libdbus上手教程

网站：https://www.freedesktop.org/wiki/Software/dbus/ 代码：https://github.com/freedesktop/dbus

What is D-Bus?

D-Bus is a message bus system, a simple way for applications to talk to one another. In addition to interprocess communication, D-Bus helps coordinate process lifecycle; it makes it simple and reliable to code a "single instance" application or daemon, and to launch applications and daemons on demand when their services are needed.

概括：

一个消息总线系统
- 一对多(signal)：类似于广播/订阅，可以携带消息数据，没有应答确认
- 一对一(method call)：有应答确认
其他功能
- 启动其它进程
- 支持单个实例进程
- 在 app 退出的时候,通知程序

系统 bus 和会话 bus

system bus：在引导时就会启动，应用场景（待定） session bus：在用户登录后启动，属于那个用户私有。它是用户的应用程序用来通信的一个会话总线。

Low-level API vs high-level binding APIs

low-level API：libdbus high-level binding APIs：GLib, Qt, Python, Mono, Java, or whatever.

区别：

low-level：依赖少，用起来更加麻烦/复杂,定制程度可以更高,更加适合自己封装,改造
high-level：用起来更加方便

Version numbers

软件版本格式：major.minor.patch

minor version 是偶数的是 stable 版本,奇数的是 dev 版本比如, development snapshots： 1.1.1, 1.1.2, 1.1.3, 1.3.4. Stable versions： 1.0, 1.0.1, 1.0.2, 1.2.1, 1.2.3

D-Bus 中的概念

通过一个一对一通信流程来解释：

启动 bus daemon, 生成 bus address(unix:path=/var/run/dbus/system_bus_socket)
获取 bus daemon 的 connect 连接, 注册 bus name(com.mycompany.TextEditor)
创建 msg, 发给哪个 bus name(进程)->obj(实例)->Interface->Method
发送 msg 到 bus daemon
bus daemon 发送到相应的 bus name 进程
接收到消息后返回数据
...

相应概念：

Address(bus) –> Bus Name(进程) –> Object path(实例) –> Interface –> Method

A...	is identified by a(n)...	which looks like...	and is chosen by...
Bus	address	`unix:path=/var/run/dbus/system_bus_socket`	system configuration
Connection	bus name	`:34-907` (unique) or `com.mycompany.TextEditor` (well-known)	D-Bus (unique) or the owning program (well-known)
Object	path	`/com/mycompany/TextFileManager`	the owning program
Interface	interface name	`org.freedesktop.Hal.Manager`	the owning program
Member	member name	`ListNames`	the owning program

D-Bus FAQ 4. What is the difference between a bus name, and object path, and an interface?
If you imagine a C++ program that implements a network service, then the bus name is the hostname of the computer running this C++ program, the object path is a C++ object instance pointer, and an interface is a C++ class (a pure virtual or abstract class, to be exact). In Java terms, the object path is an object reference, and an interface is a Java interface. People get confused because if they write an application with a single object instance and a single interface, then the bus name, object path, and interface look redundant. For example, you might have a text editor that uses the bus name org.freedesktop.TextEditor, has a global singleton object called /org/freedesktop/TextEditor, and that singleton object could implement the interface org.freedesktop.TextEditor. However, a text editor application could as easily own multiple bus names (for example, org.kde.KWrite in addition to generic TextEditor), have multiple objects (maybe /org/kde/documents/4352 where the number changes according to the document), and each object could implement multiple interfaces, such as org.freedesktop.DBus.Introspectable, org.freedesktop.BasicTextField, org.kde.RichTextDocument.

D-Bus 消息类型

消息通过 D-Bus 在进程间传递。有四类消息：

Method call 消息：将触发对象的一个 method
Method return 消息：触发的方法返回的结果
Signal 消息：通知，可以看作为事件消息。
Error 消息：触发的方法返回一个异常

相应工具

Command-line 命令行工具

dbus-daemon：Message bus daemon
dbus-launch：从 shell 脚本来启动 message bus 工具
dbus-send：发送消息到 message bus
dbus-monitor：debug probe to print message bus messages
dbus-uuidgen：生成 UUIDs 工具

