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/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* 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 */
#include "mozilla/ipc/SharedMemoryBasic.h"
#include "mozilla/Span.h"
#include <utility>
namespace mozilla::ipc {
class WritableSharedMemoryMapping;
/// A handle to shared memory.
/// See the doc comment for `WritableSharedMemoryMapping` below.
class UnsafeSharedMemoryHandle {
friend class WritableSharedMemoryMapping;
friend struct IPC::ParamTraits<UnsafeSharedMemoryHandle>;
UnsafeSharedMemoryHandle(UnsafeSharedMemoryHandle&& aOther) noexcept;
UnsafeSharedMemoryHandle& operator=(
UnsafeSharedMemoryHandle&& aOther) noexcept;
/// Attempts to allocate a shmem.
/// Returns `Nothing()` if allocation fails.
/// If `aSize` is zero, a valid empty WritableSharedMemoryMapping is returned.
static Maybe<std::pair<UnsafeSharedMemoryHandle, WritableSharedMemoryMapping>>
CreateAndMap(size_t aSize);
UnsafeSharedMemoryHandle(SharedMemoryBasic::Handle&& aHandle, uint64_t aSize)
: mHandle(std::move(aHandle)), mSize(aSize) {}
SharedMemoryBasic::Handle mHandle;
uint64_t mSize;
/// A Shared memory buffer mapping.
/// Unlike `ipc::Shmem`, the underlying shared memory buffer on each side of
/// the process boundary is only deallocated with there respective
/// `WritableSharedMemoryMapping`.
/// ## Usage
/// Typical usage goes as follows:
/// - Allocate the memory using `UnsafeSharedMemoryHandle::Create`, returning a
/// handle and a mapping.
/// - Send the handle to the other process using an IPDL message.
/// - On the other process, map the shared memory by creating
/// WritableSharedMemoryMapping via `WritableSharedMemoryMapping::Open` and
/// the received handle.
/// Do not send the shared memory handle again, it is only intended to establish
/// the mapping on each side during initialization. The user of this class is
/// responsible for managing the lifetime of the buffers on each side, as well
/// as their identity, by for example storing them in hash map and referring to
/// them via IDs in IPDL message if need be.
/// ## Empty shmems
/// An empty WritableSharedMemoryMapping is one that was created with size zero.
/// It is analogous to a null RefPtr. It can be used like a non-empty shmem,
/// including sending the handle and openning it on another process (resulting
/// in an empty mapping on the other side).
class WritableSharedMemoryMapping {
friend class UnsafeSharedMemoryHandle;
WritableSharedMemoryMapping() = default;
WritableSharedMemoryMapping(WritableSharedMemoryMapping&& aMoved) = default;
WritableSharedMemoryMapping& operator=(WritableSharedMemoryMapping&& aMoved) =
/// Open the shmem and immediately close the handle.
static Maybe<WritableSharedMemoryMapping> Open(
UnsafeSharedMemoryHandle aHandle);
// Returns the size in bytes.
size_t Size() const;
// Returns the shared memory as byte range.
Span<uint8_t> Bytes();
explicit WritableSharedMemoryMapping(
RefPtr<mozilla::ipc::SharedMemoryBasic>&& aRef);
RefPtr<mozilla::ipc::SharedMemoryBasic> mRef;
} // namespace mozilla::ipc
namespace IPC {
template <>
struct ParamTraits<mozilla::ipc::UnsafeSharedMemoryHandle> {
typedef mozilla::ipc::UnsafeSharedMemoryHandle paramType;
static void Write(IPC::MessageWriter* aWriter, paramType&& aVar);
static bool Read(IPC::MessageReader* aReader, paramType* aVar);
} // namespace IPC