Loading data into shared memory

Question

I want to know how to load data from a text file into shared memory in C or C++. I want to assign the content of the file into structs line-by-line, like:

name: gaimi dancer
id: 443432
address: 123 Southbrook Dr. Mombai,  In
PhoneNumber: 8876549300

and so on with the whole file, and then load these structs into shared memory.

I have tried everything but nothing worked. I have tried strcpy() and memcpy() and they did not work.

reader.cpp

#include <sys/shm.h>
#include <stdio.h>
#include<string>
#include<cstring>
#include<cstdlib>
using namespace std;

struct Stu_Info{
    char name[30];
    char id[20];
    char address[30];
    char PhoneNumber[20];
};

int main()
{
    // ftok for generating unique key
    key_t key = ftok("shm",6556);

    // shmget returns an identifier in shmid
    id = shmget(key,1024*4,0666|IPC_CREAT);
    //to make sure the shm made correctly
    if (id < 0){
        perror("create: shmget failed");
        exit(1);
    }else{
        cout << "The shm was craeted!!";
    }

    // shmat to attach to shared memory
    char *str = (char*) shmat(id,(void*)0,0);

    cout<<"Write Data : ";
    gets(str);

    printf("Data written in memory: %s\n",str);

    //detach from shared memory
    shmdt(str);    

    return 0;
}

writer.cpp

#include <iostream>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
using namespace std;

int main()
{
    // ftok to generate unique key
    key_t key = ftok("shmfile",65);

    // shmget returns an identifier in shmid
    int id = shmget(key,1024,0666|IPC_CREAT);

    // shmat to attach to shared memory
    char *str = (char*) shmat(id,(void*)0,0);

    printf("Data read from memory: %s\n",str);

    //detach from shared memory
    shmdt(str);

    // destroy the shared memory
    shmctl(id,IPC_RMID,NULL);

    return 0;
}

text.txt

gaimi dancer
443432
123 Southbrook Dr. Mombai,  In
8876549300
Yall S Player
465387
543 Saif Rd. Manama,  BA
7665647833
Dean Rama
456339
908 Sea Ave. Manama,  BA
4556748900
Maya Rab
677032
654 South 3rd St. Kwait,  KW
3425435456
Patricia Carey
547839
320 Old navy St. Chiacgo, IL 
1112332534

Show the code that did not work then, including any errors. This is not a code writing service. If you search "serializing in C++", you will find plenty of resources. — Quimby, Aug 30 '20 at 19:45
You should pass the *same* `id` to `shmget` in order to be able to *share* a segment. BTW why not use `mmap` instead, which is more modern and easier? See [here](https://stackoverflow.com/questions/5656530/how-to-use-shared-memory-with-linux-in-c) — rustyx, Aug 30 '20 at 19:50
Are you running on a *nix environment? shm calls won't work very well in windows — cup, Aug 30 '20 at 20:01
I am running cpp on linux. I have tried this code but did not work. — Love69, Aug 31 '20 at 11:45

score 0 · Answer 1 · answered Aug 31 '20 at 11:49

I am running cpp on Mac command line and I am required to do this using this way and will be using a semaphore way also. I have tried this code but did not work.

while (getline(myFile,line)) //getline function to get the data line by line
            {
//                  cout << line << "\n"; // to print the content of the file

                  for(int j=0;j<=15;j++){//to create 15 student structs
                       for (int i=0; i<=15;i+4){ // to iterate in every 4 lines for 15 student strcts to save tha name.
                        strcpy(student[j][i].name,line);
                        name = student[j][i].name;
//                        cout <<student[j][i].name<<"\n";
                            memcpy(str, name , strlen(name)); // to load tha data into shared memory
                        }
                        for (int i=1; i<=15;i+4){ // to iterate in every 4 lines for 15 student strcts to save the id.
                            strcpy(student[j][i].id,line);
                            id = student[j][i].id;
                            memcpy(str, id , strlen(id)); // to load tha data into shared memory
                        }
                        for (int i=2; i<=15;i+4){ // to iterate in every 4 lines for 15 student strcts to save the address.
                            strcpy(student[j][i].address,line);
                            address = student[j][i].address;
                            memcpy(str, address , strlen(address)); // to load tha data into shared memory
                        }
                        for (int i=3; i<=15;i+4){ // to iterate in every 4 lines for 15 student strcts to save phone number.
                            strcpy(student[j][i].PhoneNumber,line);
                            PhoneNumber = student[j][i].PhoneNumber;
                            memcpy(str, PhoneNumber , strlen(PhoneNumber)); // to load tha data into shared memory
                        }
                  }
            }

          myFile.close();
    } else{cout << "unable to open a file!";}

Loading data into shared memory

1 Answers1