Shared memory pointer segmentation fault

Question

Hey am trying to create a shared object between 2 processes.and trying to read and change the values from each one of them.This s my simple struct.

EDIT: I added a constructor to my struct.

struct shared{
    shared(){
        value = 10;
        name = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA";
    }
    int value;
    string name;
};

I tried both to call shmat() before and after calling fork() but nothing change it still give segmentation fault.

EDIT:And added a check after the shmat() to see if it failed.

#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <iostream>
#include <sys/shm.h>
#include <string.h>

using namespace std;

struct shared{
    shared(){
        value = 10;
        name = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA";
    }
    int value;
    string name;
};

int main(){
    int shm_id = shmget(IPC_PRIVATE,sizeof(shared),0);

    if(shm_id == -1){
        cout<<"shmget() failed "<<endl;
        return -1;
    }
    pid_t pid = fork();
    if(pid == -1){
        cout<<"fork() failed "<<endl;
        return -2;
    }

    shared* sharedPtr = (shared*)shmat(shm_id,0,0);


    if(sharedPtr == 0){
        cout<<"shmat() failed "<<endl;
    }

    cout<<"Setting up the object: "<<endl;
    sharedPtr->value = 5;
    sharedPtr->name = "aaaaaa: ";

    if(pid == 0){       //Child process

        cout<<"Child process: "<<endl;
        sharedPtr->value = 10;
        sharedPtr->name  = "bbbbbbb: ";
        cout<<"Changed the object "<<endl;
        return 0;
    }
    if(pid != 0){ //Parent process
        sleep(1);
        cout<<"Parent process: "<<endl;
        cout<< sharedPtr->name << sharedPtr->value<<endl;

    }

    return 0;
}

But I still get a segmentation fault.

Answer 1

The check for validity of the pointer (if(sharedPtr == 0){) occurs after the pointer has already been dereferenced in sharedPtr->value. So you could be getting this behaviour because you are dereferencing a null pointer. Put this check before you use the pointer.

Furthermore, you can't use shared memory like that. You are getting a statically sized shared memory pointer that represents memory equivalent to an uninitialized string and int, think about this. You essentially have this:

int : size -> 4 bytes
empty string : size -> 32 bytes

(From here, system dependent). Then when you do this (shared*)shmat(shm_id,0,0) and this sharedPtr->name = "aaaaaa: ", the string you have created in this struct never had its constructor called and therefore was not created correctly. When you do the operator= you are attempting to use a function that doesn't exist anywhere because the object you are calling it on wasn't constructed correctly. If you are using raw shared memory, you will need to be very careful about how you use it and how much size you will take up. You could also consider using a memory controller to help you, like boost::interprocess.