android mmap的使用

http://kenby.iteye.com/blog/1164700

共享内存可以说是最有用的进程间通信方式，也是最快的IPC形式, 因为进程可以直接读写内存，而不需要任何

数据的拷贝。对于像管道和消息队列等通信方式，则需要在内核和用户空间进行四次的数据拷贝，而共享内存则

只拷贝两次数据: 一次从输入文件到共享内存区，另一次从共享内存区到输出文件。实际上，进程之间在共享内

存时，并不总是读写少量数据后就解除映射，有新的通信时，再重新建立共享内存区域。而是保持共享区域，直

到通信完毕为止，这样，数据内容一直保存在共享内存中，并没有写回文件。共享内存中的内容往往是在解除映

射时才写回文件的。因此，采用共享内存的通信方式效率是非常高的。

一. 传统文件访问

UNIX访问文件的传统方法是用open打开它们, 如果有多个进程访问同一个文件, 则每一个进程在自己的地址空间都包含有该

文件的副本,这不必要地浪费了存储空间. 下图说明了两个进程同时读一个文件的同一页的情形. 系统要将该页从磁盘读到高

速缓冲区中, 每个进程再执行一个存储器内的复制操作将数据从高速缓冲区读到自己的地址空间.

二. 共享存储映射

现在考虑另一种处理方法:　进程A和进程B都将该页映射到自己的地址空间, 当进程A第一次访问该页中的数据时, 它生成一

个缺页中断. 内核此时读入这一页到内存并更新页表使之指向它.以后, 当进程B访问同一页面而出现缺页中断时, 该页已经在

内存, 内核只需要将进程B的页表登记项指向次页即可. 如下图所示: 

三、mmap()及其相关系统调用

mmap()系统调用使得进程之间通过映射同一个普通文件实现共享内存。普通文件被映射到进程地址空间后，进程可以向访

问普通内存一样对文件进行访问，不必再调用read()，write（）等操作。

 

mmap()系统调用形式如下：

void* mmap ( void * addr , size_t len , int prot , int flags , int fd , off_t offset ) 

mmap的作用是映射文件描述符fd指定文件的 [off,off + len]区域至调用进程的[addr, addr + len]的内存区域, 如下图所示:

参数fd为即将映射到进程空间的文件描述字，一般由open()返回，同时，fd可以指定为-1，此时须指定flags参数中的

MAP_ANON，表明进行的是匿名映射（不涉及具体的文件名，避免了文件的创建及打开，很显然只能用于具有亲缘关系的

进程间通信）。

len是映射到调用进程地址空间的字节数，它从被映射文件开头offset个字节开始算起。

prot 参数指定共享内存的访问权限。可取如下几个值的或：PROT_READ（可读） , PROT_WRITE （可写）, PROT_EXEC （可执行）, PROT_NONE（不可访问）。

flags由以下几个常值指定：MAP_SHARED , MAP_PRIVATE , MAP_FIXED，其中，MAP_SHARED , MAP_PRIVATE必

选其一，而MAP_FIXED则不推荐使用。

offset参数一般设为0，表示从文件头开始映射。

参数addr指定文件应被映射到进程空间的起始地址，一般被指定一个空指针，此时选择起始地址的任务留给内核来完成。函

数的返回值为最后文件映射到进程空间的地址，进程可直接操作起始地址为该值的有效地址。

四. mmap的例子

shmdata.h

#ifndef _SHMDATA_H_HEADER#define _SHMDATA_HE_HEADER#define TEXT_SZ 2048typedef struct shared_use_st{int written; // 非 0可读， 0表示 可写char text[TEXT_SZ]; // 记录 写入和 读取 的 文本 }shared_use_st;#endif

