2D arrays with shared memory - c

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);
}