c - 如何获得g_dbus_connection_signal_subscribe函数来告诉我有关预先存在的对象/接口(interface)的信息？

函数g_dbus_connection_signal_subscribe可以很好地告诉我新的dbus对象何时出现（或消失），以及接口添加的信号（或接口删除的信号）。但我需要了解预先存在的对象/接口。
我编写了下面的C代码，以便在从总线添加/删除DBus对象时提供回调。为了简单起见，省略了错误检查。

#include <stdio.h>
#include <stdlib.h>
#include <gio/gio.h>

static void signal_cb(GDBusConnection *connection,
        const gchar *sender_name,const gchar *object_path,
        const gchar *interface_name,const gchar *signal_name,
        GVariant *parameters,gpointer user_data)
{
    printf("%s: %s.%s %s\n",object_path,interface_name,signal_name,
        g_variant_print(parameters,TRUE));
}

int main(int argc,char *argv[])
{
GDBusConnection *c;
GMainLoop *loop;
int filter_id;

    c = g_bus_get_sync(G_BUS_TYPE_SYSTEM,NULL,&err);
    loop = g_main_loop_new(NULL,0);
    filter_id = g_dbus_connection_signal_subscribe(c,
        "org.bluez",NULL,NULL,NULL,NULL,
        G_DBUS_SIGNAL_FLAGS_NONE,signal_cb,NULL,NULL);
    g_main_loop_run (loop);
    g_main_loop_unref (loop);

    exit(0);
}

所以我要做的是跟踪树上的Org.Brasz分支中存在的所有DBUS对象。（它们表示可插入的蓝牙控制器和每个控制器发现的设备）。我需要知道在我的程序启动之前已经存在的DBus对象，我需要知道在我的程序启动之后出现的新对象。
上面的代码告诉我关于新对象的信息，但是没有关于已经存在的对象的信息。在GDBUS API中有一种方法可以为已经存在的对象获得“接口创建”信号吗？我想可以读取整个DBus对象层次结构，然后订阅更改，但这会导致竞争条件，如果在读取对象层次结构的时间和订阅的时间之间出现一个对象，那么我将错过这些对象。。。。
使用gdbus API实现这一点的最佳实践方法是什么？

最佳答案

如Parthiban所述，如果有人遇到这种情况，解决方案（对于实现org.freedesktop.DBus.ObjectManager接口的D-Bus服务，如BlueZ）是调用GetManagedObjects方法。用GDBus在C中编写代码是非常痛苦的，因为它需要对GVariant类型的详细理解；请参见documentation。还可以查看docsonGVariant数据类型字符串。但这是怎么做到的：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>

#include <gio/gio.h>


/* The GVariant must be of type "a{sa{sv}}" (array of interfaces, where each */
/* interface has an array of properties). */
/* This type what DBus provides with InterfacesAdded signal and is also */
/* part of the return value from the GetManagedObjects method. */
static void proc_interface_var(const gchar *objpath,GVariant *iflist)
{
GVariantIter *iter,*iter2;
gchar *ifname,*propname,*proptext;
GVariant *propvalue;

    g_variant_get(iflist,"a{sa{sv}}",&iter);
    while (g_variant_iter_loop(iter,"{sa{sv}}",&ifname,&iter2)) {
        if (strcmp(ifname,"org.bluez.Adatper1") != 0 &&
                                    strcmp(ifname,"org.bluez.Device1") != 0) {
            /* we only care about the Adatper1 and Device1 interfaces */
            continue;
        } /* if */
        printf("Interface %s added to object %s\n",ifname,objpath);
        while (g_variant_iter_loop(iter2,"{sv}",&propname,&propvalue)) {
            proptext = g_variant_print(propvalue,0);
            printf("\t%s=%s\n",propname,proptext);
            g_free(proptext);
        } /* while */
    } /* while */
    return;
} /* proc_interface_var */


/* The GVariant must be of type "a{sv}" (an array of properties). */
static void proc_property_var(const gchar *objpath,
                                        gchar *ifname,GVariant *proplist)
{
GVariantIter *iter;
gchar *propname,*proptext;
GVariant *propvalue;

    g_variant_get(proplist,"a{sv}",&iter);
    while (g_variant_iter_loop(iter,"{sv}",&propname,&propvalue)) {
        proptext = g_variant_print(propvalue,0);
        printf("\tProperty changed on object %s interface %s: %s=%s\n",
            objpath,ifname,propname,proptext);
        g_free(proptext);
    } /* while */
    return;
} /* proc_property_var */


static void signal_cb(GDBusConnection *c,
            const gchar *sender_name,const gchar *object_path,
            const gchar *interface_name,const gchar *signal_name,
            GVariant *parameters,gpointer user_data)
{
char fullsignal[200];
gchar *s,*objpath,*ifname,*propname,*proptext;
GVariant *ifvar,*propvalue;
GVariantIter *iter,*iter2;

