Shared memory is one of the IPC (interprocess communication) facilities available in every major version of Unix. It allows two or more processes to map the same set of physical-memory segments to their address space. Since the memory segments are common to all processes that are attached to them, the processes can communicate through the common data in the shared-memory segments. Thus, as the name implies, shared memory is a set of physical-memory segments that are shared among processes. When a process attaches to shared memory, it receives a pointer to the beginning of the shared segments; the process then can use the memory just like any other memory. Of course, care must be taken when a shared-memory segment is accessed or written to synchronize with another process that has access to the same physical memory.

Consider the following code, which works on most Unix systems:

//Get shared memory id
//shared memory key
const key_t ipckey = 24568;   
//shared memory permission; can be
//read and written by anybody
const int perm = 0666;
//shared memory segment size
size_t shmSize = 4096;
//Create shared memory if not
//already created with specified
//permission
int shmId = shmget
  (ipckey,shmSize,IPC_CREAT|perm);
if (shmId ==-1) {
  //Error
}

//Attach the shared memory segment

void* shmPtr = shmat(shmId,NULL,0);

struct commonData* dp =
  (struct commonData*)shmPtr;

//detach shared memory
shmdt(shmPtr);

Types of Data Structures in Shared Memory

Struct commonData {
  int sharedInt;
  float  sharedFloat;
  char* name;
Struct CommonData* next;
};

//Attach shared memory
struct commonData* dp =
  (struct commonData*) shmat
    (shmId,NULL,0);

dp->sharedInt = 5;
.
.
dp->name = new char [20];
strcpy(dp->name,"My Name");

dp->next = new struct commonData();

struct commonData* dp =
  (struct commonData*) shmat
    (shmId,NULL,0);

//count = 5;
int count = dp->sharedInt;
//problem
printf("name = [%s]\n",dp->name);
dp = dp->next;  //problem

As a result of these restrictions, data structures designed to be used in shared memory usually tend to be simple.

C++ STL Containers in Shared Memory

Consider placing an STL container in shared memory. The container itself allocates its internal data structure. It is an impossible task to construct an STL container on the heap, copy the container into shared memory, and guarantee that all the container's internal memory is pointing to the shared-memory area.

Process A does the following:

//Attach to shared memory
void* rp = (void*)shmat(shmId,NULL,0);
//Construct the vector in shared
//memory using placement new
vector<int>* vpInA = new(rp) vector<int>*;
//The vector is allocating internal data
//from the heap in process A's address
//space to hold the integer value
(*vpInA)[0] = 22;

vector<int>* vpInB =
  (vector<int>*) shmat(shmId,NULL,0);

//problem - the vector contains internal 
//pointers allocated in process A's address 
//space and are invalid here 
int i = *(vpInB)[0];

C++ STL Allocators to the Rescue

template<class T, class A = allocator<T> >
  class vector {
    //other stuff
};

//User supplied allocator myAlloc
vector<int,myAlloc<int> > alocV;  

Process A does the following:

//Attach to shared memory
void* rp = (void*)shmat(shmId,NULL,0);
//Construct the vector in shared memory
//using placement new
vector<int>* vpInA =
  new(rp) vector<int,myAlloc<int>>*;
//The vector uses myAlloc<int> to allocate
//memory for its internal data structure
//from shared memory
(*v)[0] = 22;

vector<int>* vpInB =
  (vector<int,myAlloc<int> >*) shmat
    (shmId,NULL,0);

//Okay since all of the vector is
//in shared memory
int i = *(vpInB)[0];

A Shared Memory Based STL Allocator

Listing 2 shows the Pool class definition. Pool's static member shm_ is of type shmPool. There is a single instance of shmPool per process, and it represents shared memory.shmPool's constructor creates and attaches the desired size of shared memory. Shared-memory parameters, such as the shared-memory key, number of shared-memory segments, and size of each segment, are passed to the shmPool class constructor through environmental variables. The data member segs_ is the number of shared-memory segments; segSize_ is the size of each shared-memory segment. The path_ and key_ data members are used to create a unique ipckey. shmPool creates one semaphore for each shared segment to synchronize memory-management activities among processes attached to the shared-memory segment. shmPool constructs a Chunk class in each of the shared-memory segments. Chunkrepresents a shared-memory segment. For each shared-memory segment, the shared-memory identifier shmId_, a semaphore semId_ to control access to the segment, and a pointer to the Link structure that represents the free list are kept in the Chunk class.

