Shared Memory and Semaphores

[rachitagrawal]

I was writing a code and strangely I am getting some unexpected answer.

#include <assert.h>
#include "Semaphore.h"
#include "SharedMemory.h"

using namespace std;
int main()
{
    int i, *data, *count;
    Semaphore *sem, *semCount;
    SharedMemory *shm;
    SharedMemory *shm2;

    sem = new Semaphore(1);
    semCount = new Semaphore(1);
    shm = new SharedMemory(4096);
    shm2 = new SharedMemory(4);

    data = (int*)shm->Attach();
    count = (int *)shm2->Attach();
    *count = 0;

    for(i = 0; i < 1024; i++)
    {  
        sem->getLock();
        data[i] = getpid();
        sem->ReleaseLock();
    }
    semCount->getLock();
    *count = *count + 1;
    semCount->ReleaseLock();

    while(*count < 1)
    {}

    for(i = 0; i < 1024; i++)
    {
        sem->getLock();
        cout<<"PID:"<<getpid()<<" "<<i<<": "<<data[i]<<endl;
        sem->ReleaseLock();
    }

    shm->Detach((void *)data);
    shm2->Detach((void *)count);
    return 0;
}

And the result I am getting is:
PID:10275 0: 10276
PID:10275 1: 10275
PID:10275 2: 10275
PID:10275 3: 10275
.
.
.
.
.
PID:10275 1023: 10275

The pid stored in array location 0 is alway 1 greater than the other PID values. Why so??

[Albatross]

getpid() is not defined in the scope of this forum, and I think your problem might lie there.

Oh, by the way, just curious, why are you using semaphores in this program? What other threads are accessing data?

EDIT: And what about your shared memory system? Perhaps shm->Attach() isn't working properly?

-Albatross

[rachitagrawal]

Hi Albatross,

Well the problem was that the second shared memory segment was getting attached to the one created earlier and it was incrementing the value of pid by one (*count = *count + 1). Hence I was getting one value greater than the actual pid.

Now, I have a new problem. I have 4 processes running this. When I am attempting to remove the shared memory region, it gives error if the region is already removed. How do I make sure that, it is not called if the region is already removed?

Thanks