NDK下IPC问题
由于AllJoyn的join session timeout问题一直无法解决，我们怀疑AllJoyn有些内部变量没有清理干净，因此考虑将AllJoyn相关功能放到一个单独的进程中，一旦join session timeout，就重启该进程。
这就涉及到IPC问题。
因为我们写的是通用模块，需要在DTV（arm平台linux）和Android(NDK)上运行，因此就必须考虑各种IPC手段的可用性了。
一开始我考虑直接使用socket进行IPC（肯定可用），但有人说消息队列、共享内存、管道之类的也可以用。于是俺就mmap+信号量实现了一个简单的IPC类：
/* * File: ipc.h * Author: raozf * * Created on 2013年9月17日, 上午11:04 */ #ifndef IPC_H #define IPC_H #include <semaphore.h> #include <sys/mman.h> #include <sys/stat.h> #include <fcntl.h> //semaphore name const std::string IPC_SEM = "alljoyn_sem"; //mmap length const size_t IPC_MMAP_LEN = 1 * 1024 * 1024; /* * helper class for IPC between ****forked**** processes. * implemented by semaphore and mmap */ class IPC { public: IPC():_mutex(NULL), _memory(NULL) { //unnamed semaphore should use sem_init() //open named semaphore, shared between processes _mutex = sem_open(IPC_SEM.c_str(), O_CREAT, O_RDWR, 1); if(_mutex == SEM_FAILED || _mutex == NULL) { LOGV("[ContextSharing]IPC::IPC() sem_open() failed. semaphore name:%s, %s", IPC_SEM.c_str(), strerror(errno)); return; } /* * http://www.ibm.com/developerworks/cn/linux/l-ipc/part5/index1.html */ //create a anonymous mmap _memory = (char*)mmap(NULL, IPC_MMAP_LEN, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, 0, 0); if(_memory == MAP_FAILED || _memory == NULL) { LOGV("[ContextSharing]IPC::IPC() mmap() failed. %s", strerror(errno)); deinit(); return; } } ~IPC() { deinit(); } //send msg to other process //header+message // int char* void send(const std::string& msg) { *((int*)_memory) = msg.length(); memcpy(_memory + sizeof(int), msg.c_str(), msg.length()); sem_post(_mutex); } //receive from other process std::string recv() { sem_wait(_mutex);//will block return std::string(_memory + sizeof(int), (int)(_memory)); } private: void deinit() { if(_mutex != NULL) { /* destory an unnamed semaphore, initialized by sem_init() //sem_destroy(_mutex); */ // closes the named semaphore, but still existed in system kernel sem_close(_mutex); _mutex = NULL; //remove semaphore from system kernel(if it's reference is 0--which decremented by sem_close()) //removes the named semaphore referred to by name. The semaphore name is removed immediately. //The semaphore is destroyed once all other processes that have the semaphore open close it. sem_unlink(IPC_SEM.c_str()); } if(_memory != NULL) { munmap(_memory, IPC_MMAP_LEN); _memory = NULL; } } private: sem_t* _mutex; char* _memory; }; #endif /* IPC_H */
辅助类，封装一个线程专门等待信号量、接收数据：
/* * File: ThreadedIPC.h * Author: raozf * * Created on 2013年9月17日, 下午3:21 */ #ifndef THREADEDIPC_H #define THREADEDIPC_H #include "Common/ThreadManager.h" #include "delegate.h" #include "ipc.h" static const OID OID_ThreadedIPC = { 0x8f52a31f, 0x51d5, 0x462b, { 0xa9, 0x8d, 0x40, 0x70, 0xa3, 0xf6, 0xb5, 0x7f } }; class ThreadedIPC :public CObjectRoot<CObjectMultiThreadModel> ,public IThreadClient { public: PSFRESULT FinalConstruct() { PSFRESULT res = PSF_E_FAIL; CHK_PSFRESULT(_listen_thread.Initialize()); _listen_thread.AddTask(this, NULL); CLEANUP: return res; } void FinalRelease() { _listen_thread.Terminate(); } void Send(const std::string& msg) { _sharer.send(msg); } private: void OnExecute(void* pArg) { while(true) { _delegate_recv(_sharer.recv()); } } void OnTerminate(void* pArg) { } public: delegate::Delegate<void(std::string)> _delegate_recv; private: CTaskWorker _listen_thread; IPC _sharer; BEGIN_OBJECT_INTERFACE_MAP() OBJECT_INTERFACE_ENTRY(OID_ThreadedIPC, this); END_OBJECT_INTERFACE_MAP() }; #endif /* THREADEDIPC_H */
这里面用到了我们自己写的delegate和thread类（先忽略之）。
但在android上运行时却报错：
IPC::IPC() sem_open() failed. semaphore name:alljoyn_sem, Function not implemented
看来信号量在NDK下是用不了的。
而且，不仅仅信号量，共享内存、消息队列在NDK下都不能用。
参见讨论：https://groups.google.com/forum/#!topic/android-ndk/FzJIsJIxCX4
http://stackoverflow.com/questions/18603267/cross-process-locking-with-android-ndk
（但管道可用？http://stackoverflow.com/questions/8471552/porting-c-code-which-uses-fork-to-android）
Android源代码中的文档说明：http://www.netmite.com/android/mydroid/1.6/bionic/libc/docs/SYSV-IPC.TXT
（该文件在Android4。3中似乎已经移出该文档）
Android does not support System V IPCs, i.e. the facilities provided by the following standard Posix headers: <sys/sem.h> /* SysV semaphores */ <sys/shm.h> /* SysV shared memory segments */ <sys/msg.h> /* SysV message queues */ <sys/ipc.h> /* General IPC definitions */ The reason for this is due to the fact that, by design, they lead to global kernel resource leakage.
原因就是防止内核资源泄露。
更重要的问题在于：fork()也尽量不要用。
道理很简单：我们不应该控制android的底层，这些api会造成系统的不稳定。
https://groups.google.com/forum/#!msg/android-platform/80jr-_A-9bU/nkzslcgVrfYJ
“Bear in mind that the Dalvik VM doesn't like fork() much, and goes into
conniptions if you try to do *any* work between the fork() and the
exec(). ”
https://groups.google.com/forum/#!topic/android-ndk/FzJIsJIxCX4
悲催！
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ps:学到了一个命令“ipcs”，查看系统中共享内存、信号量状态：
$ ipcs
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
------ Semaphore Arrays --------
key semid owner perms nsems
------ Message Queues --------
key msqid owner perms used-bytes messages