    snprintf(fullsignal,200,"%s.%s",interface_name,signal_name);
    if (strcmp(fullsignal,
                "org.freedesktop.DBus.ObjectManager.InterfacesAdded") == 0) {
        g_variant_get(parameters,"(o*)",&objpath,&ifvar);
        proc_interface_var(objpath,ifvar);
    } else if (strcmp(fullsignal,
                "org.freedesktop.DBus.Properties.PropertiesChanged") == 0) {
        g_variant_get(parameters,"(s*as)",&ifname,&propvalue,&iter2);
        proc_property_var(object_path,ifname,propvalue);
        while (g_variant_iter_loop(iter2,"s",&propname)) {
            printf("\tProperty changed on object %s interface %s: "
                "%s is nil\n",object_path,ifname,propname);
        } /* while */
    } else {
        printf("Ignoring unsupported signal for object %s, "
            "signal=%s.%s, param type=%s\n",
            object_path,interface_name,signal_name,
            g_variant_get_type_string(parameters));
        s = g_variant_print(parameters,TRUE);
        printf("Unsupported signal: parameters %s\n",s);
        g_free(s);
    } /* else */

    return;
} /* signal_cb */


static void bt_discover(GDBusConnection *c,const char *ctlname,int on_off)
{
GError *err=NULL;
GVariant *result;
const char *method;
char ctlpath[80];

    snprintf(ctlpath,80,"/org/bluez/%s",ctlname);
    method = on_off ? "StartDiscovery" : "StopDiscovery";
    result = g_dbus_connection_call_sync(c,"org.bluez",ctlpath,
        "org.bluez.Adapter1",method,NULL,
        G_VARIANT_TYPE("()"),       /* return-type */
        G_DBUS_CALL_FLAGS_NONE,5000,NULL,&err);
    if (result==NULL) {
        if (err) fprintf(stderr,"g_dbus_connection_call error: %s\n",
            err->message);
        exit(1);
    } /* if */
    g_variant_unref(result);
    return;
} /* bt_discover */


static void *receive_dbus_signals(void *arg)
{
GMainLoop *loop;

    printf("Receiving DBus signals...\n");
    loop = g_main_loop_new(NULL,0);
    g_main_loop_run(loop);
    g_main_loop_unref(loop);
    return NULL;
} /* receive_dbus_signals */


int main(int argc,char *argv[])
{
GError *err=NULL;
GVariant *result,*ifvar;
GVariantIter *iter;
GDBusConnection *c;
GDBusNodeInfo *node;
gchar *objpath;
pthread_t handle;
int filter_id;

    if ((c = g_bus_get_sync(G_BUS_TYPE_SYSTEM,NULL,&err)) == NULL) {
        if (err) fprintf(stderr,"g_bus_get error: %s\n",err->message);
        exit(1);
    } /* if */

    filter_id = g_dbus_connection_signal_subscribe(c,
        "org.bluez",NULL,NULL,NULL,NULL,
        G_DBUS_SIGNAL_FLAGS_NONE,signal_cb,NULL,NULL);
    if (pthread_create(&handle,NULL,receive_dbus_signals,NULL) != 0) {
        fprintf(stderr,"Failed to create DBus listen thread\n");
        exit(1);
    } /* if */

    result = g_dbus_connection_call_sync(c,"org.bluez","/",
        "org.freedesktop.DBus.ObjectManager","GetManagedObjects",NULL,
        G_VARIANT_TYPE("(a{oa{sa{sv}}})"),      /* return-type */
        G_DBUS_CALL_FLAGS_NONE,5000,NULL,&err);
    if (result==NULL) {
        if (err) fprintf(stderr,"g_dbus_connection_call error: %s\n",
            err->message);
        exit(1);
    } /* if */

    g_variant_get(result,"(a{oa{sa{sv}}})",&iter);
    /* below we replace 'a{sa{sv}}' with '*' to get it as a GVariant */
    while (g_variant_iter_loop(iter,"{o*}",&objpath,&ifvar)) {
        proc_interface_var(objpath,ifvar);
    } /* while */
    g_variant_unref(result);

    bt_discover(c,"hci0",1);
    sleep(5);
    bt_discover(c,"hci0",0);
    sleep(5);

    exit(0);
}

对于不实现org.freedesktop.DBus.ObjectManager接口的D-总线服务，需要使用D-Bus introspection并解析内省XML来查找现有对象节点的路径。

关于c - 如何获得g_dbus_connection_signal_subscribe函数来告诉我有关预先存在的对象/接口(interface)的信息？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/54131543/