使用示例：

bash

## launch message bus
$ dbus-launch --sh-syntax

## dbus-send
# send signal
$ dbus-send --type=signal /  org.signal.poweroff.test_signal string:"hello"
# send message_call
$ dbus-send --dest=org.freedesktop.ExampleName               \
            /org/freedesktop/sample/object/name              \
            org.freedesktop.ExampleInterface.ExampleMethod   \
            int32:47 string:'hello world' double:65.32       \
            array:string:"1st item","next item","last item"  \
            dict:string:int32:"one",1,"two",2,"three",3      \
            variant:int32:-8                                 \
            objpath:/org/freedesktop/sample/object/name

## dbus-uuidgen
# it just prints a new uuid
$ dbus-uuidgen
ebb40ad49f9d4d8f44faa3bc5fffdc27

# prints the machine UUID by default
$ dbus-uuidgen --get
5b86dc18cce752c5404d2f0a5ffd1885

$ dbus-uuidgen --ensure=/usr/local/var/lib/dbus/machine-id

dbus-daemon 启动脚本：

bash

#!/bin/bash
## test for an existing bus daemon, just to be safe
if test -z "$DBUS_SESSION_BUS_ADDRESS"; then
    ## if not found, launch a new one
    eval $(dbus-launch --sh-syntax --exit-with-session)
    echo "D-Bus per-session daemon address is: $DBUS_SESSION_BUS_ADDRESS"
fi

Graphical 图形调试工具

D-Feet

Bustle

libdbus 使用实例：

Method Call 消息例子：

场景：launcher 访问 db_server 获取设备信息，比如：获取 cpu_id。launcher 作为客户端，db_server 做为服务端

launcher.c 代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "dbus/dbus.h"

#define DBUS_BUS_NAME_LAUNCHER "com.vdiscovery.launcher"

// bus_name
#define DBUS_BUS_NAME_DB_SERVER "com.vdiscovery.db_server"

// object
#define DBUS_OJBECT_DB_SERVER_DEVICE_INFO "/com/vdiscovery/db_server/device_info"
// iface
#define DBUS_IFACE_DB_SERVER_DEVICE_INFO "com.vdiscovery.db_server.device_info"

// method
#define DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_GET_IMEI "get_imei"
#define DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_GET_CPU_ID "get_cpu_id"
#define DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_SET_IMEI "set_imei"
#define DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_SET_CPU_ID "set_cpu_id"

int main(int argc, char* argv[]) {
    dbus_bool_t     ret = 0;
    DBusError       err;
    DBusConnection* conn = NULL;
    dbus_error_init(&err);

    // 1.获取链接
    conn = dbus_bus_get(DBUS_BUS_SESSION, &err);
    if (dbus_error_is_set(&err)) {
        printf("bus get err : %s\n", err.message);
        return -1;
    }

    // 2.注册自己的name
    dbus_bus_request_name(conn, DBUS_BUS_NAME_LAUNCHER, DBUS_NAME_FLAG_REPLACE_EXISTING, &err);
    if (dbus_error_is_set(&err)) {
        printf("bus request name err : %s\n", err.message);
        return -1;
    }

    // 3.创建消息
    DBusMessage* req = dbus_message_new_method_call(
        DBUS_BUS_NAME_DB_SERVER,
        DBUS_OJBECT_DB_SERVER_DEVICE_INFO,
        DBUS_IFACE_DB_SERVER_DEVICE_INFO,
        DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_SET_CPU_ID);

    // 3.1.添加携带信息
    char* cpu_id = "123456";
    dbus_message_append_args(req, DBUS_TYPE_STRING, &cpu_id, DBUS_TYPE_INVALID);

    // 4.获取回应消息
    DBusMessage* reply     = dbus_connection_send_with_reply_and_block(conn, req, 1000 * 4, &err);
    char*        reply_str = NULL;
    dbus_message_get_args(reply, &err, DBUS_TYPE_STRING, &reply_str, DBUS_TYPE_INVALID);
    printf("set_cpu_id:%s, reply:reply_str = %s\n", cpu_id, reply_str);

    return 0;
}

db_server.c 代码：

#include <stdio.h>
#include <unistd.h>
#include <stdbool.h>
#include <string.h>
#include "dbus/dbus.h"

// 应用唯一表示：name
#define DBUS_BUS_NAME_DB_SERVER "com.vdiscovery.db_server"

