The magic ring buffer.

magic_ring.cpp

#define _CRT_SECURE_NO_DEPRECATE

#include <stdio.h>
#include <string.h>
#include <Windows.h>

// This allocates a "magic ring buffer" that is mapped twice, with the two
// copies being contiguous in (virtual) memory. The advantage of this is
// that this allows any function that expects data to be contiguous in
// memory to read from (or write to) such a buffer. It also means that
// block reads/writes never need to be split into two halves, and makes
// wraparound handling quite cheap.
//
// The flipside is that allocating such a beast is a bit dicey (see
// comments below) and subject to various restrictions (e.g. on Windows,
// the size of such a buffer must be a multiple of 64k). So the usual
// disclaimer applies: code responsibly, with great power comes great
// responsibility, and when you use this code to shoot yourself in the
// foot it might blow off your face instead (what with the wraparound
// and all).
class MagicRingBuffer
{
  HANDLE mapping;
  size_t size;
  char *baseptr;

public:
  MagicRingBuffer()
    : mapping(0), size(0), baseptr(0)
  {
  }

  ~MagicRingBuffer()
  {
    free();
  }

  // Allocate a magic ring buffer at a given target address.
  //   ring_size      size of one copy of the ring; must be a multiple of 64k.
  //   desired_addr   location where you'd like it.
  void *alloc_at(size_t ring_size, void *desired_addr=0)
  {
    // if we already hold one allocation, refuse to make another.
    if (baseptr)
      return 0;

    // is ring_size a multiple of 64k? if not, this won't ever work!
    if ((ring_size & 0xffff) != 0)
      return 0;

    // try to allocate and map our space
    size_t alloc_size = ring_size * 2;
    if (!(mapping = CreateFileMappingA(INVALID_HANDLE_VALUE, 0, PAGE_READWRITE, (unsigned long long)alloc_size >> 32, alloc_size & 0xffffffffu, 0)) ||
      !(baseptr = (char *)MapViewOfFileEx(mapping, FILE_MAP_ALL_ACCESS, 0, 0, ring_size, desired_addr)) ||
      !MapViewOfFileEx(mapping, FILE_MAP_ALL_ACCESS, 0, 0, ring_size, (char *)desired_addr + ring_size))
    {
      // something went wrong - clean up
      free();
    }
    else // success!
      size = ring_size;

    return baseptr;
  }

  // This function will allocate a magic ring buffer at a system-determined base address.
  //
  // Sadly, there's no way (that I can see) in the Win32 API to first reserve
  // a memory region then fill it in using mmaps; you can reserve memory via
  // VirtualAlloc, but that address range can then only be used to commit
  // memory via another VirtualAlloc, and can not be mmap'ed. Furthermore,
  // there's also no way to do the two back-to-back mmaps atomically. What we
  // do here is to reserve enough memory via VirtualAlloc, then immediately
  // free it and try to put our allocation there. This is subject to a race
  // condition - another thread might end up allocating that very memory
  // region in the interim. What this means is that even when an alloc should
  // work (i.e. there's enough memory available) it can still fail spuriously
  // sometimes. Hence the "dicey" comment above.
  //
  // What we do here is just retry the alloc a given number of times and hope
  // that we don't get screwed every single time. This increases the
  // likelihood of success, but doesn't eliminate the chance of spurious
  // failure, so be religious about checking return values!
  void *alloc(size_t ring_size, int num_retries=5)
  {
    void *ptr = 0;
    while (!ptr && num_retries-- != 0)
    {
      void *target_addr = determine_viable_addr(ring_size * 2);
      if (target_addr)
        ptr = alloc_at(ring_size, target_addr);
    }

    return ptr;
  }

  // Frees the allocated region again.
  void free()
  {
    if (baseptr)
    {
      UnmapViewOfFile(baseptr);
      UnmapViewOfFile(baseptr + size);
      baseptr = 0;
    }

    if (mapping)
    {
      CloseHandle(mapping);
      mapping = 0;
    }

    size = 0;
  }

private:
  // Determine a viable target address of "size" memory mapped bytes by
  // allocating memory using VirtualAlloc and immediately freeing it. This
  // is subject to a potential race condition, see notes above.
  static void *determine_viable_addr(size_t size)
  {
    void *ptr = VirtualAlloc(0, size, MEM_RESERVE, PAGE_NOACCESS);
    if (!ptr)
      return 0;

    VirtualFree(ptr, 0, MEM_RELEASE);
    return ptr;
  }
};

int main()
{
  static const int ringsize = 64*1024;

  MagicRingBuffer mrb;
  char *buf = (char *)mrb.alloc(ringsize);
  if (!buf)
  {
    printf("MRB allocation failed!\n");
    return 0;
  }

  // Okay, now get ready for a magic trick!
  memset(buf, 0, ringsize * 2); // clear it all to zeroes

  strcpy(buf + ringsize - 3, "Hello world!");
  printf("%s\n", buf + ringsize - 3);
  printf("%s\n", buf);

  // nothing up my sleeve!

  return 0;
}