perror()调用影响信号量锁定/解锁的问题

Question

我正在尝试进行一些基本的并行处理，使用POSIX共享内存段和未命名信号量对整数进行奇偶排序。在这一点上，我几乎所有的东西都在工作，除了最后一件事：如果我在信号量锁定/解锁后不直接执行perror（），代码的行为会有所不同（随后排序会不正确）。如果在信号量锁定和解锁后直接保留perror（）调用，代码会对整数数组进行完美排序。

int semaphoreCheck = sem_init(&(sharedData->swapSem), 1, 1);
if (semaphoreCheck == -1)
{
    perror( "failed to initialize semaphore" );
    exit(EXIT_FAILURE);
}
pid_t fork1;
fork1 = fork();
if (fork1 == 0)
{
    // original.child
    pid_t fork2;
    fork2 = fork();
    if (fork2 == 0)
    {
        // child.child
        // do a portion of the sort here
        while(sharedData->evenSwap || sharedData->oddSwap)
        {
            // obtain lock on the shared vector
            // int commandCheck = shmctl(sharedID, SHM_LOCK, NULL);
            int commandCheck = sem_wait(&(sharedData->swapSem));
            perror("semaphore lock");
            // if lock was obtained
            if (commandCheck == 0)
            {
                sharedData->evenSwap = false;
                for( int index = 1; index < arraySize - 1; index +=2)
                {
                    if( sharedData->vecData[index] > sharedData->vecData[index + 1] )   
                    {
                        int temp;
                        temp = sharedData->vecData[index];
                        sharedData->vecData[index] = sharedData->vecData[index+1];
                        sharedData->vecData[index+1] = temp;
                        sharedData->evenSwap = true;
                    }       
                }
                // release lock on the shared vector
                commandCheck = sem_post(&(sharedData->swapSem));
                perror("semaphore unlock");
                if (commandCheck == -1)
                {
                    perror("failed to unlock shared semaphore");
                }
            }
            else perror("failed to lock shared semaphore");
        }
        _exit(0);
    }
    else if (fork2 > 0)
    {
        // child.parent
        // do a portion of the sort here
        while(sharedData->evenSwap || sharedData->oddSwap)
        {
            // obtain lock on the shared vector
            int commandCheck = sem_wait(&(sharedData->swapSem));
            perror("semaphore lock");
            // if lock was obtained
            if (commandCheck == 0)
            {
                sharedData->oddSwap = false;
                for( int index = 0; index < arraySize - 1; index +=2)
                {
                    if( sharedData->vecData[index] > sharedData->vecData[index + 1] )   
                    {
                        int temp;
                        temp = sharedData->vecData[index];
                        sharedData->vecData[index] = sharedData->vecData[index+1];
                        sharedData->vecData[index+1] = temp;
                        sharedData->oddSwap = true;
                    }       
                }
                // release lock on the shared vector
                commandCheck = sem_post(&(sharedData->swapSem));
                perror("semaphore unlock");
                if (commandCheck == -1)
                {
                    perror("failed to unlock shared semaphore");
                }
            }
            else perror("failed to lock shared semaphore");
        }
        _exit(0);
    }
    else
    {
        // child.error
        // forking error.
        perror("failed to fork in child");
        exit(EXIT_FAILURE);
    }
}
else if( fork1 > 0)
{
    // original.parent
    // wait for the child process to finish.
    waitpid(fork1, NULL, 0);
}
else
{
    // forking error
    perror("failed to fork");
    exit(EXIT_FAILURE);
}

我只能猜测，如果等待无法完成，这与信号量如何阻止进程有关，但我不明白perror（）是如何调用修复它的。

Answer 1

我认为您的问题可能与您在获得信号量后检查条件是否仍然适用，或者检查条件本身是否错误有关。

您有：

    while(sharedData->evenSwap || sharedData->oddSwap)
    {
        // obtain lock on the shared vector
        int commandCheck = sem_wait(&(sharedData->swapSem));
        perror("semaphore lock");
        // if lock was obtained
        if (commandCheck == 0)
        {
            sharedData->oddSwap = false;

在获得信号量之后，您可能应该验证sharedData->evenSwap或sharedData->oddSwap是否仍然为真，如果不是，则放弃信号量。这是一个标准的成语；您可以检查、锁定和重新检查，因为从原始检查到获得锁定时，状态可能发生了变化。

在这种假设下，perror()调用改变了进程的定时，允许条件比不存在perror()调用时更长时间保持不变。所以，在某个地方，这里有一个时间问题。