On 05/08/2010 16:39, Kai Benndorf wrote:

Hi all,

I want to create a managed_shared_memory in exactly the size I needed. Therefore I do something like this:

size = sizeof( common_managed_shared_memory ); size += sizeof( MyClassInSharedMem );

managed_shared_memory segment(create_only, "name", size );

MyClassInSharedMem allocates itself some memory by segment->allocate(). I added this also to the size, but get a boost::interprocess::bad_alloc exception on allocating this additional memory.

Are there some magic bytes I forgot to add?

Yes, but you can't calculate it, because there are memory allocation bookeeping and fragmentation issues that change in runtime depending on your allocation/deallocation pattern. And shared memory is allocated by pages by the OS (4K on linux 64k on windows), so any allocation will be in practice allocated rounded to a page: managed_shared_memory segment(create_only, "name", 20); will waste the same memory as: managed_shared_memory segment(create_only, "name", 4096); Ion