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>
    #include <cstdlib> //std::system
    #include <sstream>

    int main (int argc, char *argv[])
    {
       using namespace boost::interprocess;
       if(argc == 1){  //Parent process
          //Remove shared memory on construction and destruction
          struct shm_remove
          {
             shm_remove() {  shared_memory_object::remove("MySharedMemory"); }
             ~shm_remove(){  shared_memory_object::remove("MySharedMemory"); }
          } remover;

          //Create a managed shared memory segment
          managed_shared_memory segment(create_only, "MySharedMemory", 65536);

          //Allocate a portion of the segment (raw memory)
          managed_shared_memory::size_type free_memory = segment.get_free_memory();
          void * shptr = segment.allocate(1024/*bytes to allocate*/);

          //Check invariant
          if(free_memory <= segment.get_free_memory())
             return 1;

          //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);
          std::stringstream s;
          s << argv[0] << " " << handle;
          s << std::ends;
          //Launch child process
          if(0 != std::system(s.str().c_str()))
             return 1;
          //Check memory has been freed
          if(free_memory != segment.get_free_memory())
             return 1;
       }
       else{
          //Open managed segment
          managed_shared_memory segment(open_only, "MySharedMemory");

          //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;

          //Obtain handle value
          std::stringstream s; s << argv[1]; s >> handle;

          //Get buffer local address from handle
          void *msg = segment.get_address_from_handle(handle);

          //Deallocate previously allocated memory
          segment.deallocate(msg);
       }
       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. Example:

    #include <boost/interprocess/managed_shared_memory.hpp>
    #include <cstdlib> //std::system
    #include <cstddef>
    #include <cassert>
    #include <utility>

    int main(int argc, char *argv[])
    {
       using namespace boost::interprocess;
       typedef std::pair<double, int> MyType;

       if(argc == 1){  //Parent process
          //Remove shared memory on construction and destruction
          struct shm_remove
          {
             shm_remove() { shared_memory_object::remove("MySharedMemory"); }
             ~shm_remove(){ shared_memory_object::remove("MySharedMemory"); }
          } remover;

          //Construct managed shared memory
          managed_shared_memory segment(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

          //Launch child process
          std::string s(argv[0]); s += " child ";
          if(0 != std::system(s.c_str()))
             return 1;

          //Check child has destroyed all objects
          if(segment.find<MyType>("MyType array").first ||
             segment.find<MyType>("MyType instance").first ||
             segment.find<MyType>("MyType array from it").first)
             return 1;
       }
       else{
          //Open managed shared memory
          managed_shared_memory segment(open_only, "MySharedMemory");

          std::pair<MyType*, managed_shared_memory::size_type> res;

          //Find the array
          res = segment.find<MyType> ("MyType array");
          //Length should be 10
          if(res.second != 10) return 1;

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

          //Find the array constructed from iterators
          res = segment.find<MyType> ("MyType array from it");
          //Length should be 3
          if(res.second != 3) return 1;

          //We're done, delete all the objects
          segment.destroy<MyType>("MyType array");
          segment.destroy<MyType>("MyType instance");
          segment.destroy<MyType>("MyType array from it");
       }
       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 ()
    {
       //Remove shared memory on construction and destruction
       struct shm_remove
       {
          shm_remove() { shared_memory_object::remove("MySharedMemory"); }
          ~shm_remove(){ shared_memory_object::remove("MySharedMemory"); }
       } remover;

       //Create shared memory
       managed_shared_memory segment(create_only,
                                     "MySharedMemory",  //segment name
                                     65536);

       //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());
       }
       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 special (managed) shared memory segment, declare a Boost.Interprocess allocator and construct the vector in shared memory just if it was any other object.

The class that allows this complex structures in shared memory is called `boost::interprocess::managed_shared_memory` and it's easy to use. Just execute this example without arguments:

    #include <boost/interprocess/managed_shared_memory.hpp>
    #include <boost/interprocess/containers/vector.hpp>
    #include <boost/interprocess/allocators/allocator.hpp>
    #include <string>
    #include <cstdlib> //std::system

    using namespace boost::interprocess;

    //Define an STL compatible allocator of ints that allocates from the managed_shared_memory.
    //This allocator will allow placing containers in the segment
    typedef allocator<int, managed_shared_memory::segment_manager>  ShmemAllocator;

    //Alias a vector that uses the previous STL-like allocator so that allocates
    //its values from the segment
    typedef vector<int, ShmemAllocator> MyVector;

    //Main function. For parent process argc == 1, for child process argc == 2
    int main(int argc, char *argv[])
    {
       if(argc == 1){ //Parent process
          //Remove shared memory on construction and destruction
          struct shm_remove
          {
             shm_remove() { shared_memory_object::remove("MySharedMemory"); }
             ~shm_remove(){ shared_memory_object::remove("MySharedMemory"); }
          } remover;

          //Create a new segment with given name and size
          managed_shared_memory segment(create_only, "MySharedMemory", 65536);

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

          //Construct a vector named "MyVector" in shared memory with argument alloc_inst
          MyVector *myvector = segment.construct<MyVector>("MyVector")(alloc_inst);

          for(int i = 0; i < 100; ++i)  //Insert data in the vector
             myvector->push_back(i);

          //Launch child process
          std::string s(argv[0]); s += " child ";
          if(0 != std::system(s.c_str()))
             return 1;

          //Check child has destroyed the vector
          if(segment.find<MyVector>("MyVector").first)
             return 1;
       }
       else{ //Child process
          //Open the managed segment
          managed_shared_memory segment(open_only, "MySharedMemory");

          //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");
       }

       return 0;
    }

The parent process will create an special shared memory class that allows easy construction of many complex data structures associated with a name. The parent process executes the same program with an additional argument so the child process opens the shared memory and uses the vector and erases it.

### 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;

       //Remove shared memory on construction and destruction
       struct shm_remove
       {
          shm_remove() { shared_memory_object::remove("MySharedMemory"); }
          ~shm_remove(){ shared_memory_object::remove("MySharedMemory"); }
       } remover;

       //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
       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));
       }
       return 0;
    }

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