GMainContext have ref_count > 0 after unref

Question

I am not getting ref_count to decrease properly for my GMainContext. The example program here is a small version of a large program (which uses threads, hence the need to create a context and push it on the thread).

GMainLoop *loop;
GMainContext *ctx;

struct conn
{
    GSocketClient *client;

    GSocketConnection *conn;
    GInputStream *in;
    GOutputStream *out;

    gchar data[8192];
    unsigned int count;
};

static void
read_done_cb(GObject *source_object, GAsyncResult *res, gpointer user_data)
{
    struct conn *c = (struct conn *)user_data;
    gssize len = g_input_stream_read_finish(c->in, res, NULL);

    g_input_stream_read_async(c->in, c->data, sizeof c->data / sizeof *c->data, G_PRIORITY_DEFAULT, NULL, read_done_cb, c);
    if (c->count++ == 1) {
        printf("End of life as I know it...\n");
        g_main_loop_quit(loop);
    }
}

static void
write_done_cb(GObject *source_object, GAsyncResult *res, gpointer user_data)
{
}

static void
connect_done_cb(GObject *source_object, GAsyncResult *res, gpointer user_data)
{
    printf("## %s\n", __FUNCTION__);

    struct conn *c = (struct conn *)user_data;
    c->conn = g_socket_client_connect_to_host_finish(c->client, res, NULL);

    c->in  = g_io_stream_get_input_stream(G_IO_STREAM(c->conn));
    c->out = g_io_stream_get_output_stream(G_IO_STREAM(c->conn));

    char *data = "GET /axis-cgi/mjpg/video.cgi HTTP/1.0\r\n\r\n";

    g_output_stream_write_async(c->out, data, strlen(data), G_PRIORITY_DEFAULT, NULL, write_done_cb, c);
    g_input_stream_read_async(c->in, c->data, sizeof c->data / sizeof *c->data, G_PRIORITY_DEFAULT, NULL, read_done_cb, c);
}

int
main(int argc, char **argv)
{
    g_type_init();

    struct conn *c = g_malloc0(sizeof *c);
    ctx = g_main_context_new();
    loop = g_main_loop_new(ctx, FALSE);
    g_main_context_push_thread_default(ctx);

    c->client = g_socket_client_new();
    g_socket_client_connect_to_host_async(c->client, "10.85.25.20", 80, NULL, connect_done_cb, c);

    g_main_loop_run(loop);

    g_io_stream_close(G_IO_STREAM(c->conn), NULL, NULL);
    g_object_unref(c->client);
    g_object_unref(c->conn);
    g_main_context_pop_thread_default(ctx);
    g_main_loop_unref(loop);
    g_main_context_unref(ctx);

    return 0;
}

Using gdb, inserting breakpoint just before return I can see that ctx still have one ref count:

(gdb) p ctx->ref_count
 $2 = 1

If I do another g_main_context_unref(ctx); everything shuts down as expected. I do not understand where I get this ownership though.

Thanks in advance for your help

score 1 · Accepted Answer · answered Apr 12 '12 at 18:34

I found the error. I read_done_cb I issued another g_input_stream_read_async and immediately after quitting the main loop. g_input_stream_read_async upped the ref_count but GMainLoop never got a chance to return to my callback (and decreasing the ref_count on my GMainContext).

Moving the call to g_input_stream_read_async in my callback to below the if statement

static void
read_done_cb(GObject *source_object, GAsyncResult *res, gpointer user_data)
{
    struct conn *c = (struct conn *)user_data;
    gssize len = g_input_stream_read_finish(c->in, res, NULL);

    if (c->count++ == 1) {
        printf("End of life as I know it...\n");
        g_main_loop_quit(loop);
    }

    g_input_stream_read_async(c->in, c->data, sizeof c->data / sizeof *c->data, G_PRIORITY_DEFAULT, NULL, read_done_cb, c);

}

correctly resolved the number of ref counts on my main context.

Silly mistake. Hopefully someone will find some use of my post at least.

score 0 · Answer 2 · answered Apr 12 '12 at 10:41

g_main_context_new(), g_main_loop_new(), and g_main_context_push_thread_default() all ref the context. g_main_context_pop_thread_default(), g_main_loop_unref(), and g_main_context_unref() all unref it. So your intuition is sound.

I would use a watchpoint in gdb: watch ctx->ref_count to find out where the extra reference is being added.

GMainContext have ref_count > 0 after unref

2 Answers2