片段->sdcard

section.1 main()--------sdcard.c
 1 int main(int argc, char **argv)
{
    struct fuse fuse;
    char opts[256];
    int fd;
    int res;
    const char *path = NULL;
    int i;

    for (i = 1; i < argc; i++) {
        char* arg = argv[i];
        if (!path)
            path = arg;
        else if (uid == -1)
            uid = strtoul(arg, 0, 10);
        else if (gid == -1)
            gid = strtoul(arg, 0, 10);
        else {
            ERROR("too many arguments\n");
            return usage();
        }
    }

    if (!path) {
        ERROR("no path specified\n");
        return usage();
    }
    if (uid <= 0 || gid <= 0) {
        ERROR("uid and gid must be nonzero\n");
        return usage();
    }

        /* cleanup from previous instance, if necessary */
     umount2(MOUNT_POINT, 2);//---不会立即执行umount操作，而会等挂载点退出忙碌状态时才会去卸载它。
                               //----不过此函数执行会阻止对该挂载点执行新的访问。
                               //-----之前就在访问此挂载点操作也不会强制其退出，而是会等待其自然退出。35 
    fd = open("/dev/fuse", O_RDWR);//------fuse的内核实现
    if (fd < 0){
        ERROR("cannot open fuse device (%d)\n", errno);
        return -1;
    }

    sprintf(opts, "fd=%i,rootmode=40000,default_permissions,allow_other,"
            "user_id=%d,group_id=%d", fd, uid, gid);
       //fuse 文件系统的挂载
    //MOUNT_POINT =/storage/sdcard0    
    res = mount("/dev/fuse", MOUNT_POINT, "fuse", MS_NOSUID | MS_NODEV, opts);
    if (res < 0) {
        ERROR("cannot mount fuse filesystem (%d)\n", errno);
        return -1;
    }

    if (setgid(gid) < 0) {
        ERROR("cannot setgid!\n");
        return -1;
    }
    if (setuid(uid) < 0) {
        ERROR("cannot setuid!\n");
        return -1;
    }

    fuse_init(&fuse, fd, path);//--------见section.2

    umask(0);//---------系统默认文件权限，chmod的补码形式存在
    handle_fuse_requests(&fuse);//---------see section.3
    
    return 0;
}

section.2

fuse_init()

void fuse_init(struct fuse *fuse, int fd, const char *path)---fuse文件系统的构建

---->static char *rename_node(struct node *node, const char *name)

      ---->static void node_find_actual_name(struct node *node)

section.3

void handle_fuse_requests(struct fuse *fuse)
{
    unsigned char req[256 * 1024 + 128];
    int len;
    
    for (;;) {//---------无限循环.........
        len = read(fuse->fd, req, sizeof(req));//--------阻塞的等待用户操作dev/fuse设备的请求
        if (len < 0) {
            if (errno == EINTR)
                continue;
            ERROR("handle_fuse_requests: errno=%d\n", errno);
            return;
        }
           //-------详细的处理过程
        handle_fuse_request(fuse, (void*) req, (void*) (req + sizeof(struct fuse_in_header)), len) ;
    }
}

 section.4

on early-init
    export EXTERNAL_STORAGE /storage/sdcard0
    mkdir /storage 0050 system sdcard_r//----------storage目录可读的，
                                          //就是不能往里面写东西
    mkdir /storage/sdcard0 0000 system system//------sdcard0目录连读权限都没了
    # for backwards compatibility
    symlink /storage/sdcard0 /sdcard
    symlink /storage/sdcard0 /mnt/sdcard


on post-fs-data
    # we will remap this as /storage/sdcard0 with the sdcard fuse tool
    mkdir /data/media 0770 media_rw media_rw//------目录的全新啊
    chown media_rw media_rw /data/media//------目录内文件的权限

on fs
    mount_all /fstab.tuna//----------mount system userdata cache...
    setprop ro.crypto.fuse_sdcard true//---------启动sdcard

# create virtual SD card at /storage/sdcard0, based on the /data/media directory
# daemon will drop to user/group system/media_rw after initializing
# underlying files in /data/media will be created with user and group media_rw (1023)
service sdcard /system/bin/sdcard /data/media 1023 1023
    class late_start

<!-- The <device> element should contain one or more <storage> elements.
     Exactly one of these should have the attribute primary="true".
     This storage will be the primary external storage and should have path="/mnt/sdcard".
     Each storage should have both a path and description attribute set.
     The following boolean attributes are optional:

        primary:    this storage is the primary external storage
        removable:  this is removable storage (for example, a real SD card)
        emulated:   the storage is emulated via the FUSE sdcard daemon
        mtp-reserve: number of megabytes of storage MTP should reserve for free storage//-----------!!!!!!
                     (used for emulated storage that is shared with system's data partition)

      A storage should not have both emulated and removable set to true
-->

<StorageList xmlns:android="http://schemas.android.com/apk/res/android">
    <storage android:mountPoint="/storage/sdcard0"
             android:storageDescription="@string/storage_internal"
             android:primary="true"
             android:emulated="true"
             android:mtpReserve="100" />//-------------------!!!!!!!
</StorageList>

# create virtual SD card at /storage/sdcard0, based on the /data/media directory
# daemon will drop to user/group system/media_rw after initializing
# underlying files in /data/media will be created with user and group media_rw (1023)
service sdcard /system/bin/sdcard /data/media 1023 1023//----------通过data/media目录访问sdcard0内部文件
 class late_start