C++中的共享内存对齐

Question

Is there any alignment for allocated shared memory? If yes, is it for all operating system?

For example, allocating shared memory in boost:

boost::interprocess::shared_memory_object* segment =
    new boost::interprocess::shared_memory_object(
        boost::interprocess::create_only,
        "name",
        boost::interprocess::read_write);

segment->truncate(10000);

Answer 1

The shared/virtual memory system allocates and maps memory pages and is aligned by the page size. See a list of pages sizeshere . Page size is CPU and OS specific. I am not aware of modern CPUs that use page sizes smaller than 4kB.

On a POSIX system you can find out the page size using sysconf(_SC_PAGESIZE) . In case when huge pages are in use, this call returns the smallest page size.

Answer 2

There is no shred memory in C++ itself. There are various extensions. The raw ones return page aligned blocks, but your wrapper might add a sub-block allocation system to them.

If you have a block of memory you can find an aligned pointer within the block, either manually by messing with bits (non portable), or using std::align . It is designed to make it pretty easy to "peel" off aligned sub blocks from an unaligned block.

struct aligned_peeler {
  void* raw=nullptr;
  std::size_t size=0;
  void* operator*( std::size_t align, std::size_t amt ){
    return std::align(align, amt, raw, size);
  }
};

Now an aligned_peeler can be handed a block, and you can ask for aligned pieces from it. It will fail and return nullptr when it runs out of room.

Turning this into a real free store is a lot of work, but I'd guess boost has at least one allocator that can be fed shared memory pages as a source.