Quick Guide for the Impatient

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## Quick Guide for the Impatient

### Using shared memory as a pool of unnamed memory blocks

You can just allocate a portion of a shared memory segment, copy the message to that buffer, send the offset of that portion of shared memory to another process, and you are done. Let's see the example:

    #include <boost/interprocess/managed_shared_memory.hpp>

    int main ()
    {
       using namespace boost::interprocess;

       //An special shared memory from which we are
       //able to allocate raw memory buffers.
       //First remove any old shared memory of the same name, create
       //the shared memory segment and initialize needed resources
       shared_memory_object::remove("MySharedMemory");
       try{
          managed_shared_memory segment
             (create_only,
             "MySharedMemory",  //segment name
             65536);             //segment size in bytes

          //Allocate a portion of the segment
          void * shptr   = segment.allocate(1024/*bytes to allocate*/);

          //An handle from the base address can identify any byte of the shared
          //memory segment even if it is mapped in different base addresses
          managed_shared_memory::handle_t handle = segment.get_handle_from_address(shptr);
          (void)handle;
          // Copy message to buffer
          // . . .
          // Send handle to other process
          // . . .
          // Wait response from other process
          // . . .

          //Deallocate the portion previously allocated
          segment.deallocate(shptr);
       }
       catch(...){
          shared_memory_object::remove("MySharedMemory");
          throw;
       }
       shared_memory_object::remove("MySharedMemory");
       return 0;
    }

In receiver process one just could write the following lines:

    #include <boost/interprocess/managed_shared_memory.hpp>

    int main ()
    {
       using namespace boost::interprocess;

       try{
          //An special shared memory from which we are
          //able to allocate raw memory buffers.
          //Connect to the already created shared memory segment
          //and initialize needed resources
          managed_shared_memory segment(open_only, "MySharedMemory");  //segment name

          //An handle from the base address can identify any byte of the shared
          //memory segment even if it is mapped in different base addresses
          managed_shared_memory::handle_t handle = 0;

          //Wait handle msg from the other process and put it in
          //"handle" local variable
          //Get buffer local address from handle
          void *msg = segment.get_address_from_handle(handle);
          (void)msg;
          //Do anything with msg
          //. . .
          //Send ack to sender process
       }
       catch(...){
          shared_memory_object::remove("MySharedMemory");
          throw;
       }
       shared_memory_object::remove("MySharedMemory");
       return 0;
    }

### Creating named shared memory objects

You want to create objects in a shared memory segment, giving a string name to them so that any other process can find, use and delete them from the segment when the objects are not needed anymore. Just write in one process:

    #include <boost/interprocess/managed_shared_memory.hpp>
    #include <utility>

    int main ()
    {
       using namespace boost::interprocess;
       typedef std::pair<double, int> MyType;

       try{
          //An special shared memory where we can
          //construct objects associated with a name.
          //First remove any old shared memory of the same name, create
          //the shared memory segment and initialize needed resources
          shared_memory_object::remove("MySharedMemory");
          managed_shared_memory segment
             //create       segment name    segment size
             (create_only, "MySharedMemory", 65536);

          //Create an object of MyType initialized to {0.0, 0}
          MyType *instance = segment.construct<MyType>
             ("MyType instance")  //name of the object
             (0.0, 0);            //ctor first argument

          //Create an array of 10 elements of MyType initialized to {0.0, 0}
          MyType *array = segment.construct<MyType>
             ("MyType array")     //name of the object
             [10]                 //number of elements
             (0.0, 0);            //Same two ctor arguments for all objects

          //Create an array of 3 elements of MyType initializing each one
          //to a different value {0.0, 0}, {1.0, 1}, {2.0, 2}...
          float float_initializer[3] = { 0.0, 1.0, 2.0 };
          int   int_initializer[3]   = { 0, 1, 2 };

          MyType *array_it = segment.construct_it<MyType>
             ("MyType array from it")   //name of the object
             [3]                        //number of elements
             ( &float_initializer[0]    //Iterator for the 1st ctor argument
             , &int_initializer[0]);    //Iterator for the 2nd ctor argument
       }
       catch(...){
          shared_memory_object::remove("MySharedMemory");
          throw;
       }
       shared_memory_object::remove("MySharedMemory");
       return 0;
    }

In other process execute the following:

