after calling g_seekable_truncate with offset=0, that makes segfault when calling realloc happens next time. the object can't be reused.

$ 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);
        for (i = 0; i < 1000; i++)
                g_data_output_stream_put_byte(o, 1, NULL, NULL);
        g_seekable_truncate(G_SEEKABLE (mo), 0, NULL, &error);
        if (error)
                g_print("%s\n", error->message);
        for (i = 0; i < 2000; i++)
                g_data_output_stream_put_byte(o, 1, NULL, NULL);

        return 0;
}
$ gcc t.c `pkg-config --cflags --libs gio-2.0`
$ gdb ./a.out
GNU gdb Fedora (6.8-10.fc9)
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu"...
(no debugging symbols found)
(gdb) r
Starting program: /home/tagoh/a.out 
warning: the debug information found in "/usr/lib/debug//lib64/libselinux.so.1.debug" does not match "/lib64/libselinux.so.1" (CRC mismatch).

warning: the debug information found in "/usr/lib/debug/lib64/libselinux.so.1.debug" does not match "/lib64/libselinux.so.1" (CRC mismatch).

Failed to resize memory output stream

Program received signal SIGSEGV, Segmentation fault.
_int_realloc (av=<value optimized out>, oldmem=<value optimized out>, 
    bytes=<value optimized out>) at malloc.c:4953
4953        INTERNAL_SIZE_T nextsize = chunksize(next);
Missing separate debuginfos, use: debuginfo-install libselinux.x86_64
(gdb) bt

+ Trace 201502

Giving 0 to realloc(3) behaves like free(3). so

static gboolean
array_resize (GMemoryOutputStream  *ostream,
              gsize                 size,
	      gboolean              allow_partial,
              GError              **error)
{
...
  data = priv->realloc_fn (priv->data, size);

  if (!data) 
    {
      if (allow_partial &&
	  priv->pos < priv->len)
	return TRUE; /* Short write */
      
      g_set_error (error,
                   G_IO_ERROR,
                   G_IO_ERROR_NO_SPACE,
                   _("Failed to resize memory output stream"));
      return FALSE;
    }
...
}

priv->data has invalid pointer after calling priv->realloc_fn and no updates happens in this case.