android 進程間通信---Service Manager（1）

Bind機制由4個部分組成。bind驅動，Client，ServiceManager &Service

1.Bind其實是一個基於linux系統的驅動，目的是為了實現內存共享。

 bind驅動的東西，由於偏向內核，並且bind機制的內容非常龐大，所以我們暫時略去這個部分。

2.ServiceManager

Service Manager顧名思義，是一個“管家”。更確切的說，是所有系統service 的manager。

首先從service_manager.c開始\frameworks\native\cmds\servicemanager\service_manager.c

static struct {
    unsigned uid;
    const char *name;
} allowed[] = {
    { AID_MEDIA, "media.audio_flinger" },
    { AID_MEDIA, "media.log" },
    { AID_MEDIA, "media.player" },
    { AID_MEDIA, "media.camera" },
    { AID_MEDIA, "media.audio_policy" },
    { AID_DRM,   "drm.drmManager" },
    { AID_NFC,   "nfc" },
    { AID_BLUETOOTH, "bluetooth" },
    { AID_RADIO, "radio.phone" },
    { AID_RADIO, "radio.sms" },
    { AID_RADIO, "radio.phonesubinfo" },
    { AID_RADIO, "radio.simphonebook" },
/* TODO: remove after phone services are updated: */
    { AID_RADIO, "phone" },
    { AID_RADIO, "sip" },
    { AID_RADIO, "isms" },
    { AID_RADIO, "iphonesubinfo" },
    { AID_RADIO, "simphonebook" },
    { AID_MEDIA, "common_time.clock" },
    { AID_MEDIA, "common_time.config" },
    { AID_KEYSTORE, "android.security.keystore" },
};

以上就是系統服務的一個部分。這些都是注冊在servicemanager來管理。

那service manager干那些事：

I.提供IBind對象，也就是各個service的引用，供每個進程使用，且對於每個進程來說，該Ibind對象是唯一的。

II.讓各個系統service注冊到servicemanager中。

 

 

 

這里binder驅動，不是我們通常操作系統結構里的驅動概念，可以理解為是client和ServiceManager交流的媒介。

binder驅動的本質是內存共享。

其實這是整個bind機制的前面部分，就是從client到servicemanager，這樣client可以拿到Ibind對象，進而可以直接“操作servie”。

舉個例子：

AlarmManager alarmManager = context.getSystemService(Context.ALARM_SERVICE);
        alarmManager.setExact(AlarmManager.ELAPSED_REALTIME, elapsedRealtime,
                pendingIntent);

拿到alaram service bind對象，進而操作service提供的“服務”。

而且這個操作是同步的！

就好象在操作同一個進程的東西。

下面我們看看service Manager究竟是如何做到上面說的幾點的。

2.1 Service Manager的啟動：

既然SM是管理員，那么它應該是最勤快的，也就是必須最“早”啟動。

是的，它的啟動是定義在init.rc里面的：\system\core\rootdir\init.rc

 

# adbd on at boot in emulator
on property:ro.kernel.qemu=1
    start adbd

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

Service Manager啟動后，在干什么？

還是在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)) {
        ALOGE("cannot become context manager (%s)\n", strerror(errno));
        return -1;
    }

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

binder_open打開bind驅動，並且分配128K大小。

binder_become_context_manager(bs)：

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

把自己注冊為Service 大管家。

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) {
            ALOGE("binder_loop: ioctl failed (%s)\n", strerror(errno));
            break;
        }

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

開始進入loop，和之前分析的andorid線程消息驅動機制非常相似。

讀取消息隊列，解析它們，知道出現異常。

接下來，看看bind_parse:

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++;
#if TRACE
        fprintf(stderr,"%s:\n", cmd_name(cmd));
#endif
        switch(cmd) {
        case BR_NOOP:
            break;
        case BR_TRANSACTION_COMPLETE:
            break;
        case BR_INCREFS:
        case BR_ACQUIRE:
        case BR_RELEASE:
        case BR_DECREFS:
#if TRACE
            fprintf(stderr,"  %08x %08x\n", ptr[0], ptr[1]);
#endif
            ptr += 2;
            break;
        case BR_TRANSACTION: {
            struct binder_txn *txn = (void *) ptr;
            if ((end - ptr) * sizeof(uint32_t) < sizeof(struct binder_txn)) {
                ALOGE("parse: txn too small!\n");
                return -1;
            }
            binder_dump_txn(txn);
            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;
        }
        case BR_REPLY: {
            struct binder_txn *txn = (void*) ptr;
            if ((end - ptr) * sizeof(uint32_t) < sizeof(struct binder_txn)) {
                ALOGE("parse: reply too small!\n");
                return -1;
            }
            binder_dump_txn(txn);
            if (bio) {
                bio_init_from_txn(bio, txn);
                bio = 0;
            } else {
                    /* todo FREE BUFFER */
            }
            ptr += (sizeof(*txn) / sizeof(uint32_t));
            r = 0;
            break;
        }
        case BR_DEAD_BINDER: {
            struct binder_death *death = (void*) *ptr++;
            death->func(bs, death->ptr);
            break;
        }
        case BR_FAILED_REPLY:
            r = -1;
            break;
        case BR_DEAD_REPLY:
            r = -1;
            break;
        default:
            ALOGE("parse: OOPS %d\n", cmd);
            return -1;
        }
    }

    return r;
}

 關鍵是分析：BR_TRANSACTION，BR_REPLY。

 BR_TRANSACTION中做了一些初始化，然后

res = func(bs, txn, &msg, &reply);
binder_send_reply(bs, &reply, txn->data, res);

func函數就是在service_manager.c中傳入的

int svcmgr_handler(struct binder_state *bs,
                   struct binder_txn *txn,
                   struct binder_io *msg,
                   struct binder_io *reply)

