DBusWatch and DBusTimeout examples

Question

I need to write an application in C for asynchronous sending and reading messages on the dbus message queue. I\'ve read that for doing that I should use the DBusWatch<

Answer 1

Here's something I wrote some time ago. I removed application specific code, you should just add your snippets where you handle DBus messages meant for your application and that should be it.

#include 
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#include 

#include 

struct dbus_ctx {
    DBusConnection *conn;
    struct event_base *evbase;
    struct event dispatch_ev;
    void *extra;
};

static void dispatch(int fd, short ev, void *x)
{
    struct dbus_ctx *ctx = x;
    DBusConnection *c = ctx->conn;

    logger(LOG_DEBUG "dispatching\n");

    while (dbus_connection_get_dispatch_status(c) == DBUS_DISPATCH_DATA_REMAINS)
        dbus_connection_dispatch(c);
}

static void handle_dispatch_status(DBusConnection *c,
                                   DBusDispatchStatus status, void *data)
{
    struct dbus_ctx *ctx = data;

    logger(LOG_DEBUG "new dbus dispatch status: %d\n", status);

    if (status == DBUS_DISPATCH_DATA_REMAINS) {
        struct timeval tv = {
            .tv_sec = 0,
            .tv_usec = 0,
        };
        event_add(&ctx->dispatch_ev, &tv);
    }
}

static void handle_watch(int fd, short events, void *x)
{
    struct dbus_ctx *ctx = x;
    struct DBusWatch *watch = ctx->extra;

    unsigned int flags = 0;
    if (events & EV_READ)
        flags |= DBUS_WATCH_READABLE;
    if (events & EV_WRITE)
        flags |= DBUS_WATCH_WRITABLE;
    /*if (events & HUP)
        flags |= DBUS_WATCH_HANGUP;
    if (events & ERR)
        flags |= DBUS_WATCH_ERROR;*/

    logger(LOG_DEBUG "got dbus watch event fd=%d watch=%p ev=%d\n",
           fd, watch, events);
    if (dbus_watch_handle(watch, flags) == FALSE)
        logger(LOG_ERROR "dbus_watch_handle() failed\n");

    handle_dispatch_status(ctx->conn, DBUS_DISPATCH_DATA_REMAINS, ctx);
}

static dbus_bool_t add_watch(DBusWatch *w, void *data)
{
    if (!dbus_watch_get_enabled(w))
        return TRUE;

    struct dbus_ctx *ctx = data;
    ctx->extra = w;

    int fd = dbus_watch_get_unix_fd(w);
    unsigned int flags = dbus_watch_get_flags(w);
    short cond = EV_PERSIST;
    if (flags & DBUS_WATCH_READABLE)
        cond |= EV_READ;
    if (flags & DBUS_WATCH_WRITABLE)
        cond |= EV_WRITE;

    struct event *event = event_new(ctx->evbase, fd, cond, handle_watch, ctx);
    if (!event)
        return FALSE;

    event_add(event, NULL);

    dbus_watch_set_data(w, event, NULL);

    logger(LOG_DEBUG "added dbus watch fd=%d watch=%p cond=%d\n", fd, w, cond);
    return TRUE;
}

static void remove_watch(DBusWatch *w, void *data)
{
    struct event *event = dbus_watch_get_data(w);

    if (event)
        event_free(event);

    dbus_watch_set_data(w, NULL, NULL);

    logger(LOG_DEBUG "removed dbus watch watch=%p\n", w);
}

static void toggle_watch(DBusWatch *w, void *data)
{
    logger(LOG_DEBUG "toggling dbus watch watch=%p\n", w);

    if (dbus_watch_get_enabled(w))
        add_watch(w, data);
    else
        remove_watch(w, data);
}

static void handle_timeout(int fd, short ev, void *x)
{
    struct dbus_ctx *ctx = x;
    DBusTimeout *t = ctx->extra;

    logger(LOG_DEBUG "got dbus handle timeout event %p\n", t);

    dbus_timeout_handle(t);
}

static dbus_bool_t add_timeout(DBusTimeout *t, void *data)
{
    struct dbus_ctx *ctx = data;

    if (!dbus_timeout_get_enabled(t))
        return TRUE;

    logger(LOG_DEBUG "adding timeout %p\n", t);

    struct event *event = event_new(ctx->evbase, -1, EV_TIMEOUT|EV_PERSIST,
                                    handle_timeout, t);
    if (!event) {
        logger(LOG_ERROR "failed to allocate new event for timeout\n");
        return FALSE;
    }

    int ms = dbus_timeout_get_interval(t);
    struct timeval tv = {
        .tv_sec = ms / 1000,
        .tv_usec = (ms % 1000) * 1000,
    };
    event_add(event, &tv);

    dbus_timeout_set_data(t, event, NULL);

    return TRUE;
}

static void remove_timeout(DBusTimeout *t, void *data)
{
    struct event *event = dbus_timeout_get_data(t);

    logger(LOG_DEBUG "removing timeout %p\n", t);

    event_free(event);

    dbus_timeout_set_data(t, NULL, NULL);
}

static void toggle_timeout(DBusTimeout *t, void *data)
{
    logger(LOG_DEBUG "toggling timeout %p\n", t);

    if (dbus_timeout_get_enabled(t))
        add_timeout(t, data);
    else
        remove_timeout(t, data);
}

static DBusHandlerResult handle_nameownerchanged(DBusMessage *message,
                                                 void *data)
{
    struct dbus_ctx *ctx = data;
    char *name, *old, *new;
    if (dbus_message_get_args(message, NULL,
                              DBUS_TYPE_STRING, &name,
                              DBUS_TYPE_STRING, &old,
                              DBUS_TYPE_STRING, &new,
                              DBUS_TYPE_INVALID) == FALSE) {
        logger(LOG_ERROR "spurious NameOwnerChanged signal\n");
        return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
    }
    logger(LOG_DEBUG "dbus NameOwnerChanged %s -> %s\n", old, new);

    if (new[0] != '\0')
        return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;

