如何使用从子命名信号量 [英] How to use named semaphore from child

问题描述

所以基本上我想暂停了一下孩子的过程是创造后，只是让家长prepare在共享内存为它的一些数据。我试图用一个信号，如下建议：<一href=\"http://stackoverflow.com/questions/8359322/how-to-share-semaphores-between-processes-using-shared-memory\">How要使用共享内存进程之间共享信号灯。

问题NR 1 ：孩子不能打开信号灯

问题NR 2 ：字符串错误返回一个int，而人显然strerror的说，它返回的char *

要避免出现您尝试过哪些

  SEM = sem_open（/信号，O_CREAT，0644，0）;对于（i = 0; I＆LT;民;我++）
{
    PID =叉（）;    如果（PID == 0）
    {
        SEM = sem_open（/旗语，0）;
        如果（SEM == SEM_FAILED）
        {
            的printf（错误：％d个\\ N，字符串错误（错误））;
            出口（0）;
        }
        sem_wait（SEM）;
        出口（0）;
    }
}prepare_data_for_child_processes（）;对于（i = 0; I＆LT; mpi_comm_world-＆GT; NP;我++）
    sem_post（SEM）;sem_close（SEM）;
sem_unlink（/信号量）;
 

解决方案

您不需要孩子叫 sem_open（）在所有 - 他们可以简单地 sem_wait（）他们继承 sem_t 处理。

您可能想信号灯限制只是你的工作的船员。在这种情况下，家长应打开权限受限制的信号只（O_EXCL），然后断开链接它的时候了。这将prevent诚实的错误损坏您信号灯的状态（但不会防止恶意程序）：

  ...
SEM = sem_open（/旗语，O_CREAT | O_EXCL，0644，0）; / *注意* O_EXCL /
sem_unlink（/信号量）;对于（i = 0; I＆LT;民;我++）{
  如果（叉（）== 0）{
    sem_wait（SEM）;
    /* 做一些工作 */
    出口（0）;
  }
}prepare_data_for_child_processes（）;对于（i = 0; I＆LT; mpi_comm_world-＆GT; NP;我++）
    sem_post（SEM）;sem_close（SEM）;
 

现在，如果你的实现支持它，您应该 sem_init（1,0）共享内存。这会给你只限于你的工作的船员一个真正的匿名的信号。

（是的，问题＃2是一​​个缺少包括。）

So basically I want to suspend a bit the child process after it's creation, just so the parent prepare some data for it in a shared memory. I'm trying to use a semaphore, as suggested here: How to share semaphores between processes using shared memory.

Problem nr 1: the child can't open the semaphore.

Problem nr 2: strerror returns an int, but man strerror clearly says it returns an char *.

To avoid "what have you tried":

sem = sem_open("/semaphore", O_CREAT, 0644, 0);    

for (i = 0; i < num; i++)                                        
{   
    pid = fork();                                                               

    if (pid == 0)                                                               
    {   
        sem = sem_open("/semaphore", 0);                                        
        if (sem == SEM_FAILED)                                                  
        {   
            printf( "Error : %d\n", strerror(errno ));
            exit(0);                                                            
        }
        sem_wait(sem);   
        exit(0);                                                                
    }                                                                                                                       
}

prepare_data_for_child_processes();

for (i = 0; i < mpi_comm_world->np; i++)
    sem_post(sem);

sem_close(sem);
sem_unlink("/semaphore");

解决方案

You don't need the children to call sem_open() at all — they can simply sem_wait() on their inherited sem_t handle.

You probably want to restrict the semaphore to just your "work crew". In that case, the parent should open the semaphore exclusively (O_EXCL) with restrictive permissions, and then unlink it right away. This will prevent honest mistakes from corrupting your semaphore's state (but won't protect against hostile programs):

...
sem = sem_open("/semaphore", O_CREAT|O_EXCL, 0644, 0); /* Note O_EXCL */
sem_unlink("/semaphore");

for (i = 0; i < num; i++) {
  if (fork() == 0) {
    sem_wait(sem);
    /* Do some work */
    exit(0);
  }
}

prepare_data_for_child_processes();

for (i = 0; i < mpi_comm_world->np; i++)
    sem_post(sem);

sem_close(sem);

Now, if your implementation supports it, you should instead sem_init(1, 0) in shared memory. That will give you a truly anonymous semaphore restricted just to your work crew.

(And, yes, problem #2 is a missing include.)

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