本节介绍下pkms与installd之间的联系以及installd服务的具体实现：

通过上图可以看出，他们之前的桥梁是installer，下面是installer主要的代码结构：

注意两点：
1、函数接口（左）：

2、LocalSocket通信（右）：

而服务installd在系统启动的时候，通过解析脚本文件init.rc就完成启动了：

installd的代码路径以及结构如下：

下面介绍下这两个主要文件具体代码，主要函数及流程如下：
1、installd.c

struct cmdinfo {
    const char *name;
    unsigned numargs;
    int (*func)(char **arg, char reply[REPLY_MAX]);
};

struct cmdinfo cmds[] = {
    { "ping",                 0, do_ping },
    { "install",              3, do_install },
    { "dexopt",               3, do_dexopt },
    { "movedex",              2, do_move_dex },
    { "rmdex",                1, do_rm_dex },
    { "remove",               2, do_remove },
    { "rename",               2, do_rename },
    { "fixuid",               3, do_fixuid },
    { "freecache",            1, do_free_cache },
    { "rmcache",              2, do_rm_cache },
    { "getsize",              5, do_get_size },
    { "rmuserdata",           2, do_rm_user_data },
    { "movefiles",            0, do_movefiles },
    { "linklib",              3, do_linklib },
    { "mkuserdata",           3, do_mk_user_data },
    { "rmuser",               1, do_rm_user },
    { "cloneuserdata",        3, do_clone_user_data },
};

/* Tokenize the command buffer, locate a matching command, * ensure that the required number of arguments are provided, * call the function(), return the result. */
static int execute(int s, char cmd[BUFFER_MAX])
{
    char reply[REPLY_MAX];
    char *arg[TOKEN_MAX+1];
    unsigned i;
    unsigned n = 0;
    unsigned short count;
    int ret = -1;

    // ALOGI("execute('%s')\n", cmd);

    /* default reply is "" */
    reply[0] = 0;

    /* n is number of args (not counting arg[0]) */
    // arg[0]为命令名称，命令格式：[name arg1 arg2 arg3 arg4]
    arg[0] = cmd;
    // 计算命令参数个数
    while (*cmd) {
        if (isspace(*cmd)) {
            *cmd++ = 0;
            n++;
            arg[n] = cmd;
            if (n == TOKEN_MAX) {
                ALOGE("too many arguments\n");
                goto done;
            }
        }
        cmd++;
    }

    // 根据命令名称匹配命令数组cmds中的命令
    for (i = 0; i < sizeof(cmds) / sizeof(cmds[0]); i++) {
        // 命令名称比较
        if (!strcmp(cmds[i].name,arg[0])) {
            // 判断该命令的参数个数是否满足要求
            if (n != cmds[i].numargs) {
                ALOGE("%s requires %d arguments (%d given)\n",
                     cmds[i].name, cmds[i].numargs, n);
            } else {
                // 调用命令执行函数，执行命令
                ret = cmds[i].func(arg + 1, reply);
            }
            goto done;
        }
    }
    ALOGE("unsupported command '%s'\n", arg[0]);

done:
    // 格式化返回结果
    if (reply[0]) {
        n = snprintf(cmd, BUFFER_MAX, "%d %s", ret, reply);
    } else {
        n = snprintf(cmd, BUFFER_MAX, "%d", ret);
    }
    if (n > BUFFER_MAX) n = BUFFER_MAX;
    // 返回结果数据长度
    count = n;

    // ALOGI("reply: '%s'\n", cmd);
    // 写结果数据长度
    if (writex(s, &count, sizeof(count))) return -1;
    // 写结果数据
    if (writex(s, cmd, count)) return -1;
    return 0;
}

int initialize_globals() {
    // Get the android data directory.
    // 从环境变量中读取数据存储目录，在Android启动脚本init.rc中配置了ANDROID_DATA
    // 环境变量，export ANDROID_DATA /data ，因此变量android_data_dir=/data/
    if (get_path_from_env(&android_data_dir, "ANDROID_DATA") < 0) {
        return -1;
    }

    // Get the android app directory.
    // 得到应用程序安装目录android_app_dir=/data/app/
    if (copy_and_append(&android_app_dir, &android_data_dir, APP_SUBDIR) < 0) {
        return -1;
    }

    // Get the android protected app directory.
    // 得到应用程序私有目录android_app_private_dir=/data/app-private/
    if (copy_and_append(&android_app_private_dir, &android_data_dir, PRIVATE_APP_SUBDIR) < 0) {
        return -1;
    }

    // Get the android app native library directory.
    if (copy_and_append(&android_app_lib_dir, &android_data_dir, APP_LIB_SUBDIR) < 0) {
        return -1;
    }

