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Android系统匿名共享内存（Anonymous Shared Memory）C++调用接口分析
一、Ashmem驱动程序
~/Android/kernel/goldfish
----include
----linux
----ashmem.h
----mm
----ashmem.c
驱动程序详解请看《Android系统源代码情景分析》，作者罗升阳。
二、运行时库cutils的匿名共享内存访问接口
~/Android/system/core
----libcutils
----ashmem-dev.c
详解请看《Android系统源代码情景分析》，作者罗升阳。
三、MemoryHeapBase
1、Server端实现
我们只分析下面3个类
IMemoryHeap--->IMemory.h，IMemory.cpp
BnMemoryHeap--->IMemory.h，IMemory.cpp
MemoryHeapBase--->MemoryHeapBase.h，MemoryHeapBase.cpp
详解请看《Android系统源代码情景分析》，作者罗升阳，这里只列出代码。
IMemoryHeap(IMemory.h)
class IMemoryHeap : public IInterface
{
public:
DECLARE_META_INTERFACE(MemoryHeap);
// flags returned by getFlags()
enum {
READ_ONLY = 0x00000001
};
virtual int getHeapID() const = 0;
virtual void* getBase() const = 0;
virtual size_t getSize() const = 0;
virtual uint32_t getFlags() const = 0;
// these are there just for backward source compatibility
int32_t heapID() const { return getHeapID(); }
void* base() const { return getBase(); }
size_t virtualSize() const { return getSize(); }
}; BnMemoryHeap(IMemory.cpp)
status_t BnMemoryHeap::onTransact(
uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
switch(code) {
case HEAP_ID: {
CHECK_INTERFACE(IMemoryHeap, data, reply);
reply->writeFileDescriptor(getHeapID());
reply->writeInt32(getSize());
reply->writeInt32(getFlags());
return NO_ERROR;
} break;
default:
return BBinder::onTransact(code, data, reply, flags);
}
}
MemoryHeapBase(MemoryHeapBase.cpp)
MemoryHeapBase::MemoryHeapBase(size_t size, uint32_t flags, char const * name)
: mFD(-1), mSize(0), mBase(MAP_FAILED), mFlags(flags),
mDevice(0), mNeedUnmap(false)
{
const size_t pagesize = getpagesize();
size = ((size + pagesize-1) & ~(pagesize-1));
int fd = ashmem_create_region(name == NULL ? "MemoryHeapBase" : name, size);
LOGE_IF(fd<0, "error creating ashmem region: %s", strerror(errno));
if (fd >= 0) {
if (mapfd(fd, size) == NO_ERROR) {
if (flags & READ_ONLY) {
ashmem_set_prot_region(fd, PROT_READ);
}
}
}
}
......
status_t MemoryHeapBase::mapfd(int fd, size_t size, uint32_t offset)
{
.....
if ((mFlags & DONT_MAP_LOCALLY) == 0) {
void* base = (uint8_t*)mmap(0, size,
PROT_READ|PROT_WRITE, MAP_SHARED, fd, offset);
......
mBase = base;
mNeedUnmap = true;
} else {
.....
}
mFD = fd;
mSize = size;
return NO_ERROR;
}
.........
int MemoryHeapBase::getHeapID() const {
return mFD;
}
void* MemoryHeapBase::getBase() const {
return mBase;
}
size_t MemoryHeapBase::getSize() const {
return mSize;
}
uint32_t MemoryHeapBase::getFlags() const {
return mFlags;
}2、Client端实现
我们只分析下面2个类
IMemoryHeap--->IMemory.h，IMemory.cpp
BpMemoryHeap--->IMemory.cpp
详解请看《Android系统源代码情景分析》，作者罗升阳。
BpMemoryHeap(IMemory.cpp)
BpMemoryHeap::BpMemoryHeap(const sp<IBinder>& impl)
: BpInterface<IMemoryHeap>(impl),
mHeapId(-1), mBase(MAP_FAILED), mSize(0), mFlags(0), mRealHeap(false)
{
}
.......