JNI_Shm.c

#include<stdio.h>#include<stdlib.h>#include<unistd.h>#include<jni.h>#include <sys/types.h> /* for open */#include <sys/stat.h> /* for open */#include <fcntl.h>     /* for open */#include<signal.h>#include <sys/mman.h>#include "shmdata.h"#include "JNIHelp.h"#include<android/log.h>#define LOGI(...) ((void)__android_log_print(ANDROID_LOG_INFO, "native-activity", __VA_ARGS__))#define LOGW(...) ((void)__android_log_print(ANDROID_LOG_WARN, "native-activity", __VA_ARGS__))#define LOGE(...) ((void)__android_log_print(ANDROID_LOG_ERROR, "native-activity", __VA_ARGS__))#ifndef NELEM# define NELEM(x) ((int) (sizeof(x) / sizeof((x)[0])))#endifstatic char CLASS_NAME[] = "com/sailor/ShareMem";static shared_use_st *use_st = NULL;static jint throwException(JNIEnv* env, jobject clazz, const char *clsname, const char *msg){jclass cls;    cls = ( *env)->FindClass(env, clsname);    if(cls == NULL){return -1;    }    (*env)->ThrowNew(env,cls, msg);return -1;} static jint openMem(JNIEnv* env, jobject clazz, jstring name, jint length){shared_use_st *st = NULL;const char* namestr = (name ? (*env)->GetStringUTFChars(env,name, NULL) : NULL);int result = -1;if(access(namestr, F_OK) == 0){result = open(namestr, O_RDWR);}else{result = open(namestr, O_RDWR|O_CREAT);if(result >= 0){st = (shared_use_st *)malloc(sizeof(shared_use_st));if(st == NULL){LOGE("open malloc failed");}if(write(result, st, sizeof(shared_use_st)) < 0){LOGE("open write failed");}free(st);}}(*env)->ReleaseStringUTFChars(env, name, namestr);return result;}static jint getMem(JNIEnv* env, jobject clazz,int fd){struct stat sb;int result = -1;if(fd >= 0){if((fstat(fd, &sb)) == -1){LOGE("readMem size failed");return result;}if((use_st = (shared_use_st *)mmap(NULL, sb.st_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)) == (void *)-1){LOGE("readMem mmap failed");return result;}result = (jint)use_st->text;LOGE("memory address:%0x", result);}return result;}static jint writeMem(JNIEnv* env, jobject clazz, int fd, jbyteArray buffer, int size, jint address){int ret = 0;if(use_st != NULL && address == (int)use_st->text){(*env)->GetByteArrayRegion(env, buffer, 0, size, (jbyte *)address);ret = size;}return ret;}static jbyteArray readMem(JNIEnv* env, jobject clazz, int fd, int address){if(fd >= 0){if(use_st != NULL){jbyteArray array = (*env)->NewByteArray(env, TEXT_SZ);(*env)->SetByteArrayRegion(env, array, 0, TEXT_SZ, (char *)address);return array;}}return NULL;}static void setModeMem(JNIEnv* env, jobject clazz, int mode){if(use_st != NULL){use_st->written = mode;}}static int getModeMem(JNIEnv* env, jobject clazz){int nRet = -1;if(use_st != NULL){nRet = use_st->written;}return nRet;}static  void closeMem(JNIEnv* env, jobject clazz, int fd){close(fd);if(use_st != NULL){if ((munmap((void *)use_st, sizeof(shared_use_st))) == -1) {          LOGE("munmap failed");    }  }}static  jboolean flushMem(JNIEnv* env, jobject clazz){jboolean isFailed = 1;if(use_st != NULL){if ((msync((void *)use_st, sizeof(shared_use_st), MS_SYNC)) == -1) {         LOGE("msync failed");     }  else{isFailed = 0;}}return !isFailed;}static JNINativeMethod mehods[] = {{ "open", "(Ljava/lang/String;I)I", (void *) openMem },{ "read", "(II)[B", (void *) readMem },{ "write", "(I[BII)I", (void *) writeMem },{ "get", "(I)I", (void *) getMem },{ "setMode", "(I)V", (void *) setModeMem },{ "getMode", "()I", (void *) getModeMem },{ "flush", "()Z", (void *) flushMem },{ "close", "(I)V", (void *) closeMem }};static int registerNativeMethods(JNIEnv *env, const char* className,                                 const JNINativeMethod* methods, int numMethods){    int rc;    jclass clazz;    clazz = (*env)->FindClass(env, className);    if (clazz == NULL) {        LOGE("Native registration unable to find class '%s'\n", className);        return -1;    }    if (rc = ((*env)->RegisterNatives(env, clazz, methods, numMethods)) < 0) {        LOGE("RegisterNatives failed for '%s' %d\n", className, rc);        return -1;    }    return 0;}static int register_jni(JNIEnv *env){return registerNativeMethods(env, CLASS_NAME, mehods, NELEM(mehods));}JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved){     JNIEnv* env = NULL;     jint result = -1;          //获取JNI版本     if ((*vm)->GetEnv(vm, (void**)&env, JNI_VERSION_1_4) != JNI_OK)      {         LOGE("GetEnv failed!");             return result;     }     InitHelp(env, NULL);     if (register_jni(env) < 0) {LOGE("register method failed!");             return result;     }     return JNI_VERSION_1_4;}

