Arithmetic on 'void *' should not be done. I know some versions of
gcc allow it (provided some warning level/option is enabled) but
that doesn't mean it is valid.
All relevant GCC versions support it (going back to 2.x at least).
It is _always_ allowed, whatever compiler options you use; -pedantic
or -Wpointer-arith will warn (which you can upgrade to an error).
The Linux kernel is not built with these warnings enabled.
A char* is incorrect because a char could be more
than one byte, in theory.
It is somewhat difficult to untangle the standard, but
sizeof (char) is defined to be one.
A char takes exactly one byte. A byte could be more than eight bits,
of course.
In GCC, sizeof(void) is 1 as well.
Of course, the C language doesn't actually require that
you can converts between pointers to different types in
any well-defined manner.
It does actually, see 6.3.2.3/7. You can convert any pointer to object
to a pointer to a different object type, as long as it is properly
aligned.
You cannot in general access the object as that new type of course, but
it is perfectly well-defined; in particular, you can convert it back to
the original type and get the same value again, and you can walk
consecutive
bytes of the object by using a pointer to character type.
Segher
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