    /* XXX handle disconnecting clients */

    return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}

static DBusHandlerResult msg_filter(DBusConnection *connection,
                                    DBusMessage *message, void *data)
{
    if (dbus_message_is_signal(message, DBUS_INTERFACE_DBUS,
                               "NameOwnerChanged"))
        return handle_nameownerchanged(message, data);

    logger(LOG_DEBUG "got dbus message %d %s -> %s %s/%s/%s %s\n",
           dbus_message_get_type(message),
           dbus_message_get_sender(message),
           dbus_message_get_destination(message),
           dbus_message_get_path(message),
           dbus_message_get_interface(message),
           dbus_message_get_member(message),
           dbus_message_get_type(message) == DBUS_MESSAGE_TYPE_ERROR ?
           dbus_message_get_error_name(message) : "");

    return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}

static void unregister_func(DBusConnection *connection, void *data)
{
}

static DBusHandlerResult message_func(DBusConnection *connection,
                                      DBusMessage *message, void *data)
{
    struct dbus_ctx *ctx = data;

    logger(LOG_DEBUG "got dbus message sent to %s %s %s\n",
           dbus_message_get_destination(message),
           dbus_message_get_interface(message),
           dbus_message_get_path(message));

    /* XXX handle DBus message */

    return DBUS_HANDLER_RESULT_HANDLED;
}

static DBusObjectPathVTable dbus_vtable = {
    .unregister_function = unregister_func,
    .message_function = message_func,
};

struct dbus_ctx *dbus_init(struct event_base *eb)
{
    DBusConnection *conn = NULL;
    struct dbus_ctx *ctx = calloc(1, sizeof(struct dbus_ctx));
    if (!ctx) {
        logger_perror("can't allocate dbus_ctx\n");
        goto out;
    }

    conn = dbus_bus_get_private(DBUS_BUS_SESSION, NULL);
    if (conn == NULL) {
        logger(LOG_ERROR "failed to get bus\n");
        goto out;
    }

    dbus_connection_set_exit_on_disconnect(conn, FALSE);

    ctx->conn = conn;
    ctx->evbase = eb;
    event_assign(&ctx->dispatch_ev, eb, -1, EV_TIMEOUT, dispatch, ctx);

    if (!dbus_connection_set_watch_functions(conn, add_watch, remove_watch,
                                             toggle_watch, ctx, NULL)) {
        logger(LOG_ERROR "dbus_connection_set_watch_functions() failed\n");
        goto out;
    }

    if (!dbus_connection_set_timeout_functions(conn, add_timeout,
                                               remove_timeout, toggle_timeout,
                                               ctx, NULL)) {
        logger(LOG_ERROR "dbus_connection_set_timeout_functions() failed\n");
        goto out;
    }

    if (dbus_connection_add_filter(conn, msg_filter, ctx, NULL) == FALSE) {
        logger(LOG_ERROR "dbus_connection_add_filter() failed\n");
        goto out;
    }

    dbus_connection_set_dispatch_status_function(conn, handle_dispatch_status,
                                                 ctx, NULL);

    char match[256];
    snprintf(match,
             sizeof(match),
             "type='signal',interface='%s',member='NameOwnerChanged'",
             DBUS_INTERFACE_DBUS);
    DBusError error;
    dbus_error_init(&error);
    dbus_bus_add_match(conn, match, &error);
    if (dbus_error_is_set(&error)) {
        logger(LOG_ERROR "dbus_bus_add_match() %s failed: %s\n",
               "NameOwnerChanged", error.message);
        dbus_error_free(&error);
        goto out;
    }

    snprintf(match,
             sizeof(match),
             "type='signal',interface='%s',member='%s'",
             GNP_IPC_INTERFACE, GNP_IPC_SIGNAL_DELIVER_SA);
    dbus_error_init(&error);
    dbus_bus_add_match(conn, match, &error);
    if (dbus_error_is_set(&error)) {
        logger(LOG_ERROR "dbus_bus_add_match() %s failed: %s\n",
               GNP_IPC_SIGNAL_DELIVER_SA, error.message);
        dbus_error_free(&error);
        goto out;
    }

    if (dbus_connection_register_object_path(conn, GNP_IPC_PATH, &dbus_vtable,
                                             ctx) != TRUE) {
        logger(LOG_ERROR "failed to register object path\n");
        goto out;
    }

    return ctx;

out:
    if (conn) {
        dbus_connection_close(conn);
        dbus_connection_unref(conn);
    }
    if (ctx)
        free(ctx);
    return NULL;
}

void dbus_close(struct dbus_ctx *ctx)
{
    if (ctx && ctx->conn) {
        dbus_connection_flush(ctx->conn);
        dbus_connection_close(ctx->conn);
        dbus_connection_unref(ctx->conn);
        event_del(&ctx->dispatch_ev);
    }
    if (ctx)
        free(ctx);
}