com.sailor.ShareMem.java:

package com.sailor;public class ShareMem {static {// 加载动态库System.loadLibrary("JNIShm");}public native int open(String path, int length);public native void close(int fd);public native int get(int fd);public native byte[] read(int fd, int address);public native int write(int fd, byte[] buffer, int size, int address);public native boolean flush();public native void setMode(int mode);public native int getMode();}

android1 app:

public void startMem3(View view){mem = new ShareMem();int fd = mem.open(Environment.getExternalStorageDirectory().getAbsolutePath()+"/test.txt",0);Log.d("jacklam", "startMem fd:" + fd);int address = mem.get(fd);Log.d("jacklam", "startMem address:" + address);boolean isFlush = mem.flush();Log.d("jacklam", "flush buffer:" + isFlush);mem.close(fd);}public void startMem2(View view){mem = new ShareMem();int fd = mem.open(Environment.getExternalStorageDirectory().getAbsolutePath()+"/test.txt",0);Log.d("jacklam", "startMem fd:" + fd);int address = mem.get(fd);Log.d("jacklam", "startMem address:" + address);byte[] buffer = mem.read(fd, address);Log.d("jacklam", "read buffer:" + new String(buffer));/*mem.close(fd);*/}public void startMem1(View view){mem = new ShareMem();int fd = mem.open(Environment.getExternalStorageDirectory().getAbsolutePath()+"/test.txt",0);Log.d("jacklam", "startMem fd:" + fd);int address = mem.get(fd);Log.d("jacklam", "startMem address:" + address);byte[] buffer = {'I','I','t'};mem.write(fd, buffer, buffer.length, address);/*mem.close(fd);*/}

android app2:

public void startMem3(View view){mem = new ShareMem();int fd = mem.open(Environment.getExternalStorageDirectory().getAbsolutePath()+"/test.txt",0);Log.d("jacklam", "startMem fd:" + fd);int address = mem.get(fd);Log.d("jacklam", "startMem address:" + address);boolean isFlush = mem.flush();Log.d("jacklam", "flush buffer:" + isFlush);mem.close(fd);}public void startMem2(View view){mem = new ShareMem();int fd = mem.open(Environment.getExternalStorageDirectory().getAbsolutePath()+"/test.txt",0);Log.d("jacklam", "startMem fd:" + fd);int address = mem.get(fd);Log.d("jacklam", "startMem address:" + address);byte[] buffer = mem.read(fd, address);Log.d("jacklam", "read buffer:" + new String(buffer));mem.close(fd);}public void startMem1(View view){mem = new ShareMem();int fd = mem.open(Environment.getExternalStorageDirectory().getAbsolutePath()+"/test.txt",0);Log.d("jacklam", "startMem fd:" + fd);int address = mem.get(fd);Log.d("jacklam", "startMem address:" + address);byte[] buffer = {'I','o','t'};mem.write(fd, buffer, buffer.length, address);mem.close(fd);}