从Tutorial中学习基本概念后,进入实战阶段,通过学习,我们给出连个小例子,同步例子和异步例子。
类型和GType的映射
在D-Bus编程中,基础类型和GType的映射表格如下。在后面的程序小例子中我们会看到具体如何对应。
D-Bus basic type GType Free function Notes
BYTE G_TYPE_BOOLEAN INT16 G_TYPE_INT Will be changed to a G_TYPE_INT16 once GLib has it UINT16 G_TYPE_UINT Will be changed to a G_TYPE_UINT16 once GLib has it INT32 G_TYPE_INT Will be changed to a G_TYPE_INT32 once GLib has it UINT32 G_TYPE_UINT Will be changed to a G_TYPE_UINT32 once GLib has it INT64 G_TYPE_GINT64 UINT64 G_TYPE_GUINT64 DOUBLE G_TYPE_DOUBLE STRING G_TYPE_STRING g_free OBJECT_PATH DBUS_TYPE_G_PROXY g_object_unref The returned proxy does not have an interface set; use dbus_g_proxy_set_interface to invoke methods 在D-Bus编程中,container类型和GType的映射表格如下:
D-Bus type signature Description GType C typedef Free function Notes
as Array of strings G_TYPE_STRV char ** g_strfreev v Generic value container G_TYPE_VALUE GValue * g_value_unset The calling conventions for values expect that method callers have allocated return values; see below. a{ss} Dictionary mapping strings to strings DBUS_TYPE_G_STRING_ STRING_HASHTABLE GHashTable * g_hash_table_destroy Makefile
在D-Bus中,使用了dbus-1 dbus-glib-1 glib-2.0(以Moblin 2.1为例)。Makefile的例子如下:
CC = gcc
CFLAGS += -Wall -g `pkg-config dbus-1 dbus-glib-1 glib-2.0 --cflags` LIBS += -Wall -g `pkg-config dbus-1 dbus-glib-1 glib-2.0 --libs` TARGET = sample
OBJ = $(TARGET).o all: $(TARGET)
%o: %c
$(CC) $(CFLAGS) -c $< -o $@ $(TARGET): $(OBJ)
$(CC) $(LIBS) -o $(TARGET) $(OBJ) 同步的例子
同步即程序发出method call消息,等待method_return消息。下面是一个小例子,如果我们用dbus-send命令,可以使用:
dbus-send --session --print-reply --type=method_call --dest=org.freedesktop.Notifications / org.freedesktop.DBus.Introspectable.Introspect
在上一次学习中学习过。它的返回GType类型为G_TYPE_STRING,在C程序中对应char *。
#include <stdio.h>
#include <stdlib.h> #include <dbus/dbus-glib.h> int main( int argc , char ** argv)
{ GError * error; DBusGConnection * connection; DBusGProxy * proxy; char * string; /* GType初始化 */
g_type_init(); error = NULL; /* dbus_g_bus_get用来建立连接,这里和session bus连接,也可以通过DBUS_BUS_SYSTEM与系统总线连接*/
connection = dbus_g_bus_get (DBUS_BUS_SESSION, & error); if(connection == NULL){ g_printerr ("Failed to open connection to bus : %s\n",error->message); g_error_free( error); exit( 1 ); } /* Create a proxy object用来代表远端org.freedesktop.Notifications是系统带有的,可以使用DBUS_INTERFACE_INTROSPECTABLE等定义来标识它*/
proxy = dbus_g_proxy_new_for_name (connection, "org.freedesktop.Notifications" /* service */ , "/" /* path */ , "org.freedesktop.DBus.Introspectable" /* interface,可以使用宏定义DBUS_INTERFACE_INTROSPECTABLE */ ); error = NULL;
/* 采用同步方式,第二个参数是method,第三个参数是error,后面是不定长的参数,显示输入,后为输出,以G_TYPE_INVALID标识接收,每个参数由GType类型和存贮参数值的地址构成。在这个例子中,输入没有参数,输入有一个参数,是string*/
if( !dbus_g_proxy_call (proxy,"Introspect",&error, G_TYPE_INVALID,G_TYPE_STRING, &string, G_TYPE_INVALID) ){ if(error->domain == DBUS_GERROR && error->code == DBUS_GERROR_REMOTE_EXCEPTION) g_printerr("Caught remote method exception %s:%s",dbus_g_error_get_name(error),error->message); else g_printerr("Error : %s\n", error->message); g_error_free(error); exit (1); } g_print("Message Method return from bus:\n%s\n",string);
g_free(string);
g_object_unref(proxy); return 0; } 异步的例子
异步中,程序将不等返回消息,继续执行,等有返回消息的时候,触发一个回调函数 。下面是同样的操作,但是用异步的方式来实现:
/* Test for invoke method asynchronously */
#include <stdio.h> #include <stdlib.h> #include <dbus/dbus-glib.h> static GMainLoop * main_loop;
/* 下面是回调函数,收到method return消息触发函数 */
static void my_callback_func (DBusGProxy *proxy, DBusGProxyCall *call_id, void *user_data)
{ GError * error = NULL; gchar * string = NULL; /* 结束一个消息的收发,处理收到的消息,获取返回值或者error信息 */ dbus_g_proxy_end_call (proxy,call_id,&error,G_TYPE_STRING,&string,G_TYPE_INVALID); if(error != NULL){ g_print("Error in method call : %s\n", error->message); g_error_free(error); }else{ g_print("SUCCESS,it is now %s\n",string); } g_main_loop_quit(main_loop); } int main( int argc , char ** argv)
{ GError * error = NULL; DBusGConnection * connection; DBusGProxy * proxy; g_type_init(); main_loop = g_main_loop_new(NULL,TRUE); connection = dbus_g_bus_get (DBUS_BUS_SESSION, & error);
if(connection == NULL){ g_printerr ("Failed to open connection to bus : %s\n", error->message); g_error_free( error); exit( 1 ); } /* Create a proxy object for the 'bus driver' named org.freedesktop.DBus */
proxy = dbus_g_proxy_new_for_name (connection, "org.freedesktop.Notifications", "/", DBUS_INTERFACE_INTROSPECTABLE); /* 异步触发,也可以带上一个超时的时间限制,使用dbus_g_proxy_call_with_timeout 。这里的参数只需带上输入的情况。第四个参数为携带到回调函数的user_data,第五个参数标识释放user_data的函数,例如g_free等*/ dbus_g_proxy_begin_call (proxy,"Introspect",my_callback_func, NULL,NULL, G_TYPE_INVALID); g_main_loop_run(main_loop); return 0; } 在上面的例子中,使用了GMainLoop的方式,对于线程D-Bus还提供了dbus_g_thread_init 来处理。
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