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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef mozilla_ipc_Shmem_h
#define mozilla_ipc_Shmem_h
#include "mozilla/Attributes.h"
#include "base/basictypes.h"
#include "base/process.h"
#include "nscore.h"
#include "nsDebug.h"
#include "mozilla/ipc/SharedMemory.h"
#include "mozilla/UniquePtr.h"
* |Shmem| is one agent in the IPDL shared memory scheme. The way it
works is essentially
* (1) C++ code calls, say, |parentActor->AllocShmem(size)|
* (2) IPDL-generated code creates a |mozilla::ipc::SharedMemory|
* wrapping the bare OS shmem primitives. The code then adds the new
* SharedMemory to the set of shmem segments being managed by IPDL.
* (3) IPDL-generated code "shares" the new SharedMemory to the child
* process, and then sends a special asynchronous IPC message to the
* child notifying it of the creation of the segment. (What this
* means is OS specific.)
* (4a) The child receives the special IPC message, and using the
* |SharedMemory{Basic}::Handle| it was passed, creates a
* |mozilla::ipc::SharedMemory| in the child
* process.
* (4b) After sending the "shmem-created" IPC message, IPDL-generated
* code in the parent returns a |mozilla::ipc::Shmem| back to the C++
* caller of |parentActor->AllocShmem()|. The |Shmem| is a "weak
* reference" to the underlying |SharedMemory|, which is managed by
* IPDL-generated code. C++ consumers of |Shmem| can't get at the
* underlying |SharedMemory|.
* If parent code wants to give access rights to the Shmem to the
* child, it does so by sending its |Shmem| to the child, in an IPDL
* message. The parent's |Shmem| then "dies", i.e. becomes
* inaccessible. This process could be compared to passing a
* "shmem-access baton" between parent and child.
namespace mozilla {
namespace layers {
class ShadowLayerForwarder;
} // namespace layers
namespace ipc {
template <typename P>
struct IPDLParamTraits;
class Shmem final {
friend struct IPDLParamTraits<mozilla::ipc::Shmem>;
#ifdef DEBUG
// For ShadowLayerForwarder::CheckSurfaceDescriptor
friend class mozilla::layers::ShadowLayerForwarder;
typedef int32_t id_t;
// Low-level wrapper around platform shmem primitives.
typedef mozilla::ipc::SharedMemory SharedMemory;
typedef SharedMemory::SharedMemoryType SharedMemoryType;
// Shmem objects should only be constructed directly from SharedMemory
// objects by the Shmem implementation itself, or by a select few functions
// in ProtocolUtils.{h,cpp}. You should not need to add new instances of
// this token.
struct PrivateIPDLCaller {};
Shmem() : mSegment(nullptr), mData(nullptr), mSize(0), mId(0) {}
Shmem(const Shmem& aOther) = default;
#if !defined(DEBUG)
Shmem(PrivateIPDLCaller, SharedMemory* aSegment, id_t aId)
: mSegment(aSegment), mData(aSegment->memory()), mSize(0), mId(aId) {
mSize = static_cast<size_t>(*PtrToSize(mSegment));
Shmem(PrivateIPDLCaller, SharedMemory* aSegment, id_t aId);
~Shmem() {
// Shmem only holds a "weak ref" to the actual segment, which is
// owned by IPDL. So there's nothing interesting to be done here
Shmem& operator=(const Shmem& aRhs) = default;
bool operator==(const Shmem& aRhs) const { return mSegment == aRhs.mSegment; }
// Returns whether this Shmem is writable by you, and thus whether you can
// transfer writability to another actor.
bool IsWritable() const { return mSegment != nullptr; }
// Returns whether this Shmem is readable by you, and thus whether you can
// transfer readability to another actor.
bool IsReadable() const { return mSegment != nullptr; }
// Return a pointer to the user-visible data segment.
template <typename T>
T* get() const {
return reinterpret_cast<T*>(mData);
// Return the size of the segment as requested when this shmem
// segment was allocated, in units of T. The underlying mapping may
// actually be larger because of page alignment and private data,
// but this isn't exposed to clients.
template <typename T>
size_t Size() const {
return mSize / sizeof(T);
// These shouldn't be used directly, use the IPDL interface instead.
id_t Id(PrivateIPDLCaller) const { return mId; }
SharedMemory* Segment(PrivateIPDLCaller) const { return mSegment; }
#ifndef DEBUG
void RevokeRights(PrivateIPDLCaller) {}
void RevokeRights(PrivateIPDLCaller);
void forget(PrivateIPDLCaller) {
mSegment = nullptr;
mData = nullptr;
mSize = 0;
mId = 0;
static already_AddRefed<Shmem::SharedMemory> Alloc(PrivateIPDLCaller,
size_t aNBytes,
SharedMemoryType aType,
bool aUnsafe,
bool aProtect = false);
// Prepare this to be shared with another process. Return an IPC message that
// contains enough information for the other process to map this segment in
// OpenExisting() below. Return a new message if successful (owned by the
// caller), nullptr if not.
UniquePtr<IPC::Message> MkCreatedMessage(PrivateIPDLCaller,
int32_t routingId);
// Stop sharing this with another process. Return an IPC message that
// contains enough information for the other process to unmap this
// segment. Return a new message if successful (owned by the
// caller), nullptr if not.
UniquePtr<IPC::Message> MkDestroyedMessage(PrivateIPDLCaller,
int32_t routingId);
// Return a SharedMemory instance in this process using the descriptor shared
// to us by the process that created the underlying OS shmem resource. The
// contents of the descriptor depend on the type of SharedMemory that was
// passed to us.
static already_AddRefed<SharedMemory> OpenExisting(
PrivateIPDLCaller, const IPC::Message& aDescriptor, id_t* aId,
bool aProtect = false);
static void Dealloc(PrivateIPDLCaller, SharedMemory* aSegment);
template <typename T>
void AssertAligned() const {
if (0 != (mSize % sizeof(T))) MOZ_CRASH("shmem is not T-aligned");
#if !defined(DEBUG)
void AssertInvariants() const {}
static uint32_t* PtrToSize(SharedMemory* aSegment) {
char* endOfSegment =
reinterpret_cast<char*>(aSegment->memory()) + aSegment->Size();
return reinterpret_cast<uint32_t*>(endOfSegment - sizeof(uint32_t));
void AssertInvariants() const;
RefPtr<SharedMemory> mSegment;
void* mData;
size_t mSize;
id_t mId;
} // namespace ipc
} // namespace mozilla
#endif // ifndef mozilla_ipc_Shmem_h