Thread: How to communicate between two processes

Hi everyone. I am new to Cboard. This is my first post.
I am practicing Linux Programming. I am doing an Addressbook program in C using IPC. This program does Insert, Display, Delete and Modify.
My task is to create two applications.
All the selections for insert, delete, modify & delete will be done in application1.
For Insertion:
application1 has to take the input data(like name, address & phone#) and send them to application2. Application2 must receive the data and save them to a file.
For Deletion: Application2 must be doing the deletion part
For Display: Application2 sends data stored in file to Application1 for displaying them
For Modify: Application2 does the modification part.

I am using System() to create a new process and using Shared memory for IPC.

I am successful in sending the data to second process and saving them to a file. I can also display the contents in the file. Can anyone please help in how to delete and modify the data.
This is my code what i have done upto now. Thanks in Advance.

address.c

Code:

#include<stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <errno.h>

typedef struct 
{
  int select;
  char lastname[25];
  char firstname[25];
  char address[25];
  char phonenumber[25];
}addressbook;

#define SIZE 3
addressbook a[SIZE]={0,0};

void insert(void);
void display(void);
void delete(void);
void modify(void);
char *shared_memory;
int i;

int main()
{
  int select;
  int segment_id;
  //char* shared_memory;
  int segment_size;
  key_t shm_key;
  const int shared_segment_size = 0x6500;
  shm_key = ftok("/home/madan/programs/shm_tok",'X');
  if(shm_key < 0) 
  {
    printf("failed to create the key %s\n",strerror(errno));
  }
  /* Allocate a shared memory segment. */
  segment_id = shmget (shm_key, shared_segment_size, IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR);
  if(segment_id < 0) 
   {
     printf("error geting the segment id %s\n",strerror(errno));
   }
    printf("segment ID:%d\n", segment_id);
   /* Attach the shared memory segment. */
   shared_memory = (char*) shmat (segment_id, 0, 0);
   printf ("shared memory attached at address %p\n", shared_memory);
  do 
  {
  printf("Address book: \n Press\n 1: Insert \n 2: Display \n 3: Delete \n 4: Modify\n Enter:");
  scanf("%d", &select);
  switch(select)
  {
    case 1:
      insert();
      break;
    case 2:
      display();
      break;
    case 3:
      delete();
      break;
    case 4:
     modify();
      break;  
    default:
      printf("wrong selection\n");
      break;
  
  }
  if((select > 4) || (select < 1))
    break;
  }while(1);
      /* Detach the shared memory segment. */
   shmdt (shared_memory);
   /* Deallocate the shared memory segment.*/
   shmctl (segment_id, IPC_RMID, 0);  
  return 0;
}

void insert()
{  
   
   for(i=0; i<SIZE; i++)
   {
    a[i].select=1;
    printf("enter details\n");
    printf("enter lastname of person %d:\n", i);
    scanf("%s", a[i].lastname);
    printf("enter firstname of person %d:\n", i);
    scanf("%s", a[i].firstname);
    printf("enter address of person %d:\n", i);
    scanf("%s", a[i].address);
    printf("enter phone number of person %d:\n", i);
    scanf("%s", a[i].phonenumber);
    memcpy(shared_memory, &a[i], sizeof(addressbook));
    system("./address-insert");
   }
}

void display()
{
 //for(i=0; i<SIZE; i++)
 // {
   a[i].select=2;
 // printf("enter last name of the person to display the details:\n");
//  scanf("%s", a[i].lastname);
  memcpy(shared_memory, &a, sizeof(addressbook));
  system("./address-insert");
  memcpy(&a, shared_memory, sizeof(addressbook));
  for(i=0; i<SIZE; i++)
  {
  printf("lastname of person %d: %s\n", i,a[i].lastname);
  printf("firstname of person %d: %s\n", i, a[i].firstname);
  printf("address of person %d: %s\n", i, a[i].address);
  printf("phone number of person %d: %s\n", i, a[i].phonenumber);
  }
  
