Wireshark-dev: Re: [Wireshark-dev] cast increases required alignment of target type in packet-d
I think the cast to void* is OK.
the GArray is created given sizeof(struct contained_t) so it is going
to mallocate a block of N*sizeof(struct contained_t) and the very
first struct contained_t will be aligned o the base of the block given
by malloc (a void*), taking into account that compilers are supposed
to size structs into valid pointer boundaries, my guess is that every
struct contained_t in the GArray will be propperly aligned.
On Tue, Jul 15, 2008 at 4:50 PM, Jeff Morriss <jeff.morriss.ws@xxxxxxxxx> wrote:
>
> Hi folks,
>
> My Solaris/SPARC compiles (with gcc-3.4.6) die with an alignment warning
> in packet-diameter.c:
>
>> packet-diameter.c: In function `dissect_diameter_avp':
>> packet-diameter.c:340: warning: cast increases required alignment of target type
>
> (followed by several more such warnings)
>
> The relevant code for the first warning is:
>
>> 109 typedef struct _diam_vnd_t {
>> 110 guint32 code;
>> 111 GArray* vs_avps;
>> 112 GArray* vs_cmds;
>> 113 } diam_vnd_t;
> [...]
>> 126 #define VND_AVP_VS(v) ((value_string*)((v)->vs_avps->data))
> [...]
>> 319 const diam_vnd_t* vendor;
> [...]
>> 340 vendor_avp_vs = VND_AVP_VS(vendor);
>
> GArray comes from GLIB's garray.h:
>
>> 34 typedef struct _GArray GArray;
> [...]
>> 38 struct _GArray
>> 39 {
>> 40 gchar *data;
>> 41 guint len;
>> 42 };
>
>
> The Solaris buildbot is not getting this warning but I think the warning
> is valid: an array of chars may not be aligned correctly to be accessed
> as a 'value_string' (a guint32 followed by a pointer). And it may cause
> bus errors because GArrays make copies of the data passed in--so we have
> no guarantee how it will be aligned.
>
> Unfortunately GArrays come in only 3 flavors: with pointers to chars,
> pointers to guint8s, and pointers to pointers.
>
> Is there an easy way to solve this?
>
> Adding an intermediate cast to void*:
>
> #define VND_AVP_VS(v) ((value_string*)(void*)((v)->vs_avps->data))
>
> solves the warning and it appears this is the approach used by the glib
> team:
>
> http://svn.gnome.org/viewvc/glib?view=revision&revision=6092
>
> but I don't think it's correct. But at the moment I also think the
> example (storing gint values) used in the GArrays documentation:
>
> http://library.gnome.org/devel/glib/unstable/glib-Arrays.html
>
> is also not correct/safe on SPARCs. Can someone please tell me I'm wrong?
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