    // Get the sd-card ASEC mount point.
    // 从环境变量中取得sd-card ASEC的挂载点，在启动脚本init.rc中也有配置：export ASEC_MOUNTPOINT /mnt/asec
    // 因此android_asec_dir=/mnt/asec/
    if (get_path_from_env(&android_asec_dir, "ASEC_MOUNTPOINT") < 0) {
        return -1;
    }

    // Get the android media directory.
    if (copy_and_append(&android_media_dir, &android_data_dir, MEDIA_SUBDIR) < 0) {
        return -1;
    }

    // Take note of the system and vendor directories.
    android_system_dirs.count = 2;

    // 定义android_system_dirs变量并分配存储空间
    android_system_dirs.dirs = calloc(android_system_dirs.count, sizeof(dir_rec_t));
    if (android_system_dirs.dirs == NULL) {
        ALOGE("Couldn't allocate array for dirs; aborting\n");
        return -1;
    }

    // system
    // 从环境变量中取得android根目录，在启动脚本init.rc中也有配置：export ANDROID_ROOT /system
    // 因此android_system_dirs.dirs[0]=/system/
    if (get_path_from_env(&android_system_dirs.dirs[0], "ANDROID_ROOT") < 0) {
        free_globals();
        return -1;
    }

    // append "app/" to dirs[0]
    char *system_app_path = build_string2(android_system_dirs.dirs[0].path, APP_SUBDIR);
    // android_system_dirs.dirs[0]=/system/app/
    android_system_dirs.dirs[0].path = system_app_path;
    android_system_dirs.dirs[0].len = strlen(system_app_path);

    // vendor
    // TODO replace this with an environment variable (doesn't exist yet)
    android_system_dirs.dirs[1].path = "/vendor/app/";
    android_system_dirs.dirs[1].len = strlen(android_system_dirs.dirs[1].path);

    return 0;
}

int initialize_directories() {
    int res = -1;

    // Read current filesystem layout version to handle upgrade paths
    char version_path[PATH_MAX];
    snprintf(version_path, PATH_MAX, "%s.layout_version", android_data_dir.path);

    int oldVersion;
    if (fs_read_atomic_int(version_path, &oldVersion) == -1) {
        oldVersion = 0;
    }
    int version = oldVersion;

    // /data/user
    char *user_data_dir = build_string2(android_data_dir.path, SECONDARY_USER_PREFIX);
    // /data/data
    char *legacy_data_dir = build_string2(android_data_dir.path, PRIMARY_USER_PREFIX);
    // /data/user/0
    char *primary_data_dir = build_string3(android_data_dir.path, SECONDARY_USER_PREFIX, "0");
    if (!user_data_dir || !legacy_data_dir || !primary_data_dir) {
        goto fail;
    }

    // Make the /data/user directory if necessary
    if (access(user_data_dir, R_OK) < 0) {
        if (mkdir(user_data_dir, 0711) < 0) {
            goto fail;
        }
        // 修改目录权限及所有属性
        if (chown(user_data_dir, AID_SYSTEM, AID_SYSTEM) < 0) {
            goto fail;
        }
        if (chmod(user_data_dir, 0711) < 0) {
            goto fail;
        }
    }
    // Make the /data/user/0 symlink to /data/data if necessary
    if (access(primary_data_dir, R_OK) < 0) {
        if (symlink(legacy_data_dir, primary_data_dir)) {
            goto fail;
        }
    }

    if (version == 0) {
        // Introducing multi-user, so migrate /data/media contents into /data/media/0
        ALOGD("Upgrading /data/media for multi-user");

        // Ensure /data/media
        if (fs_prepare_dir(android_media_dir.path, 0770, AID_MEDIA_RW, AID_MEDIA_RW) == -1) {
            goto fail;
        }

        // /data/media.tmp
        char media_tmp_dir[PATH_MAX];
        snprintf(media_tmp_dir, PATH_MAX, "%smedia.tmp", android_data_dir.path);

        // Only copy when upgrade not already in progress
        if (access(media_tmp_dir, F_OK) == -1) {
            if (rename(android_media_dir.path, media_tmp_dir) == -1) {
                ALOGE("Failed to move legacy media path: %s", strerror(errno));
                goto fail;
            }
        }

        // Create /data/media again
        if (fs_prepare_dir(android_media_dir.path, 0770, AID_MEDIA_RW, AID_MEDIA_RW) == -1) {
            goto fail;
        }

        // /data/media/0
        char owner_media_dir[PATH_MAX];
        snprintf(owner_media_dir, PATH_MAX, "%s0", android_media_dir.path);

        // Move any owner data into place
        if (access(media_tmp_dir, F_OK) == 0) {
            if (rename(media_tmp_dir, owner_media_dir) == -1) {
                ALOGE("Failed to move owner media path: %s", strerror(errno));
                goto fail;
            }
        }