}

void delete()
{
  a[i].select=3;
  memcpy(shared_memory, &a, sizeof(addressbook));
  system("./address-insert");
}

void modify()
{
  a[i].select=4;
    memcpy(shared_memory, &a, sizeof(addressbook));
  system("./address-insert");
}

-------------------------------------------------------------------------------------------------------------------------------------
address-insert.c

Code:

#include <arpa/inet.h>
#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <string.h>
#include <errno.h>

typedef struct 
{
  int select;
  char lastname[25];
  char firstname[25];
  char address[25];
  char phonenumber[25];
} addressbook;
#define SIZE 3
addressbook a[SIZE]={0,0};

void insert(void);
void display(void);
void delete(void);
void modify(void);
char* shared_memory;
 FILE *fp;
 int i;
 char *name;
int main ()
{
  int segment_id;
  
  
  
  int segment_size;
  key_t shm_key;
  shm_key = ftok("/home/madan/programs/shm_tok",'X');
  const int shared_segment_size = 0x6500;
  /* Allocate a shared memory segment. */
  segment_id = shmget (shm_key, shared_segment_size, S_IRUSR | S_IWUSR);
  
  if(segment_id < 0) 
  {
    printf("error:[%s]",strerror(errno));
  }
  printf("segment id %d\n",segment_id);
  /* Attach the shared memory segment. */
  shared_memory = (char*) shmat (segment_id, 0, 0);
  if(shared_memory == NULL)
  {
    printf("failed to attach the shared memory %s",strerror(errno));
  }
  printf ("shared memory2 attached at address %p\n", shared_memory);
  
  memcpy(&a[i], shared_memory, sizeof(addressbook));
  printf("select=%d\n", a[i].select);
  
 switch(a[i].select)
  {
    case 1:
      insert();
      break;
    case 2:
      display();
      break;
    case 3:
      delete();
      break;
    case 4:
      modify();
      break;
    default:
      printf("wrong selection\n");
      break;
  }
   /* Detach the shared memory segment. */
  shmdt (shared_memory);
  return 0;
}

  void insert()
  {
   /* To insert data*/  
     fp = fopen("addressbook.dat","a+");
     fwrite(&a[i], sizeof(addressbook), 1, fp);
     fclose(fp);
  }
  
  void display()
  {
    /* To delete data */ 
    fp = fopen("addressbook.dat", "r");
    for(i=0; i<SIZE; i++)
    {
      fread(&a, sizeof(addressbook), 1, fp); 
      //memcpy(shared_memory, &a, sizeof(addressbook));
    }
  fclose(fp);
  }
  
  void delete()
  {
  /*To display data */
  printf("enter last name of the person to delete:\n");
  scanf("%s", name);
  printf("contact to be deleted is: %s", name);
  fp = fopen("addressbook.dat", "a+");
  for(i=0; i<SIZE; i++)
  {
    fread(&a[i], sizeof(addressbook), 1, fp);
    if(a[i].lastname == name)
    {
      printf("found\n");
    }
    else
      printf("not found\n");
  }
  fclose(fp);
  
  //memcpy(shared_memory, &a, sizeof(addressbook));
  
  }
  
  void modify()
  {
  /*To modify data */
  printf("enter last name of the person to modify the details:\n");
  scanf("%s", a[i].lastname);
  memcpy(shared_memory, &a, sizeof(addressbook));
  
  }

The program is running with no errors. insert() and display() are working. Can someone help me the best way to send select variable to second application and how to do delete and modify part.
Thanks in advance.

Surely at some point you learned to use strcmp instead of == when dealing with strings?

yeah it make sense. thank you @tabstop.
Can anyone please tell me how to pass the select option along with data to the second application. I am getting something wrong in passing the select option to application2.

@anduril462 thank you. it was a blunder i made (a[i].select=3). yes, variable 'i' has to be synchronized between two applications.
yes what you said was correct, after delete and modify it must re-read the addressbook.

