CODEWITHSUNDEEP
c++containersglobal
Squall Leonhart
Squall Leonhart

Reputation: 117

C++: Global container not being modified

I have the following C++ code:

map<string, map<string, deque<Event> > > events;

int main(){
        //set up socket and stuff here (omitted for clarity)
        while(1){
                newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &client_len);
                if (newsockfd < 0)
                        error("ERROR on accept");

                pid = fork();
                if (pid < 0)
                        error("ERROR on event server fork");

                if (pid == 0){  //Child Process
                        close(sockfd);

                        //do stuff here
                        read_and_send_ack (newsockfd);
                        exit(0);
                }

                else //Parent Process
                        close(newsockfd);
}

void read_and_send_ack (int sockfd)
{
        // buffer is basically a string
        //do some non important stuff here, omitted for clarity
        event_received(buffer, &events);
}

void event_received(char* event_info, map<string, map<string, deque<Event> > >* events)
{
        Event event = Event(event_info);
        (*events)[string(event.device)][string(event.dev_id)].push_back(event);

        //print keys to test if events are being modified each turn
        for (map<string, map<string, deque<Event> > >::iterator it = events->begin(); it!=events->end(); ++it)
                cout << it->first;
}

I expect the global container "events" to be modified each time the while loop receives something to add to the container. But the change does not persist. As a result, whenever I print out the keys, only one key is in the container. What is going on? I thought all processes/threads can access the same global variable in C++.

Upvotes: 0

Views: 168

Answers (1)

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171167

Turning my comment into an answer.

When the child process is created by fork(), it gets its own copy of the process state, including global variables.

If you want the parent and child process to communicate, you'll have to set up some form of channel for them to do so (such as a pipe, a file, shared memory or something similar).

Upvotes: 1

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