c - 如何在没有外部机制的情况下通信树的进程

Question

我需要使用 fork() 在 C 中构建具有以下形状的进程树:

我必须在它们之间发送信号，所以我还想知道是否有任何方法可以将进程的 PID 存储在数组或其他内容中，这样每个进程都有其他进程的 PID。问题是我有一些限制，例如不使用管道、文件或其他外部机制在进程之间共享数据。 sleep和exec都不能使用。

这就是我在它们之间发送信号的方式:

Answer 1

So I just have to create them in a certain order so at the moment they are created, the PID of the process they will send a signal is already created?

对 - 特别是 H4 必须在 H3/N3 之前 fork ，以便 N3 知道 H4。演示:

#include <signal.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>

void handler(int signum, siginfo_t *si, void *u)
{
    printf("%d received signal from %d %s\n", getpid(), si->si_pid,
                                                        si->si_value.sival_ptr);
}

main()
{
    // for demo, defer signal delivery until process unmasks the signal
    sigset_t set, oldset;
    sigemptyset(&set);
    sigaddset(&set, SIGRTMIN);
    sigprocmask(SIG_BLOCK, &set, &oldset);

    sigaction(SIGRTMIN, &(struct sigaction){ .sa_sigaction = handler,
                                             .sa_flags = SA_SIGINFO }, NULL);
    pid_t P = getpid();
    pid_t H1 = fork();          if (H1 < 0) perror("H1"), exit(1);
    if (H1 == 0)
    {
        // use sigqueue() instead of kill(), so can pass sender ID
        sigqueue(P, SIGRTMIN, (union sigval){.sival_ptr = "H1"});
        sigsuspend(&oldset);
        exit(0);
    }
    pid_t H2 = fork();          if (H2 < 0) perror("H2"), exit(1);
    if (H2 == 0)
    {
        pid_t N2 = fork();      if (N2 < 0) perror("N2"), exit(1);
        if (N2 == 0)
        {
            sigqueue(H1, SIGRTMIN, (union sigval){.sival_ptr = "N2"});
            sigsuspend(&oldset);
            exit(0);
        }
        sigqueue(N2, SIGRTMIN, (union sigval){.sival_ptr = "H2"});
        sigsuspend(&oldset);
        exit(0);
    }
    sigqueue(H2, SIGRTMIN, (union sigval){.sival_ptr = "P"});
    pid_t H4 = fork();          if (H4 < 0) perror("H4"), exit(1);
    if (H4 == 0)
    {
        sigqueue(P, SIGRTMIN, (union sigval){.sival_ptr = "H4"});
        sigsuspend(&oldset);
        exit(0);
    }
    pid_t H3 = fork();          if (H3 < 0) perror("H3"), exit(1);
    if (H3 == 0)
    {
        pid_t N3 = fork();      if (N3 < 0) perror("N3"), exit(1);
        if (N3 == 0)
        {
            sigqueue(H4, SIGRTMIN, (union sigval){.sival_ptr = "N3"});
            sigsuspend(&oldset);
            exit(0);
        }
        sigqueue(N3, SIGRTMIN, (union sigval){.sival_ptr = "H3"});
        sigsuspend(&oldset);
        exit(0);
    }
    sigqueue(H3, SIGRTMIN, (union sigval){.sival_ptr = "P"});
    sigprocmask(SIG_UNBLOCK, &set, NULL);
    do ; while (wait(NULL) > 0 || errno != ECHILD);
}

示例输出:

1074 received signal from 1072 P
1072 received signal from 1073 H1
1072 received signal from 1076 H4
1075 received signal from 1074 H2
1073 received signal from 1075 N2
1077 received signal from 1072 P
1076 received signal from 1078 N3
1078 received signal from 1077 H3