You should definitely avoid using 'i' as a global variable, since it's such a commonly used variable for loops, etc. Call it something descriptive and possibly prefix it with a 'g' to signify global, e.g. g_address_entry_number. Use a local 'i' in your functions where you need loop processing. Make as many things local as possible. Technically, you can even move your array and address_entry_number into main too, and pass them to the functions as necessary (you can do this in both programs).

@anduril462 yeah sure i will make changes.
Can we use use linked lists in this program, so that it will be easy to modify and delete contacts from the file. Please anyone suggest me how to use linked lists for this program.

I thought you wanted to save these to a file as you go. Does this mean you want the whole addressbook in memory at one time, and only writing to a file at the end of the day? (Linked lists and flat database-like files don't work well together, necessarily.)

If you do want a linked list, then you modify your struct to contain a link to the next struct.

@tabstop. We have to save the data to files whenever we insert, modify or delete them immediately. So that if we display them, it should display the updated data(like address book in phone).

@MK27.

It seems to me this is a straightforward client-server model, application 1 being the client (a user interface) and application 2 being a database server.

What you said is correct. This program should be like a address book in a phone. It should be capable of parallel processing(like in phone, though we open a contact in address book and make a call still we can look for another contacts).
If we delete a contact it should be automatically updated when we display the contacts. So if we what to delete a particular contact, I think it is better to use linked lists.
@tapstop & @ MK27 thank you for your suggestions.

Originally Posted by chaituchemico

@MK27.
What you said is correct. This program should be like a address book in a phone. It should be capable of parallel processing(like in phone, though we open a contact in address book and make a call still we can look for another contacts).
If we delete a contact it should be automatically updated when we display the contacts. So if we what to delete a particular contact, I think it is better to use linked lists.
@tapstop & @ MK27 thank you for your suggestions.

Linked lists are great if:
1. You keep all data in memory, nothing resides in a database or on disk (Pointers don't match up between different programs or different runs of the same program).
2. You only process the list in order and don't need random access (such as looking up a specific individual).

In your situation, you might want to consider an array, since it's easy sort and search for elements, which will improve lookups and edits, and it's easy to write to disk, so you can read it back without worrying about pointers not jiving. To "delete" something, you just do a lazy delete -- just mark the record as deleted/invalid, but leave the contents in there. Only throw them away when you need to reuse that spot.

Also, as MK27 pointed out, shared memory is not the best IPC mechanism for this, but if you're just out for learning, then by all means, keep it up.

In your situation, you might want to consider an array, since it's easy sort and search for elements, which will improve lookups and edits, and it's easy to write to disk, so you can read it back without worrying about pointers not jiving. To "delete" something, you just do a lazy delete -- just mark the record as deleted/invalid, but leave the contents in there. Only throw them away when you need to reuse that spot.

Thank you anduril462. In my code above, I have used arrays to store the data. If I want to delete or modify, can anyone help me how to write code for delete and modify in my program so that if I display the data it should display with the updated data. Thank you

If you intend to do lazy delete, as mentioned above, you'll need another field in your struct for "valid" or the like to make sure it's still there, and then when outputting (whether to screen or to file) if the valid field is invalid, then skip that record.

I don't know what you're missing for modify, exactly. You'll need to walk through the array until you find the record to be modified, then modify it.

thank you tabstop.
Is there any other way to pass the 'select' option to the second application, other than defining it in the struct and equating to particular value in its function as i did.

Code:

typedef struct 
{
  int select;
  char lastname[25];
  char firstname[25];
  char address[25];
  char phonenumber[25];
}addressbook;

void insert()
{  
   
   for(i=0; i<SIZE; i++)
   {
    a[i].select=1;
    printf("enter details\n");
    printf("enter lastname of person %d:\n", i);
    scanf("%s", a[i].lastname);
    printf("enter firstname of person %d:\n", i);
    scanf("%s", a[i].firstname);
    printf("enter address of person %d:\n", i);
    scanf("%s", a[i].address);
    printf("enter phone number of person %d:\n", i);
    scanf("%s", a[i].phonenumber);
    memcpy(shared_memory, &a[i], sizeof(addressbook));
    system("./address-insert");
   }
}