在分叉进程之间共享运行时创建的对象

Question

我有一个节点类，它包含两个嵌套类，服务器和客户机，以及指向每个类实例的指针。

#include "../hdr/g04.h"
#include <sys/shm.h>
#include <sys/wait.h>

void fatal(string s) {
    cerr << "Error: " << s << " . . .\n";
    exit(-1);   
}

int main() {

    int shmkey = 12345, shmid;
    Node **nodeLocation, *thisNode;
    pid_t client_pid, server_pid;

    // get a shared mem segment
    if ((shmid = shmget(shmkey, sizeof(Node *), IPC_CREAT | 0666)) < 0)
        fatal("shmget");
    if ((nodeLocation = (Node **) shmat(shmid, NULL, 0)) == (Node **) - 1)
        fatal("shmat");

    // instantiate the Node
    // assign shared memory address
    thisNode = new Node();
    *nodeLocation = thisNode;

    // Node configuration / initialization
    if (configure(*thisNode) < 0) fatal("configuration");
    if (thisNode->read_seeds() < 0) fatal("seed file");
    if (thisNode->read_catalogue() < 0) fatal("catalogue");
    thisNode->init();

    // test
    thisNode->server->test = 10;
    cout << "parent: " << thisNode 
        << "\n\t" << thisNode->server->test << endl;


    // create a client operations process
    if ((client_pid = fork()) < 0) fatal("fork");
    if (client_pid == 0) {

        // test
        thisNode->server->test = 20;
        cout << "client: " << thisNode 
            << "\n\t" << thisNode->server->test << endl;

        // do client stuff
        exit(0);
    }

    // test
    wait(NULL);
    cout << "parent: " << thisNode 
        << "\n\t" << thisNode->server->test << endl;


    // create a server operations process
    if ((server_pid = fork()) < 0) fatal("fork");
    if (server_pid == 0) {

        // test
        thisNode->server->test = 30;
        cout << "server: " << thisNode 
            << "\n\t" << thisNode->server->test << endl;

        // do server stuff
        exit(0);
    }

    // test
    wait(NULL);
    cout << "parent: " << thisNode 
        << "\n\t" << thisNode->server->test << endl;


    delete thisNode;
    return 0;
}

parent: 0x7f2532683390
        10
client: 0x7f2532683390
        20
parent: 0x7f2532683390
        10
server: 0x7f2532683390
        30
parent: 0x7f2532683390
        10

parent: 0x7f2532683390
        10
client: 0x7f2532683390
        20
parent: 0x7f2532683390
        20
server: 0x7f2532683390
        30
parent: 0x7f2532683390
        30

我的猜测是上下文切换正在发生，因此每个进程都有自己的副本被加载到相同的地址中。

我如何改变这一点，使每个进程访问该节点的完全相同的副本？

更新：

#include <sys/shm.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <errno.h>

void fatal(string s) {
    if (errno)
        perror(s.c_str());
    else
        cerr << "Error: " << s << " . . .\n";
    exit(-1);   
}

void test(string proc, Node *thisNode) {
    cout << proc << thisNode 
        << "\n\t" << thisNode->server->test << endl;
}

int main() {

    Node *tmp = new Node();
    key_t shmkey;
    int shmid;
    Node *thisNode, *nodeLocation;
    pid_t client_pid, server_pid;

    if ((shmkey = ftok("shared_obj.dat", 'R')) == (key_t)-1)
        fatal("ftok: ");
    if ((shmid = shmget(shmkey, sizeof(*tmp), 0644 | IPC_CREAT)) < 0)
        fatal("shmget: ");
    if ((nodeLocation = (Node *) shmat(shmid, (void *)0, 0)) < 0)
        fatal("shmat: ");
    thisNode = new (nodeLocation) Node();


    // Node configuration / initialization
    if (configure(*thisNode) < 0) fatal("configuration");
    if (thisNode->read_seeds() < 0) fatal("seed file");
    if (thisNode->read_catalogue() < 0) fatal("catalogue");
    thisNode->init();

    // test
    thisNode->server->test = 10;
    test("parent: ", thisNode);

    // create a client operations process
    if ((client_pid = fork()) < 0) fatal("fork");
    if (client_pid == 0) {

        thisNode->server->test = 20;
        test("client: ", thisNode); 
        // do client stuff
        exit(0);
    }

    // test
    wait(NULL);
    test("parent: ", thisNode);


    // create a server operations process
    if ((server_pid = fork()) < 0) fatal("fork");
    if (server_pid == 0) {

        thisNode->server->test = 30;
        test("server: ", thisNode); 
        // do server stuff
        exit(0);
    }

    // test
    wait(NULL);
    test("parent: ", thisNode);


    // delete thisNode;
    return 0;
}

Answer 1

您没有共享节点对象，每个进程都有自己的节点副本。它们在分叉时间是相同的，但一个过程中的变化不会反映在另一个过程中。要做到这一点，您必须实际实例化共享内存中的节点对象。你需要做一个“放置新的”

https://isocpp.org/wiki/faq/dtors#放置-新建