所以bind_loop最終實現分析的函數是傳入的函數！

至此整個service_manager的流程已經清楚。

事件驅動機制：

1.從bind驅動讀取消息

2.處理消息

3.進入looper，永遠不會主動退出，直到出現致命錯誤。

 

int svcmgr_handler(struct binder_state *bs,
                   struct binder_txn *txn,
                   struct binder_io *msg,
                   struct binder_io *reply)
{
    struct svcinfo *si;
    uint16_t *s;
    unsigned len;
    void *ptr;
    uint32_t strict_policy;
    int allow_isolated;

//    ALOGI("target=%p code=%d pid=%d uid=%d\n",
//         txn->target, txn->code, txn->sender_pid, txn->sender_euid);

    if (txn->target != svcmgr_handle)
        return -1;

    // Equivalent to Parcel::enforceInterface(), reading the RPC
    // header with the strict mode policy mask and the interface name.
    // Note that we ignore the strict_policy and don't propagate it
    // further (since we do no outbound RPCs anyway).
    strict_policy = bio_get_uint32(msg);
    s = bio_get_string16(msg, &len);
    if ((len != (sizeof(svcmgr_id) / 2)) ||
        memcmp(svcmgr_id, s, sizeof(svcmgr_id))) {
        fprintf(stderr,"invalid id %s\n", str8(s));
        return -1;
    }

    switch(txn->code) {
    case SVC_MGR_GET_SERVICE:
    case SVC_MGR_CHECK_SERVICE:
        s = bio_get_string16(msg, &len);
        ptr = do_find_service(bs, s, len, txn->sender_euid);
        if (!ptr)
            break;
        bio_put_ref(reply, ptr);
        return 0;

    case SVC_MGR_ADD_SERVICE:
        s = bio_get_string16(msg, &len);
        ptr = bio_get_ref(msg);
        allow_isolated = bio_get_uint32(msg) ? 1 : 0;
        if (do_add_service(bs, s, len, ptr, txn->sender_euid, allow_isolated))
            return -1;
        break;

    case SVC_MGR_LIST_SERVICES: {
        unsigned n = bio_get_uint32(msg);

        si = svclist;
        while ((n-- > 0) && si)
            si = si->next;
        if (si) {
            bio_put_string16(reply, si->name);
            return 0;
        }
        return -1;
    }
    default:
        ALOGE("unknown code %d\n", txn->code);
        return -1;
    }

    bio_put_uint32(reply, 0);
    return 0;
}

switch語句，查詢和獲取service 或者注冊。

查找svclist里面是否有相同name的服務。

svclist是鏈表的方式，與線程的消息隊列一樣！

struct svcinfo *find_svc(uint16_t *s16, unsigned len)
{
    struct svcinfo *si;

    for (si = svclist; si; si = si->next) {
        if ((len == si->len) &&
            !memcmp(s16, si->name, len * sizeof(uint16_t))) {
            return si;
        }
    }
    return 0;
}

接下來我們看看void *do_find_service(struct binder_state *bs, uint16_t *s, unsigned len, unsigned uid)

return的到底是什么？

注冊服務：SVC_MGR_ADD_SERVICE：

 

int do_add_service(struct binder_state *bs,
                   uint16_t *s, unsigned len,
                   void *ptr, unsigned uid, int allow_isolated)
{
    struct svcinfo *si;
    //ALOGI("add_service('%s',%p,%s) uid=%d\n", str8(s), ptr,
    //        allow_isolated ? "allow_isolated" : "!allow_isolated", uid);

    if (!ptr || (len == 0) || (len > 127))
        return -1;

    if (!svc_can_register(uid, s)) {
        ALOGE("add_service('%s',%p) uid=%d - PERMISSION DENIED\n",
             str8(s), ptr, uid);
        return -1;
    }

    si = find_svc(s, len);
    if (si) {
        if (si->ptr) {
            ALOGE("add_service('%s',%p) uid=%d - ALREADY REGISTERED, OVERRIDE\n",
                 str8(s), ptr, uid);
            svcinfo_death(bs, si);
        }
        si->ptr = ptr;
    } else {
        si = malloc(sizeof(*si) + (len + 1) * sizeof(uint16_t));
        if (!si) {
            ALOGE("add_service('%s',%p) uid=%d - OUT OF MEMORY\n",
                 str8(s), ptr, uid);
            return -1;
        }
        si->ptr = ptr;
        si->len = len;
        memcpy(si->name, s, (len + 1) * sizeof(uint16_t));
        si->name[len] = '\0';
        si->death.func = svcinfo_death;
        si->death.ptr = si;
        si->allow_isolated = allow_isolated;
        si->next = svclist;
        svclist = si;
    }

    binder_acquire(bs, ptr);
    binder_link_to_death(bs, ptr, &si->death);
    return 0;
}

int svc_can_register(unsigned uid, uint16_t *name)

判斷是否在allowed表格里面。

先看看是否在列表里面？

si = find_svc(s, len);

如果不再的話，就注冊一個新的si，到svclist。

至此service_manager就啟動起來了。