    #include <boost/interprocess/managed_shared_memory.hpp>
    #include <cstddef>
    #include <utility>
    #include <cassert>

    int main ()
    {
       using namespace boost::interprocess;
       typedef std::pair<double, int> MyType;

       try{
          //An special shared memory where we can
          //construct objects associated with a name.
          //Connect to the already created shared memory segment
          //and initialize needed resources
          managed_shared_memory segment(open_only, "MySharedMemory");

          std::pair<MyType*, std::size_t> res;

          //Find the array
          res = segment.find<MyType> ("MyType array");
          //Length should be 10
          assert(res.second == 10);

          //Find the object
          res = segment.find<MyType> ("MyType instance");
          //Length should be 1
          assert(res.second == 1);

          //Find the array constructed from iterators
          res = segment.find<MyType> ("MyType array from it");
          //Length should be 3
          assert(res.second == 3);

          //Use data
          // . . .

          //We're done, delete all the objects
          segment.destroy<MyType>("MyType array");
          segment.destroy<MyType>("MyType instance");
          segment.destroy<MyType>("MyType array from it");
       }
       catch(...){
          shared_memory_object::remove("MySharedMemory");
          throw;
       }
       shared_memory_object::remove("MySharedMemory");
       return 0;
    }

### Using an offset smart pointer for shared memory

Boost.Interprocess offers offset_ptr smart pointer family as an offset pointer that stores the distance between the address of the offset pointer itself and the address of the pointed object. When offset_ptr is placed in a shared memory segment, it can point safely objects stored in the same shared memory segment, even if the segment is mapped in different base addresses in different processes.

This allows placing objects with pointer members in shared memory. For example, if we want to create a linked list in shared memory:

    #include <boost/interprocess/managed_shared_memory.hpp>
    #include <boost/interprocess/offset_ptr.hpp>

    using namespace boost::interprocess;

    //Shared memory linked list node
    struct list_node
    {
       offset_ptr<list_node> next;
       int                   value;
    };

    int main ()
    {
       //Destroy any previous shared memory with the name to be used.
       //Create an special shared memory from which we can
       //allocate buffers of raw memory.
       shared_memory_object::remove("MySharedMemory");
       try{
          managed_shared_memory segment(
             create_only,
             "MySharedMemory", //segment name
             65536);           //segment size in bytes

          //Create linked list with 10 nodes in shared memory
          offset_ptr<list_node> prev = 0, current, first;

          int i;
          for(i = 0; i < 10; ++i, prev = current){
             current = static_cast<list_node*>(segment.allocate(sizeof(list_node)));
             current->value = i;
             current->next  = 0;

             if(!prev)
                first = current;
             else
                prev->next = current;
          }

          //Communicate list to other processes
          //. . .
          //When done, destroy list
          for(current = first; current; /**/){
             prev = current;
             current = current->next;
             segment.deallocate(prev.get());
          }
       }
       catch(...){
          shared_memory_object::remove("MySharedMemory");
          throw;
       }
       shared_memory_object::remove("MySharedMemory");
       return 0;
    }

To help with basic data structures, Boost.Interprocess offers containers like vector, list, map, so you can avoid these manual data structures just like with standard containers.

### Creating vectors in shared memory

Boost.Interprocess allows creating complex objects in shared memory and memory mapped files. For example, we can construct STL-like containers in shared memory. To do this, we just need to create a an special (managed) shared memory segment, declare a Boost.Interprocess allocator and construct the vector in shared memory just if it was any other object. Just execute this first process:

    #include <boost/interprocess/managed_shared_memory.hpp>
    #include <boost/interprocess/containers/vector.hpp>
    #include <boost/interprocess/allocators/allocator.hpp>

    int main ()
    {
       using namespace boost::interprocess;
       try{
          //Shared memory front-end that is able to construct objects
          //associated with a c-string. Erase previous shared memory with the name
          //to be used and create the memory segment at the specified address and initialize resources
          shared_memory_object::remove("MySharedMemory");
          managed_shared_memory segment
             (create_only
             ,"MySharedMemory" //segment name
             ,65536);          //segment size in bytes

          //Alias an STL compatible allocator of ints that allocates ints from the managed
          //shared memory segment.  This allocator will allow to place containers
          //in managed shared memory segments
          typedef allocator<int, managed_shared_memory::segment_manager>
             ShmemAllocator;

          //Alias a vector that uses the previous STL-like allocator
          typedef vector<int, ShmemAllocator> MyVector;

