ServiceManager

这里单纯说一下ServiceManager的Bn端，与其他服务的native不同，ServiceManager的native单纯由c实现，并没有使用binder c++中的框架结构。

代码位于：

frameworks/base/cmds/servicemanager/

文件也很少：

Android.mk
bctest.c
binder.c
binder.h
service_manager.c

编译时将生成 /system/bin/servicemanager，并通过init.rc启动

service servicemanager /system/bin/servicemanager
    class core
    user system
    group system
    critical
    onrestart restart healthd
    onrestart restart zygote
    onrestart restart media
    onrestart restart surfaceflinger
    onrestart restart drm
    onrestart restart iss_daemon

main函数位于 service_manager.c中：

int main(int argc, char **argv)
{
    struct binder_state *bs;
    void *svcmgr = BINDER_SERVICE_MANAGER;

    bs = binder_open(128*1024);

    if (binder_become_context_manager(bs)) {
        LOGE("cannot become context manager (%s)\n", strerror(errno));
        return -1;
    }

    svcmgr_handle = svcmgr;
    binder_loop(bs, svcmgr_handler);
    return 0;
}

可以看到，启动时此进程将首先通过 binder_open与binder driver通信，把所有信息（binder驱动句柄、mmap信息）都保存在 binder_state这个结构体中：

struct binder_state
{
    int fd;
    void *mapped;
    unsigned mapsize;
};

struct binder_state *binder_open(unsigned mapsize)
{
    struct binder_state *bs;
    bs = malloc(sizeof(*bs));
    bs->fd = open("/dev/binder", O_RDWR);
    bs->mapsize = mapsize;
    bs->mapped = mmap(NULL, mapsize, PROT_READ, MAP_PRIVATE, bs->fd, 0);
    return bs;
}

接下来，是将自己注册成service manager:

int binder_become_context_manager(struct binder_state *bs)
{
    return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);
}

然后是进入 binder_loop这个循环中，开始提供服务注册/获取等相关服务。

void binder_loop(struct binder_state *bs, binder_handler func)
{
    int res;
    struct binder_write_read bwr;
    unsigned readbuf[32];

    bwr.write_size = 0;
    bwr.write_consumed = 0;
    bwr.write_buffer = 0;
    
    readbuf[0] = BC_ENTER_LOOPER;
    binder_write(bs, readbuf, sizeof(unsigned));

    for (;;) {
        bwr.read_size = sizeof(readbuf);
        bwr.read_consumed = 0;
        bwr.read_buffer = (unsigned) readbuf;

        res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);

        if (res < 0) {
            LOGE("binder_loop: ioctl failed (%s)\n", strerror(errno));
            break;
        }

        res = binder_parse(bs, 0, readbuf, bwr.read_consumed, func);
        if (res == 0) {
            LOGE("binder_loop: unexpected reply?!\n");
            break;
        }
        if (res < 0) {
            LOGE("binder_loop: io error %d %s\n", res, strerror(errno));
            break;
        }
    }
}

可以看到，在进入for循环后，这里只是不停地从 binder driver中读取其他程序给自己的数据，然后通过 binder_parse进行处理。

我们知道，所有的binder数据传输都是从客户端由 BC_TRANSACTION发起，到本地端对应的接收到 BR_TRANSACTION，这里我们就只看　BR_TRANSACTION这个最重要的case：

int binder_parse(struct binder_state *bs, struct binder_io *bio,
                 uint32_t *ptr, uint32_t size, binder_handler func)
{
    int r = 1;
    uint32_t *end = ptr + (size / 4);

    while (ptr < end) {
        uint32_t cmd = *ptr++;

        switch(cmd) {
        case BR_TRANSACTION: {
            struct binder_txn *txn = (void *) ptr;
            if (func) {
                unsigned rdata[256/4];
                struct binder_io msg;
                struct binder_io reply;
                int res;

                bio_init(&reply, rdata, sizeof(rdata), 4);
                bio_init_from_txn(&msg, txn);
                res = func(bs, txn, &msg, &reply);
                binder_send_reply(bs, &reply, txn->data, res);
            }
            ptr += sizeof(*txn) / sizeof(uint32_t);
            break;
        }
        }
    }

    return r;
}

这里首先从 ptr中取出 cmd（比如这里就是 BR_TRANSACTION）,然后将剩余的数据转化为 binder_txn；

而通过学习 binder driver可以知道，binder数据的传输结构为 binder_write_read，其中 write_buffer和 read_buffer成员指向的数据正是 cmd + binder_transaction_data，那这里 binder_transaction_data可以直接转换成 binder_txn吗？

我们来对比一下这两者的定义：

struct binder_transaction_data {
	union {
		__u32	handle;	/* target descriptor of command transaction */
		binder_uintptr_t ptr;	/* target descriptor of return transaction */
	} target;
	binder_uintptr_t	cookie;	/* target object cookie */
	__u32		code;		/* transaction command */

	__u32	        flags;
	pid_t		sender_pid;
	uid_t		sender_euid;
	binder_size_t	data_size;	/* number of bytes of data */
	binder_size_t	offsets_size;	/* number of bytes of offsets */
	union {
		struct {
			binder_uintptr_t	buffer;
			binder_uintptr_t	offsets;
		} ptr;
		__u8	buf[8];
	} data;
};

struct binder_txn
{
    void *target;
    void *cookie;
    uint32_t code;
    uint32_t flags;

    uint32_t sender_pid;
    uint32_t sender_euid;

    uint32_t data_size;
    uint32_t offs_size;
    void *data;
    void *offs;
};