1 // 2 // Copyright 2005 The Android Open Source Project 3 // 4 // Inter-process shared memory. 5 // 6 #ifndef __LIBS_SHMEM_H 7 #define __LIBS_SHMEM_H 8 9 #ifdef HAVE_ANDROID_OS 10 #error DO NOT USE THIS FILE IN THE DEVICE BUILD 11 #endif 12 13 #include "Semaphore.h" 14 15 namespace android { 16 17 /* 18 * Platform-independent shared memory. Each object can be used to 19 * create a chunk of memory that is shared between processes. 20 * 21 * For convenience, a semaphore is associated with each segment. 22 * (Whether this should have been done in a subclass is debatable.) 23 * 24 * The "key" is usually the process ID of the process that created the 25 * segment. The goal is to avoid clashing with other processes that are 26 * trying to do the same thing. It's a little awkward to use when you 27 * want to have multiple shared segments created in one process. In 28 * SysV you can work around this by using a "private" key and sharing 29 * the shmid with your friends, in Win32 you can use a string, but we're 30 * in lowest-common-denominator mode here. Assuming we have 16-bit PIDs, 31 * the upper 16 bits can be used to serialize keys. 32 * 33 * When the object goes out of scope, the shared memory segment is 34 * detached from the process. If the object was responsible for creating 35 * the segment, it is also marked for destruction on SysV systems. This 36 * will make it impossible for others to attach to. 37 * 38 * On some systems, the length returned by getLength() may be different 39 * for parent and child due to page size rounding. 40 */ 41 class Shmem { 42 public: 43 Shmem(void); 44 virtual ~Shmem(void); 45 46 /* 47 * Create a new shared memory segment, with the specified size. If 48 * "deleteExisting" is set, any existing segment will be deleted first 49 * (useful for SysV IPC). 50 * 51 * Returns "true" on success, "false" on failure. 52 */ 53 bool create(int key, long size, bool deleteExisting); 54 55 /* 56 * Attach to a shared memory segment. Use this from the process that 57 * didn't create the segment. 58 * 59 * Returns "true" on success, "false" on failure. 60 */ 61 bool attach(int key); 62 63 /* 64 * Get the memory segment address and length. These will not change 65 * for the lifetime of the object, so it's okay to cache the results. 66 * 67 * On failure, getAddr() returns NULL and getLength() returns -1. 68 */ 69 void* getAddr(void); 70 long getLength(void); 71 72 /* 73 * Lock or unlock the shared memory segment. This is useful if you 74 * are updating pieces of shared data. The segment is initially 75 * "unlocked". 76 * 77 * This does *not* lock down the segment in the virtual paging system. 78 * It's just a mutex. 79 */ 80 void lock(void); 81 void unlock(void); 82 bool tryLock(void); 83 84 private: 85 Semaphore mSem; // uses the same value for "key" 86 unsigned long mHandle; // shmid(int) or HANDLE 87 void* mAddr; // address 88 long mLength; // length of segment (cached) 89 bool mCreator; // true if we created the segment 90 int mKey; // key passed in as arg 91 }; 92 93 }; // namespace android 94 95 #endif // __LIBS_SHMEM_H 96
