After adding support for Bluez5 in the latest version of gattlib, the Glib warning message GLib-WARNING **: Invalid file descriptor was printed every time we were closing a gattlib connection.
Even if the error warning was looking harmless, a warning message should never occurs after a normal operation.

Here are the following steps to fix a Glib warning or error:

1. Ensure debug support is present in the gattlib based applications:

   cd <gattlib_root>
   mkdir build && cd build
   cmake -DCMAKE_BUILD_TYPE=Debug ..
   make
   

2. Tell Glib library to treat warnings as error:

   export G_DEBUG=fatal_warnings
   

3. Debug one of the gattlib examples using gdb:

   gdb ./examples/read_write/read_write
   

4. Start the program and pass its arguments:

   (gdb) run  DA:94:40:95:E0:87 read 0x2a38
   Starting program: /home/olivier/gattlib/build/examples/read_write/read_write DA:94:40:95:E0:87 read 0x2a38
   [Thread debugging using libthread_db enabled]
   Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".
   [New Thread 0xb7bdab40 (LWP 3594)]
   Read UUID completed: 03 
   
   (process:3590): GLib-WARNING **: Invalid file descriptor.
   
   
   Thread 1 "read_w" received signal SIGTRAP, Trace/breakpoint trap.
   0xb7cbc749 in g_logv () from /lib/i386-linux-gnu/libglib-2.0.so.0
   

5. Inspect the callstack:

   (gdb) where
   #0  0xb7cbc749 in g_logv () from /lib/i386-linux-gnu/libglib-2.0.so.0
   #1  0xb7cbc805 in g_log () from /lib/i386-linux-gnu/libglib-2.0.so.0
   #2  0xb7ca7c28 in g_io_channel_error_from_errno () from /lib/i386-linux-gnu/libglib-2.0.so.0
   #3  0xb7cfcbee in ?? () from /lib/i386-linux-gnu/libglib-2.0.so.0
   #4  0xb7ca84ea in g_io_channel_shutdown () from /lib/i386-linux-gnu/libglib-2.0.so.0
   #5  0xb7ca86cd in g_io_channel_unref () from /lib/i386-linux-gnu/libglib-2.0.so.0
   #6  0xb7fb988c in io_destroy (io=0xb7200fe8) at /home/olivier/gattlib/bluez5/src/shared/io-glib.c:132
   #7  0xb7fa90cb in bt_att_free (att=0xb7202a00) at /home/olivier/gattlib/bluez5/src/shared/att.c:955
   #8  0xb7fa94be in bt_att_unref (att=0xb7202a00) at /home/olivier/gattlib/bluez5/src/shared/att.c:1065
   #9  0xb7f7f224 in g_attrib_unref (attrib=0xb7200fc0) at /home/olivier/gattlib/bluez5/attrib/gattrib.c:195
   #10 0xb7f7932c in gattlib_disconnect (connection=0x804cef8) at /home/olivier/gattlib/src/gattlib_connect.c:359
   #11 0x08048c72 in main (argc=4, argv=0xbfffee74) at /home/olivier/gattlib/examples/read_write/read_write.c:93
   (gdb)
   

6. Fix the issue!