Shared memory segment with different variable types C

Question

I'm using Linux.

Using shared memory to store a static array of structs between two different programs.

Here's a code snippet showing how I create the shared memory block.

typedef struct {
 int ID;
 int nData;
 int time;
} strPrintJob;

size = (sizeof(strPrintJob) * lRetMaxJobs) + (sizeof(int) * 2);
strPrintJob *shmPrintJob;

    //Create data segment
    if((nShmid = shmget(nKey, size, IPC_CREAT | 0666)) < 0)
    {
        perror("shmget");
        exit(1);
    }

    shmPrintJob = (strPrintJob *) shmat(nShmid, NULL, 0);
    if (shmPrintJob == (strPrintJob *)(-1))
    {
        perror("shmat");
        exit(1);
    }

So far everything is working fine, and the two programs communicate: One modifying data within the structs and the other printing it out.

I would also like to use two integers within the shared memory to act as 'flags' but how would I attach and access them? Something along the lines of?

int *shmnFlagOne, *nPtr;

if((shmnFlagOne = shmat(nShmid, NULL, 0)) == -1)
{
   perror("shmat");
   exit(1);
}

nPtr = shmnFlagOne;

And then set the pointer to go after the array of structs in the shared memory?

Answer 1

You are right on track. You can place these items within your memory as you see fit. It looks like you have already allocated the storage for your flags with

2 * sizeof(int)

You can access them many ways all of which involve pointer casting.

It would look something like:

void *shared_memory = shmat(nShmid, NULL, 0);
strPrintJob *shmPrintJob = (strPrintJob *) shared_memory;
int *flags = (int *) shared_memory[sizeof(strPrintJob) * lRetMaxJobs / sizeof(int)];

OK, totally gross. But that would be the minimal change. An alternative is to create another struct to wrap your struct:

typedef struct {
    strPrintJob[lRetMaxJobs] printJobs;
    int flags[2];
} PrintJobWrapper;

Now use PrintJobWrapper to access your shared memory as you were before:

PrintJobWrapper *print_wrapper = (PrintJobWrapper *) shmat(nShmid, NULL, 0);
print_wrapper->flags[0] = xxx;