        // Ensure media directories for any existing users
        DIR *dir;
        struct dirent *dirent;
        char user_media_dir[PATH_MAX];

        dir = opendir(user_data_dir);
        if (dir != NULL) {
            while ((dirent = readdir(dir))) {
                if (dirent->d_type == DT_DIR) {
                    const char *name = dirent->d_name;

                    // skip "." and ".."
                    if (name[0] == '.') {
                        if (name[1] == 0) continue;
                        if ((name[1] == '.') && (name[2] == 0)) continue;
                    }

                    // /data/media/<user_id>
                    snprintf(user_media_dir, PATH_MAX, "%s%s", android_media_dir.path, name);
                    if (fs_prepare_dir(user_media_dir, 0770, AID_MEDIA_RW, AID_MEDIA_RW) == -1) {
                        goto fail;
                    }
                }
            }
            closedir(dir);
        }

        version = 1;
    }

    // /data/media/obb
    char media_obb_dir[PATH_MAX];
    snprintf(media_obb_dir, PATH_MAX, "%sobb", android_media_dir.path);

    if (version == 1) {
        // Introducing /data/media/obb for sharing OBB across users; migrate
        // any existing OBB files from owner.
        ALOGD("Upgrading to shared /data/media/obb");

        // /data/media/0/Android/obb
        char owner_obb_path[PATH_MAX];
        snprintf(owner_obb_path, PATH_MAX, "%s0/Android/obb", android_media_dir.path);

        // Only move if target doesn't already exist
        if (access(media_obb_dir, F_OK) != 0 && access(owner_obb_path, F_OK) == 0) {
            if (rename(owner_obb_path, media_obb_dir) == -1) {
                ALOGE("Failed to move OBB from owner: %s", strerror(errno));
                goto fail;
            }
        }

        version = 2;
    }

    if (ensure_media_user_dirs(0) == -1) {
        ALOGE("Failed to setup media for user 0");
        goto fail;
    }
    if (fs_prepare_dir(media_obb_dir, 0770, AID_MEDIA_RW, AID_MEDIA_RW) == -1) {
        goto fail;
    }

    // Persist layout version if changed
    if (version != oldVersion) {
        if (fs_write_atomic_int(version_path, version) == -1) {
            ALOGE("Failed to save version to %s: %s", version_path, strerror(errno));
            goto fail;
        }
    }

    // Success!
    res = 0;

fail:
    free(user_data_dir);
    free(legacy_data_dir);
    free(primary_data_dir);
    return res;
}

int main(const int argc, const char *argv[]) {
    char buf[BUFFER_MAX];
    struct sockaddr addr;
    socklen_t alen;
    int lsocket, s, count;

    ALOGI("installd firing up\n");

    //初始化一些全局变量
    if (initialize_globals() < 0) {
        ALOGE("Could not initialize globals; exiting.\n");
        exit(1);
    }

    //初始化安装目录
    if (initialize_directories() < 0) {
        ALOGE("Could not create directories; exiting.\n");
        exit(1);
    }

    drop_privileges();

    //取得installd套接字的句柄，系统中的所有socket以ANDROID_SOCKET_[name]为键，
    //socket句柄为值的方式保存在//环境变量中
    lsocket = android_get_control_socket(SOCKET_PATH);
    if (lsocket < 0) {
        ALOGE("Failed to get socket from environment: %s\n", strerror(errno));
        exit(1);
    }
    if (listen(lsocket, 5)) {
        ALOGE("Listen on socket failed: %s\n", strerror(errno));
        exit(1);
    }
    //修改该socket的属性
    fcntl(lsocket, F_SETFD, FD_CLOEXEC);

    for (;;) {
        alen = sizeof(addr);
        //循环等待接收客户端的请求
        s = accept(lsocket, &addr, &alen);
        if (s < 0) {
            ALOGE("Accept failed: %s\n", strerror(errno));
            continue;
        }
        //接收到客户端的请求后，修改客户端请求socket属性
        fcntl(s, F_SETFD, FD_CLOEXEC);

        ALOGI("new connection\n");
        //循环读取客户端socket中的内容，直到读取内容为空为止
        //客户端发送的数据格式：| 数据长度 | 数据内容 |
        for (;;) {
            unsigned short count;

            //读取数据长度，读取成功返回0，反之返回-1
            if (readx(s, &count, sizeof(count))) {
                ALOGE("failed to read size\n");
                break;
            }
            //如果读取成功，但是读取的数据长度超出1024字节，同样停止读取
            if ((count < 1) || (count >= BUFFER_MAX)) {
                ALOGE("invalid size %d\n", count);
                break;
            }
            //读取数据内容，读取成功返回0，反之返回-1
            if (readx(s, buf, count)) {
                ALOGE("failed to read command\n");
                break;
            }
            buf[count] = 0;
            //执行客户端发送过来的命令
            if (execute(s, buf)) break;
        }
        ALOGI("closing connection\n");
        //执行完客户端的请求后，关闭该socket连接，继续进入接收请求模式
        close(s);
    }

    return 0;
}

2、commands.c
//主要包括安装，卸载，apk优化

int install(const char *pkgname, uid_t uid, gid_t gid)
{
    char pkgdir[PKG_PATH_MAX];
    char libsymlink[PKG_PATH_MAX];
    char applibdir[PKG_PATH_MAX];
    struct stat libStat;