Placing an STL Container in Shared Memory

Conclusion

The complete source code is available for download at <www.cuj.com/code>.

References

Matthew H. Austern. Generic Programming and the STL: Using and Extending the C++ Standard Template Library (Addison-Wesley, 1999).

About the Author

Listing 1: An implementation of the C++ Standard STL allocator

template<class T>class SharedAllocator {
    private:
        Pool pool_;    // pool of elements of sizeof(T)
    public:
        typedef T value_type;
        typedef unsigned int  size_type;
        typedef ptrdiff_t difference_type;
        typedef T* pointer;
        typedef const T* const_pointer;
        typedef T& reference;
        typedef const T& const_reference;
        pointer address(reference r) const { return &r; }
        const_pointer address(const_reference r) const {return &r;}
        SharedAllocator() throw():pool_(sizeof(T)) {}
        template<class U> SharedAllocator
            (const SharedAllocator<U>& t) throw():
                        pool_(sizeof(T)) {}
        ~SharedAllocator() throw() {};
        // space for n Ts
        pointer allocate(size_t n, const void* hint=0)
        {
            return(static_cast<pointer> (pool_.alloc(n)));
        }
        // deallocate n Ts, don't destroy
        void deallocate(pointer p,size_type n)
        {
            pool_.free((void*)p,n);
            return;
        }
        // initialize *p by val
        void construct(pointer p, const T& val) { new(p) T(val); }
        // destroy *p but don't deallocate
        void destroy(pointer p) { p->~T(); }
        size_type max_size() const throw()
        {
            pool_.maxSize();
        }
        template<class U>    
        // in effect: typedef SharedAllocator<U> other
        struct rebind { typedef SharedAllocator<U> other; };
};

template<class T>bool operator==(const SharedAllocator<T>& a,
    const SharedAllocator<T>& b) throw()
{
        return(a.pool_ == b.pool_);
}
template<class T>bool operator!=(const SharedAllocator<T>& a,
    const SharedAllocator<T>& b) throw()
{
        return(!(a.pool_ == b.pool_));
}

class Pool {
    private:
        class shmPool {
            private:
                struct Container {
                    containerMap* cont;
                };
                class Chunk {
                    public:
                        Chunk()
                        Chunk(Chunk&);
                        ~Chunk() {}
                        void* alloc(size_t size);
                        void free (void* p,size_t size);
                    private:
                        int shmId_;
                        int semId_;
                        int lock_()
                };
                int key_;
                char* path_;
                Chunk** chunks_;
                size_t segs_;
                size_t segSize_;
                Container* contPtr_;
                int contSemId_;
            public:
                shmPool();
                ~shmPool();
                size_t maxSize();
                void* alloc(size_t size);
                void free(void* p, size_t size);
                int shmPool::lockContainer()
                int unLockContainer()
                containerMap* getContainer()
                void shmPool::setContainer(containerMap* container)
        };

    private:
        static shmPool shm_;
        size_t elemSize_;
    public:
        Pool(size_t elemSize);
        ~Pool() {}
        size_t maxSize();
        void* alloc(size_t size);
        void free(void* p, size_t size);
        int lockContainer();
        int unLockContainer();
        containerMap* getContainer();
        void setContainer(containerMap* container);
};
inline bool operator==(const Pool& a,const Pool& b)
{
    return(a.compare(b));
}

Listing 3: A container factory

struct keyComp {
    bool operator()(const char* key1,const char* key2)
    {
        return(strcmp(key1,key2)<0);
    }
};
class containerMap: public map<char*,void*,keyComp,SharedAllocator<char* > > {};
class containerFactory {
    public:
        containerFactory():pool_(sizeof(containerMap)){}
        ~containerFactory() {}
        template<class Container> Container* createContainer
            (char* key,Container* c=NULL);
        template<class Container> Container* getContainer
            (char* key,Container* c=NULL);
        template<class Container> int removeContainer
            (char* key,Container* c=NULL); 
    private:
        Pool pool_;
        int lock_();
        int unlock_();
};