Void pointers and pointer arithmetic

[T. William Wells]

In article <1055 at esatst.yc.estec.nl> arne at yc.estec.nl (Arne Lundberg) writes:
:
: I am trying to write some code that tries to access members in a structure
: by knowing the start address and the offset to a particular member element.
: The following program shows a small example (the real program does not
: hardcode the offsets etc.) Is this program legal in ANSI C, will it
: produce the desired result?
:
: The (Non-ANSI) compilers I have tested either gives the value for `a'
: three times or complains about ``unknown size for pointer to void''.
: It works perfectly well if I change the type of p to be ``char *''.
:
: ---------------------------------------
: struct x {
:     int a, b, c;
: } x = {
:     1, 2, 3
: };
:
: main()
: {
:     void *p = &x;
:     printf("a %d\n", *(int *)(p + 0));
:     printf("b %d\n", *(int *)(p + 4));
:     printf("c %d\n", *(int *)(p + 8));
: }
: ---------------------------------------

This is not legal ANSI C. About the only things you can do with
void * are cast them, assign them, and pass them as function
arguments.

: is it possible to write an ANSI compatible ``offsetof'' macro
: for traditional C compilers?

There is no way to write an offsetof macro that will work on all
compilers. There are a number of ways to write it that will work
on most non-ANSI compilers.

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