2D arrays with shared memory

Question

I have 2 programs that communicate with each other using a block of shared memory.

The first program takes an argument from the command line and forks the number of times specified, the process ID of each child and a randomly generated number are stored in a 2D array that is then supposed to be passed to the second program via the attached memory block. Problem is I have no idea how to do this and would appreciate some help as I am a bit of a novice when it comes to this.

Here is the code for the first program so far and is fully tested and working:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/wait.h>
#include <unistd.h>
#include <time.h>

int main(int argc, char *argv[])
{
    if (argc < 2)
    {
        printf("\nError with command line argument!\n");
        exit(1);
    }

    /* Program is simulation of a printer queue, forked child processes
       act as jobs in the queue with pids being the job number and a
       randomly generated value being the priority order (this comes into
       play in program 2) */

    srand(time(null));
    pid_t pid;

    int amount, i, pID, rNum;

    amount = atoi(argv[1]);

    int procID[amount]; /* 1D array that will hold pid for each process */
    int randNum[amount]; /* 1D array that will hold random number to
                            determine priority level of the print jobs */

    int rows = amount;
    int cols = 2;

    int printDetails[rows][cols]; /* 2D array that will hold the values
                                     from both 1D arrays, displaying all
                                     data from print queue when output */

    printf("\nPrint queue started:");
    getchar();

    for (i = 0; i < amount; i++)
    {
         pid = fork();

         if (pid < 0)
         {
             perror("Error with fork!");
             exit(1);
         }

         if (pid == 0)
         {
             pID = getpid();
             rNum = rand()%50;

             printf("\nPrint Job : %d", pID);
             printf("\nPriority Level : %d\n", rNum);

             procID[i] = pID;
             randNum[i] = rNum;

             sleep(1);
         }

         else
         {
             wait(NULL);
             return 0;
         }
    }

    printf("\n%d successful print jobs created\n", amount);

    printf("\n-----PRINT DETAILS-----\n");
    printf("\nJob No:\tPriority:\n");

    for (i = 0; i < rows; i++)
    {
        printDetails[i][0] = procID[i];
        printDetails[i][1] = randNum[i];

        printf("%d\t%d\n", printDetails[i][0], printDetails[i][1];
    }

    printf("\n-----END OF LIST-----\n");

    /* Create shared memory segment using shmget and shmat,
       how do I insert the array above into this, like I said
       complete noob! */

}

Sorry for the huge wall of code, just to help understand what I'm working with, like I said, any help in regards to the shared memory would be much appreciated as I'm kinda outta my depth here!

Answer 1

Copy on write mechanism will allocate a new pointer the moment you changed it on the second process... And when it dies it will take the new allocated memory with different data with it...the solution is allocating a dynamic pointer... When you change it's values it won't allocate a new one dynamically and use the old one and change its data

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/wait.h>
#include <unistd.h>
#include <time.h>

int main(int argc, char *argv[])
{
    if (argc < 2)
    {
        printf("\nError with command line argument!\n");
        exit(1);
    }

    /* Program is simulation of a printer queue, forked child processes
       act as jobs in the queue with pids being the job number and a
       randomly generated value being the priority order (this comes into
       play in program 2) */

    srand(time(NULL)); // null -> NULL
    pid_t pid;

    int amount, i, pID, rNum;

    amount = atoi(argv[1]);

    int* procID =(int*) calloc(amount,sizeof(int)); /* 1D array that will hold pid for each process */
    if(!procID)
    return -1;
    int* randNum =(int*) calloc (amount,sizeof(int)); /* 1D array that will hold random number to
                            determine priority level of the print jobs */
    if(!randNum)
    return -1;
    int rows = amount;
    int cols = 2;
    int k;
    int** printDetails = (int**) calloc (rows, sizeof(int*)); /* 2D array that will hold the values
                                     from both 1D arrays, displaying all
                                     data from print queue when output */
    if(!printDetails)
    return -1;
    for(k=0; k<rows;k++)
    {
        printDetails[k] = (int*) calloc (cols, sizeof(int));
        if(!printDetails[k])
        return -1;
    }
    printf("\nPrint queue started:");
    getchar();

    for (i = 0; i < amount; i++)
    {
         pid = fork();

         if (pid < 0)
         {
             perror("Error with fork!");
             exit(1);
         }

         if (pid == 0)
         {
             pID = getpid();
             rNum = rand()%50;

             printf("\nPrint Job : %d", pID);
             printf("\nPriority Level : %d\n", rNum);

             procID[i] = pID;
             randNum[i] = rNum;

             sleep(1);
         }

         else
         {
             wait(NULL);
             return 0;
         }
    }

    printf("\n%d successful print jobs created\n", amount);

    printf("\n-----PRINT DETAILS-----\n");
    printf("\nJob No:\tPriority:\n");

    for (i = 0; i < rows; i++)
    {
        printDetails[i][0] = procID[i];
        printDetails[i][1] = randNum[i];

        printf("%d\t%d\n", printDetails[i][0], printDetails[i][1]);
    }

    printf("\n-----END OF LIST-----\n");

    /* Create shared memory segment using shmget and shmat,
       how do I insert the array above into this, like I said
       complete noob! */
    for(k=0; k<rows; k++)
        free(printDetails[k]);
    free(printDetails);
    free(randNum);
    free(procID);
}