void BpMemoryHeap::assertMapped() const
{
if (mHeapId == -1) {
sp<IBinder> binder(const_cast<BpMemoryHeap*>(this)->asBinder());
sp<BpMemoryHeap> heap(static_cast<BpMemoryHeap*>(find_heap(binder).get()));
heap->assertReallyMapped();
if (heap->mBase != MAP_FAILED) {
Mutex::Autolock _l(mLock);
if (mHeapId == -1) {
mBase = heap->mBase;
mSize = heap->mSize;
android_atomic_write( dup( heap->mHeapId ), &mHeapId );
}
} else {
// something went wrong
free_heap(binder);
}
}
}
void BpMemoryHeap::assertReallyMapped() const
{
if (mHeapId == -1) {
// remote call without mLock held, worse case scenario, we end up
// calling transact() from multiple threads, but that's not a problem,
// only mmap below must be in the critical section.
Parcel data, reply;
data.writeInterfaceToken(IMemoryHeap::getInterfaceDescriptor());
status_t err = remote()->transact(HEAP_ID, data, &reply);
int parcel_fd = reply.readFileDescriptor();
ssize_t size = reply.readInt32();
uint32_t flags = reply.readInt32();
.........
int fd = dup( parcel_fd );
........
int access = PROT_READ;
if (!(flags & READ_ONLY)) {
access |= PROT_WRITE;
}
Mutex::Autolock _l(mLock);
if (mHeapId == -1) {
mRealHeap = true;
mBase = mmap(0, size, access, MAP_SHARED, fd, 0);
if (mBase == MAP_FAILED) {
LOGE("cannot map BpMemoryHeap (binder=%p), size=%ld, fd=%d (%s)",
asBinder().get(), size, fd, strerror(errno));
close(fd);
} else {
mSize = size;
mFlags = flags;
android_atomic_write(fd, &mHeapId);
}
}
}
}
int BpMemoryHeap::getHeapID() const {
assertMapped();
return mHeapId;
}
void* BpMemoryHeap::getBase() const {
assertMapped();
return mBase;
}
size_t BpMemoryHeap::getSize() const {
assertMapped();
return mSize;
}
uint32_t BpMemoryHeap::getFlags() const {
assertMapped();
return mFlags;
}四、MemoryBase
1、Server端实现
我们只分析下面3个类
IMemory--->IMemory.h，IMemory.cpp
BnMemory--->IMemory.h，IMemory.cpp
MemoryBase--->MemoryBase.h，MemoryBase.cpp
详解请看《Android系统源代码情景分析》，作者罗升阳。
2、Client端实现
我们只分析下面2个类
IMemory--->IMemory.h，IMemory.cpp
BpMemory--->IMemory.cpp
详解请看《Android系统源代码情景分析》，作者罗升阳。
五、应用实例
~/Android/external/ashmem
----common
----ISharedBuffer.h
----ISharedBuffer.cpp
----server
----SharedBufferServer.cpp
----Android.mk
----client
----SharedBufferClient.cpp
----Android.mk
1、Server端实现
想象上面的图。
我们只分析下面3个类
ISharedBuffer--->ISharedBuffer.h，ISharedBuffer.cpp
BnSharedBuffer--->ISharedBuffer.h，ISharedBuffer.cpp
SharedBufferService--->SharedBufferServer.cpp
详解请看《Android系统源代码情景分析》，作者罗升阳。
2、Client端实现
想象上面的图。我们只分析下面2个类
ISharedBuffer--->ISharedBuffer.h，ISharedBuffer.cpp
BpSharedBuffer--->ISharedBuffer.cpp
详解请看《Android系统源代码情景分析》，作者罗升阳。
3、实现代码
SharedBufferServer.cppclass SharedBufferService : public BnSharedBuffer
{
public:
SharedBufferService()
{
sp<MemoryHeapBase> heap = new MemoryHeapBase(SHARED_BUFFER_SIZE, 0, "SharedBuffer");
if(heap != NULL)
{
mMemory = new MemoryBase(heap, 0, SHARED_BUFFER_SIZE);
int32_t* data = http://www.mamicode.com/(int32_t*)mMemory->pointer();>
未完。