http://library.gnome.org/devel/gio/stable/GMemoryOutputStream.html#g-memory-output-stream-get-size says that I could achieve with g_seekable_seek() and g_seekable_tell() to get the number of bytes written in GMemoryOutputStream. but what I can get with g_seekable_tell() is the number of bytes allocated, not written.

$ cat t.c
#include <gio/gio.h>

int
main(void)
{
        GOutputStream *mo;
        GDataOutputStream *o;
        int i;
        GError *error = NULL;

        g_type_init();
        mo = g_memory_output_stream_new(NULL, 0, g_realloc, g_free);
        o = g_data_output_stream_new(mo);
        g_data_output_stream_put_byte(o, 1, NULL, NULL);
        g_seekable_seek(G_SEEKABLE (mo), 0, G_SEEK_END, NULL, NULL);
        g_print("%d\n", g_seekable_tell(G_SEEKABLE (mo)));
        g_object_unref(o);
        g_object_unref(mo);

        return 0;
}

$ gcc t.c `pkg-config --cflags --libs gio-2.0`
$ ./a.out
16