// device_info
#define DBUS_OJBECT_DB_SERVER_DEVICE_INFO "/com/vdiscovery/db_server/device_info"
#define DBUS_IFACE_DB_SERVER_DEVICE_INFO "com.vdiscovery.db_server.device_info"

#define DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_GET_IMEI "get_imei"
#define DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_GET_CPU_ID "get_cpu_id"
#define DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_SET_IMEI "set_imei"
#define DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_SET_CPU_ID "set_cpu_id"

// kv store
#define DBUS_OJBECT_DB_SERVER_KV_STORE "/com/vdiscovery/db_server/kv_store"
#define DBUS_IFACE_DB_SERVER_KV_STORE "com.vdiscovery.db_server.kv_store"

#define DBUS_METHOD_CALL_DB_SERVER_KV_PUT "put"
#define DBUS_METHOD_CALL_DB_SERVER_KV_GET "get"
#define DBUS_METHOD_CALL_DB_SERVER_KV_DELETE "delete"

// content
#define DBUS_OJBECT_DB_SERVER_CONTENT "/com/vdiscovery/db_server/content"

DBusHandlerResult _get_imei(DBusConnection* connection, DBusMessage* message, void* user_data) {
    // todo: do something: 从数据库取
    DBusMessage* reply    = dbus_message_new_method_return(message);
    char*        imei_str = "imei:fasdfasdflfasdk";
    dbus_message_append_args(reply, DBUS_TYPE_STRING, &imei_str, DBUS_TYPE_INVALID);
    dbus_connection_send(connection, reply, NULL);
    return DBUS_HANDLER_RESULT_HANDLED;
}

DBusHandlerResult _get_cpu_id(DBusConnection* connection, DBusMessage* message, void* user_data) {
    // todo: do something: 从数据库取
    DBusMessage* reply      = dbus_message_new_method_return(message);
    char*        cpu_id_str = "cpu_id:231412341";
    dbus_message_append_args(reply, DBUS_TYPE_STRING, &cpu_id_str, DBUS_TYPE_INVALID);
    dbus_connection_send(connection, reply, NULL);
    return DBUS_HANDLER_RESULT_HANDLED;
}

DBusHandlerResult _set_imei(DBusConnection* connection, DBusMessage* message, void* user_data) {
    DBusError error;
    dbus_error_init(&error);
    char* req_str = NULL;  // json
    dbus_message_get_args(message, &error, DBUS_TYPE_STRING, &req_str, DBUS_TYPE_INVALID);
    printf("req_str: %s\n", req_str);

    // todo: do something with req_str, 存到数据库
    DBusMessage* reply    = dbus_message_new_method_return(message);
    char*        succ_str = "store success";
    dbus_message_append_args(reply, DBUS_TYPE_STRING, &succ_str, DBUS_TYPE_INVALID);
    dbus_connection_send(connection, reply, NULL);
    return DBUS_HANDLER_RESULT_HANDLED;
}

DBusHandlerResult _set_cpu_id(DBusConnection* connection, DBusMessage* message, void* user_data) {
    DBusError error;
    dbus_error_init(&error);
    char* req_str = NULL;  // json
    dbus_message_get_args(message, &error, DBUS_TYPE_STRING, &req_str, DBUS_TYPE_INVALID);
    printf("req_str: %s\n", req_str);

    // todo: do something with req_str, 存到数据库
    DBusMessage* reply    = dbus_message_new_method_return(message);
    char*        succ_str = "store success";
    dbus_message_append_args(reply, DBUS_TYPE_STRING, &succ_str, DBUS_TYPE_INVALID);
    dbus_connection_send(connection, reply, NULL);
    return DBUS_HANDLER_RESULT_HANDLED;
}

DBusHandlerResult device_info_handler(DBusConnection* connection, DBusMessage* message, void* user_data) {
    if (dbus_message_is_method_call(message, DBUS_IFACE_DB_SERVER_DEVICE_INFO, DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_GET_IMEI)) {
        return _get_imei(connection, message, user_data);
    } else if (dbus_message_is_method_call(message, DBUS_IFACE_DB_SERVER_DEVICE_INFO, DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_GET_CPU_ID)) {
        return _get_cpu_id(connection, message, user_data);
    } else if (dbus_message_is_method_call(message, DBUS_IFACE_DB_SERVER_DEVICE_INFO, DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_SET_IMEI)) {
        return _set_imei(connection, message, user_data);
    } else if (dbus_message_is_method_call(message, DBUS_IFACE_DB_SERVER_DEVICE_INFO, DBUS_METHOD_CALL_DB_SERVER_DEVICE_INFO_SET_CPU_ID)) {
        return _set_cpu_id(connection, message, user_data);
    }  // ...
    return DBUS_HANDLER_RESULT_HANDLED;
}

