Getting Zero in out as value of shared memory variable - c

Question

I have a parent program which will fork the following child program and increment, display, decrement and display the variable 'test' (initially 0) 15 times. I tried to run it a lot of times and see what output I get after every few lines, however I'm not able to know how my 'test' variable is showing only zero, 15 times.
//Include required preprocessors
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <unistd.h>
#define SHARED_MEM 1024 //shared memory size
int main(void)
{
/* The child process' new program. This program replaces the parent's */
/* program when 'fork()' is called */
printf("Process[%d]: child in execution ... \n",getpid());
int MAX = 15;
int shmID, shmID2;
key_t key = 1234, key2 = 2345;
int *test, *counter;
shmID = shmget(key, SHARED_MEM, 0666);
printf("\nShmID: %d", shmID);
shmID2 = shmget(key2, SHARED_MEM, 0666);
printf("\nshmID2: %d", shmID2);
test = (int *) shmat(shmID, 0, 0);
counter = (int *) shmat(shmID2, 0, 0);
printf("\ntEST before assignment: %d", *test);
printf("\nCounter: %d", *counter);
*test = 0;
*counter = 1;
printf("\ntest: %d", *test);
printf("\nCounter%d", *counter);
printf("\nAlmost there...");
if (*counter == 1){
for(int i=0; i < MAX; i++){
printf("\MAX: %d", MAX);
printf("%d", *test);
*test++;
}
*counter++; //to enter second condition of second child process
printf("\nCounter: %d", counter);
}
else if(*counter == 2){
for(int j = 0; j < MAX; j++){
printf("%d", *test);
*test++;
}
*counter--;
}
sleep(1);
printf("Process[%d]: child terminating ... \n", getpid());
shmdt(test);
shmdt(counter);
return 0;
}

Answer 1

I can see several things.
Independently of how is code structured, I'd check if there's an error in shmget and shmat:
I'd change
shmID = shmget(key, SHARED_MEM, 0666)
shmID2 = shmget(key2, SHARED_MEM, 0666);
by
if ( (shmID = shmget(key, SHARED_MEM, 0666)) < 0)
{
perror("shmget");
exit(1);
}
if ( (shmID2 = shmget(key2, SHARED_MEM, 0666) < 0)
{
perror("shmat");
exit(1);
}
Note that you're creating SHM with 0666 privileges but without IPC_CREAT | IPC_EXCL.
I recommend you create first time with IPC_CREAT | IPC_EXCL | 0666 flags.
In the same way that 1st point, I'll check also problems with shmat:
See example for test. Should be the same for counter.
if ((test = (int *) shmat(shmID, 0, 0)) == -1)
{
perror("shmat");
exit(1);
}
You can check in cli if there's something wrong with shm, for instance, is already created, using command ipcs to check or ipcrm shm to delete and re-initialize:
ipcs shm | grep [your_shm_key or shmid]