Wireshark-dev: Re: [Wireshark-dev] cast increases required alignment of target type in packet-d
2008/7/15 Jeff Morriss <
jeff.morriss.ws@
gmail.com>:
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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An easy way to solve alignment issues like this is to add a variable of the data type that is of a size equal to that of the platform's pointer size. This variable should be added as a member of a union with the original data (an array of characters in this case).
Example:
The original data layout is this:
char someData[50];
Casting someData to a data type other than char is never safe from an alignment perspective. However if the data layout instead would be like this:
union AUnion
{
char someData[50];
guint64 *ensuresProperAlignmentOfSomeData;
};
This will force the compiler to allocate someData aligned on an 8-byte boundary (safe to cast to any pointer on up to 64-bit platforms). Replacing guint64 with a data type that varies in size with the platform (so that it will be 32 bits on a 32 bit platform for instance) would of course be the better approach, however I cannot recall if there is such a data type in glib (there probably is one).
/ Peter