          //Initialize shared memory STL-compatible allocator
          const ShmemAllocator alloc_inst (segment.get_segment_manager());

          //Construct a shared memory
          MyVector *myvector =
             segment.construct<MyVector>("MyVector") //object name
                                        (alloc_inst);//first ctor parameter

          //Insert data in the vector
          for(int i = 0; i < 100; ++i){
             myvector->push_back(i);
          }
       }
       catch(...){
          shared_memory_object::remove("MySharedMemory");
          throw;
       }
       shared_memory_object::remove("MySharedMemory");
       return 0;
    }

After this process is executed we can search the constructed vector and use it with STL algorithms:

    #include <boost/interprocess/managed_shared_memory.hpp>
    #include <boost/interprocess/containers/vector.hpp>
    #include <boost/interprocess/allocators/allocator.hpp>
    #include <algorithm>

    int main ()
    {
       using namespace boost::interprocess;
       try{
          //An special shared memory where we can
          //construct objects associated with a name.
          //Connect to the already created shared memory segment
          //and initialize needed resources
          managed_shared_memory segment
             (open_only
             ,"MySharedMemory");  //segment name

          //Alias an STL compatible allocator of ints that allocates ints from the managed
          //shared memory segment.  This allocator will allow to place containers
          //in managed shared memory segments
          typedef allocator<int, managed_shared_memory::segment_manager>
             ShmemAllocator;

          //Alias a vector that uses the previous STL-like allocator
          typedef vector<int, ShmemAllocator> MyVector;

          //Find the vector using the c-string name
          MyVector *myvector = segment.find<MyVector>("MyVector").first;

          //Use vector in reverse order
          std::sort(myvector->rbegin(), myvector->rend());
          // . . .

          //When done, destroy the vector from the segment
          segment.destroy<MyVector>("MyVector");
       }
       catch(...){
          shared_memory_object::remove("MySharedMemory");
          throw;
       }
       shared_memory_object::remove("MySharedMemory");
       return 0;
    }

### Creating maps in shared memory

Just like a vector, Boost.Interprocess allows creating maps in shared memory and memory mapped files. The only difference is that like standard associative containers, Boost.Interprocess's map needs also the comparison functor when an allocator is passed in the constructor:

    #include <boost/interprocess/managed_shared_memory.hpp>
    #include <boost/interprocess/containers/map.hpp>
    #include <boost/interprocess/allocators/allocator.hpp>
    #include <functional>
    #include <utility>

    int main ()
    {
       using namespace boost::interprocess;
       //Shared memory front-end that is able to construct objects
       //associated with a c-string. Erase previous shared memory with the name
       //to be used and create the memory segment at the specified address and initialize resources
       shared_memory_object::remove("MySharedMemory");

       try{
          managed_shared_memory segment
             (create_only
             ,"MySharedMemory" //segment name
             ,65536);          //segment size in bytes

          //Note that map<Key, MappedType>'s value_type is std::pair<const Key, MappedType>,
          //so the allocator must allocate that pair.
          typedef int    KeyType;
          typedef float  MappedType;
          typedef std::pair<const int, float> ValueType;

          //Alias an STL compatible allocator of for the map.
          //This allocator will allow to place containers
          //in managed shared memory segments
          typedef allocator<ValueType, managed_shared_memory::segment_manager>
             ShmemAllocator;

          //Alias a map of ints that uses the previous STL-like allocator.
          //Note that the third parameter argument is the ordering function
          //of the map, just like with std::map, used to compare the keys.
          typedef map<KeyType, MappedType, std::less<KeyType>, ShmemAllocator> MyMap;

          //Initialize the shared memory STL-compatible allocator
          ShmemAllocator alloc_inst (segment.get_segment_manager());

          //Construct a shared memory map.
          //Note that the first parameter is the comparison function,
          //and the second one the allocator.
          //This the same signature as std::map's constructor taking an allocator
          MyMap *mymap =
             segment.construct<MyMap>("MyMap")      //object name
                                        (std::less<int>() //first  ctor parameter
                                        ,alloc_inst);     //second ctor parameter

          //Insert data in the map
          for(int i = 0; i < 100; ++i){
             mymap->insert(std::pair<const int, float>(i, (float)i));
          }
       }
       catch(...){
          shared_memory_object::remove("MySharedMemory");
          throw;
       }
       shared_memory_object::remove("MySharedMemory");
       return 0;
    }

For a more advanced example including containers of containers, see the section Containers of containers.