BSP Developer's Guide

VxWorks 5.5
BSP Developer’s Guide
sufficiently for an item of type type. The test is normally done by examining
low-order bits of the pointer’s value.
void * pVoid;
if (!WRS_ALIGN_CHECK(pVoid, long))
{
printf (“Error: pVoid is not longword aligned”);
}
Unaligned Accesses and Copying
You may need to access data that may not be correctly aligned at runtime. It is
recommended in this situation that you copy the data to a structure or other area
that is properly aligned. After the data has been copied, it can be accessed without
the possibility of causing unaligned access exceptions. The macro provided for this
purpose is _WRS_UNALIGNED_COPY(pSrc, pDst, size). While the standard
VxWorks bcopy( ) function could be used for this purpose, most compilers can do
short copies more efficiently on their own. Using the macro is therefore desirable
for performance reasons.
The following example shows both _WRS_ALIGN_CHECK and
_WRS_UNALIGNED_COPY used together to check an unknown pointer. If it is
sufficiently aligned, the pointer can be cast to some other type and the item can be
accessed directly. If the pointer is not aligned, the unaligned macro is used to copy
the data element to a usable variable location.
struct structA {
long item1;
long item2;
char item3;
} itemA;
void * pVoid;
long aLong;
if (WRS_ALIGN_CHECK(pVoid, (struct structA)))
{
/* Alignment is okay, reference directly */
aLongItem = ((struct structA *)pVoid)->item2;
}
else
{
/* alignment is not okay, use unaligned copy */
_WRS_UNALIGNED_COPY(pVoid,&aLong,sizeof(aLong));
}
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