    // 权限判断
    if ((uid < AID_SYSTEM) || (gid < AID_SYSTEM)) {
        ALOGE("invalid uid/gid: %d %d\n", uid, gid);
        return -1;
    }

    // 组合应用程序安装目录pkgdir=/data/data/应用程序包名
    if (create_pkg_path(pkgdir, pkgname, PKG_DIR_POSTFIX, 0)) {
        ALOGE("cannot create package path\n");
        return -1;
    }

    // 组合应用程序库目录libdir=/data/data/应用程序包名/lib
    if (create_pkg_path(libsymlink, pkgname, PKG_LIB_POSTFIX, 0)) {
        ALOGE("cannot create package lib symlink origin path\n");
        return -1;
    }

    if (create_pkg_path_in_dir(applibdir, &android_app_lib_dir, pkgname, PKG_DIR_POSTFIX)) {
        ALOGE("cannot create package lib symlink dest path\n");
        return -1;
    }

    // 创建目录pkgdir=/data/data/应用程序包名
    if (mkdir(pkgdir, 0751) < 0) {
        ALOGE("cannot create dir '%s': %s\n", pkgdir, strerror(errno));
        return -1;
    }
    // 修改/data/data/应用程序包名目录的权限
    if (chmod(pkgdir, 0751) < 0) {
        ALOGE("cannot chmod dir '%s': %s\n", pkgdir, strerror(errno));
        unlink(pkgdir);
        return -1;
    }

    if (lstat(libsymlink, &libStat) < 0) {
        if (errno != ENOENT) {
            ALOGE("couldn't stat lib dir: %s\n", strerror(errno));
            return -1;
        }
    } else {
        if (S_ISDIR(libStat.st_mode)) {
            if (delete_dir_contents(libsymlink, 1, 0) < 0) {
                ALOGE("couldn't delete lib directory during install for: %s", libsymlink);
                return -1;
            }
        } else if (S_ISLNK(libStat.st_mode)) {
            if (unlink(libsymlink) < 0) {
                ALOGE("couldn't unlink lib directory during install for: %s", libsymlink);
                return -1;
            }
        }
    }

    if (symlink(applibdir, libsymlink) < 0) {
        ALOGE("couldn't symlink directory '%s' -> '%s': %s\n", libsymlink, applibdir,
                strerror(errno));
        unlink(pkgdir);
        return -1;
    }

#ifdef HAVE_SELINUX
    if (selinux_android_setfilecon(pkgdir, pkgname, uid) < 0) {
        ALOGE("cannot setfilecon dir '%s': %s\n", pkgdir, strerror(errno));
        unlink(libsymlink);
        unlink(pkgdir);
        return -1;
    }
#endif

    if (chown(pkgdir, uid, gid) < 0) {
        ALOGE("cannot chown dir '%s': %s\n", pkgdir, strerror(errno));
        unlink(libsymlink);
        unlink(pkgdir);
        return -1;
    }

    return 0;
}

int uninstall(const char *pkgname, uid_t persona)
{
    char pkgdir[PKG_PATH_MAX];

    // 根据包名得到应用程序安装目录路径
    if (create_pkg_path(pkgdir, pkgname, PKG_DIR_POSTFIX, persona))
        return -1;

    /* delete contents AND directory, no exceptions */
    // 删除安装目录及目录下的文件
    return delete_dir_contents(pkgdir, 1, NULL);
}

static void run_dexopt(int zip_fd, int odex_fd, const char* input_file_name,
    const char* dexopt_flags)
{
    static const char* DEX_OPT_BIN = "/system/bin/dexopt";
    static const int MAX_INT_LEN = 12;      // '-'+10dig+'\0' -OR- 0x+8dig
    char zip_num[MAX_INT_LEN];
    char odex_num[MAX_INT_LEN];

    sprintf(zip_num, "%d", zip_fd);
    sprintf(odex_num, "%d", odex_fd);

    // 调用/system/bin/dexopt工具来优化apk文件
    execl(DEX_OPT_BIN, DEX_OPT_BIN, "--zip", zip_num, odex_num, input_file_name,
        dexopt_flags, (char*) NULL);
    ALOGE("execl(%s) failed: %s\n", DEX_OPT_BIN, strerror(errno));
}