Is it safe to free a glib buffer twice?

Question

Is it safe or forbidden to free twice a buffer allocated by glib g_malloc function?

char *buffer = g_malloc(10);
g_free(buffer);
g_free(buffer);

Answer 1

From glib/gmem.c (Assuming you didn't do g_mem_set_vtable to something fancy):

static void
standard_free (gpointer mem)
{
  free (mem);
}
...
/* --- variables --- */
static GMemVTable glib_mem_vtable = {
  standard_malloc,
  standard_realloc,
  standard_free,
  standard_calloc,
  standard_try_malloc,
  standard_try_realloc,
};
...
void
g_free (gpointer mem)
{
  if (G_UNLIKELY (!g_mem_initialized))
    g_mem_init_nomessage();
  if (G_LIKELY (mem))
    glib_mem_vtable.free (mem);
  TRACE(GLIB_MEM_FREE((void*) mem));
}

The glib_mem_vtable.free(mem) will call standard_free(mem) which will just call free(mem). As it is invalid to do:

 void *mem = malloc(1);
 free(mem);
 free(mem); // undefined behavior

As much it is invalid to call g_free on the same memory pointer twice, as it internally calls free on it's argument.