信号量无法等待，原因是“资源暂时不可用”

Question

我试图让父进程在父进程继续之前等待多个子进程发出信号，使用未命名信号量数组（每个子进程一个信号量）。但是，当使用sem_wait()时，父进程将无限期地等待，而sem_trywait()返回“资源暂时不可用”错误，并在子进程没有发出信号的情况下继续。sem_init()和sem_post()都不返回错误。

int numsems = concurrent_instrs.size();
std::cout << "Num sems: " << numsems << "\n";
// create semaphores
sem_t* sems = new sem_t[numsems];
for (int i = 0; i < numsems; i++)
{
    if (sem_init(&sems[i], 1, 0) < 0)
    {
        perror("sem initialization failed");
        exit(1);
    }
}

int child_num = 0;

// perform all calculations in block concurrently
for(unsigned int i = 0; i < concurrent_instrs.size() && !isChild; i++)
{
    int pid = fork();
    if (pid == -1)
    {
        perror("Error forking:");
        exit(1);
    }
    if (pid == 0)
    {
        isChild = true;
        instr = concurrent_instrs[i];
    }
    else
    {
        child_num++;
    }
}
if (isChild)
{
    std::cout << "Child process " << child_num << " calculating: " << instr << "\n";
    perform_calculation(instr, input_vars, internal_vars, shm_input, shm_internal);
    std::cout << "Child process " << child_num << " finished calculating\n";

    if (sem_post(&sems[child_num]) < 0)
    {
        perror("Child signal failed");
    }

    std::cout << "Child "<< child_num << " signalled\n";

    // detach from shared memory
    if (shmdt(shm_input) < 0)
    {
        perror("child shm_input detach failed");
    }
    if (shmdt(shm_internal) < 0)
    {
        perror("child shm_internal detach failed");
    }
    exit(0);
}
else
{
    // parent waits for all children to finish
    for (int i = 0; i < numsems; i++)
    {
        std::cout << "Waiting on subprocess " << i << " of " << numsems << "\n";
        if (sem_trywait(&sems[i]) < 0)
            perror("Parent wait failed");
        else
            std::cout << "Parent wait " << i << " working\n";
    }
    std::cout << "Finished waiting\n";

    // destroy semaphores
    for (int i = 0; i < numsems; i++)
    {
        if(sem_destroy(&sems[i]) < 0)
        {
            perror("Sem destroy failed");
            exit(2);
        }
        else
        {
            std::cout << "Sem " << i << " destroyed\n";
        }
    }

    delete[] sems;
}

编辑以添加:sem_wait()遇到错误，无论子进程在等待之前还是之后调用sem_post()。

Answer 1

与sem_t*sems=new sem_t[numsems]；一起分配的信号量不在共享内存中。因此，每个进程都有自己的副本，在子进程中过帐不会影响父进程。

父级的副本保持锁定。sem_trywait失败，导致eAgain，这将转换为资源暂时不可用解释。