DBusHandlerResult kv_store_handler(DBusConnection* connection, DBusMessage* message, void* user_data) {
    // todo
    return DBUS_HANDLER_RESULT_HANDLED;
}

DBusHandlerResult content_handler(DBusConnection* connection, DBusMessage* message, void* user_data) {
    // todo
    return DBUS_HANDLER_RESULT_HANDLED;
}

DBusObjectPathVTable g_device_info_table = {.message_function = device_info_handler};
DBusObjectPathVTable g_kv_table          = {.message_function = kv_store_handler};
DBusObjectPathVTable g_content_table     = {.message_function = content_handler};

int main(int argc, char* argv[]) {
    dbus_bool_t     ret = false;
    DBusError       err;
    DBusConnection* conn = NULL;

    dbus_error_init(&err);

    // 1.获取链接
    conn = dbus_bus_get(DBUS_BUS_SESSION, &err);
    if (dbus_error_is_set(&err)) {
        printf("bus get err : %s\n", err.message);
        return -1;
    }

    // 2.注册自己的name
    dbus_bus_request_name(conn, DBUS_BUS_NAME_DB_SERVER, DBUS_NAME_FLAG_REPLACE_EXISTING, &err);
    if (dbus_error_is_set(&err)) {
        printf("bus request name err : %s\n", err.message);
        return -1;
    }

    // 3.注册自己的Object
    ret = dbus_connection_register_object_path(conn, DBUS_OJBECT_DB_SERVER_DEVICE_INFO, &g_device_info_table, NULL);
    ret = dbus_connection_register_object_path(conn, DBUS_OJBECT_DB_SERVER_KV_STORE, &g_kv_table, NULL);
    ret = dbus_connection_register_object_path(conn, DBUS_OJBECT_DB_SERVER_CONTENT, &g_content_table, NULL);
    if (!ret) {
        printf("Failed to register a object path for 'TestObject'\n");
        return -1;
    }

    // 4.read/write/despatch消息
    while (true) {
        dbus_connection_read_write_dispatch(conn, 0);
    }
    return 0;
}

Signal 消息例子：

todo：

配置文件：

启动其它服务：

其它：

我当时是怎么学习 dbus 的？

核心结构体DBusConnection和DBusMessage

D-Bus FAQ

Dbus 是如果连接上 dbus-daemon，调用`dbus_bus_get(DBUS_BUS_SESSION, &err);`就 get 到了？

换一种方式：如果把参数传给程序

参数写死
main 参数argv
特定环境变量（程序自定义）
配置文件（配置非常多）

DbusMessage：

References：

网站：https://www.freedesktop.org/wiki/Software/dbus/
dbus-specification 规范：https://dbus.freedesktop.org/doc/dbus-specification.html
D-Bus FAQ：https://dbus.freedesktop.org/doc/dbus-faq.html
README：https://dbus.freedesktop.org/doc/README
相关中文介绍文章：https://thebigdoc.readthedocs.io/en/latest/dbus/dbus.html

libdbus上手教程 ​

What is D-Bus? ​

概括： ​

系统 bus 和会话 bus ​

Low-level API vs high-level binding APIs ​

Version numbers ​

D-Bus 中的概念 ​

D-Bus FAQ 4. What is the difference between a bus name, and object path, and an interface? ​

D-Bus 消息类型 ​

相应工具 ​

Command-line 命令行工具 ​

Graphical 图形调试工具 ​

libdbus 使用实例： ​

Method Call 消息例子： ​

Signal 消息例子： ​

配置文件： ​

启动其它服务： ​

其它： ​

我当时是怎么学习 dbus 的？ ​

Dbus 是如果连接上 dbus-daemon，调用dbus_bus_get(DBUS_BUS_SESSION, &err);就 get 到了？ ​

DbusMessage： ​

References： ​