process and child process
    pid=fork();         
    int st=0;
    if(pid<0){
        printf("\n Error ");
        exit(1);
    }
    /*
      *Child will create threads that will run queries concurrently and communicate to parent process (daemon) 
    via child process 
      * child and threads communcate through shared memory 'shared_tc' of type SHAREDM
      * parent(daemon) and child also communicate through shared memory 'shared_pc' of type SHAREDM 
    */
    else if(pid==0) {    

        pthread_t threads[20];
        long taskids[20];   
        for(int i=0;i<20;i++){
               taskids[i]=i+1;
               pthread_create( &threads[i], NULL, query, (void*)taskids[i]);
        }
        int t=10;
        while(t--){
            printf("Hello from child\n");
            // child will check continuously whether daemon has written in the shared memory
            printf("%s\n", shrd_pc->pmsg);

            //child will check continuouly whether threads has written something in the shared memory
            printf("%s\n", shrd_tc.tmsg);

            sleep(1);   
        }


        for(int i=0;i<20;i++){
             pthread_join(threads[i],NULL);
        }

    }
    else{
        //unmask the file mode
        umask(0);
        int sid;
        //set new session
        sid = setsid();
        if(sid < 0){
        // Return failure   
           exit(1);
        }
        // Change the current working directory to root.
        chdir("/");
        // Close stdin. stdout and stderr

        close(STDIN_FILENO);
        close(STDOUT_FILENO);
        close(STDERR_FILENO);
        int t=10;
        while(t--){
            //doemon will write continuous in shared memory
            sprintf(shrd_pc->pmsg,"%s","Hello I am parent process");
            sleep(1);
        }  
    }



    return 0;
}
void* query(void* thread_id){
     long tid=(long)thread_id;   
     int T=10;
     while(T--){
          //printf("Hello from thread %ld\n",tid);
          //thread writing something to shared memory between child and threads
          sprintf(shrd_tc.tmsg,"%s %ld","Hello I am thread ",tid);
          sleep(1);
     } 
     pthread_exit(0);
}

当我试图让守护进程向 shrd_pc->pmsg 写入内容时，它无法这样做。我无法在子进程中读取 shrd_pc->pmsg。如何让这个守护进程写入 shrd_pc->pmsg 以便稍后孩子可以读取相同的内容？

Answer 1

您的子进程内存是父进程的副本，不与线程共享。

要在进程之间进行通信，有a lot 的可用方法，但您可能需要查看this 的答案，以了解如何与您的孩子共享您的部分地址空间。