[erlang-questions] Passing data between Erlang and C port while preserving padding and alignment?

[Frank Muller]

Thanks Mikael.
Any others thoughts ?

/Frank

You'll need to use C's offsetof() and sizeof() to discover (a) any
> internal padding between end of a previous field and the start of a
> new field, basically offsetof(struct, f2) - (offsetof(struct, f1) +
> sizeof(struct.f1)) gives that internal padding, and (2) any trailing
> padding, given by sizeof(struct) - (offsetof(struct, lastField) +
> sizeof(struct.lastField)).  Beware that the contents of padding is
> unspecified, so might not be all-bits-zero.
>
> You'll need to derive these values via the C compiler.  In theory you
> could derive them in pure Erlang given the types involved, but that'd
> require not only a C type parser but also a complete description of
> the machine's C ABI.  I would't go that way.
> On Fri, Dec 7, 2018 at 9:49 AM Frank Muller <frank.muller.erl@REDACTED>
> wrote:
> >
> > Hello Vance
> >
> > Fantastic!!!
> >
> > It would be great if you can share some code.
> > I’m totally lost with this tricky problem and no one was able to help me.
> >
> > Thanks in advance...
> >
> > /Frank
> >>
> >> On Fri, Dec 7, 2018 at 9:51 AM Frank Muller <frank.muller.erl@REDACTED>
> wrote:
> >> > Question: how can I derive the GCC's padding, and automatically add
> it to my Erlang binary?
> >>
> >> Frank,
> >>
> >> I used to write linked-in drivers for embedded systems which provided
> >> C library APIs. Back in those days I was building for 32 & 64 bit, x86
> >> & SPARC. I used GNU autotools to figure out the target environment and
> >> generate macros in my Erlang header files. It all worked quite
> >> smoothly in the end.  I'd be happy to send you an example project if
> >> you think it'd helpful.
> >>
> >> To your specific question I think the autoconf macro AC_CHECK_ALIGNOF
> >> does what you want:
> >>
> https://www.gnu.org/software/autoconf/manual/autoconf-2.67/html_node/Generic-Compiler-Characteristics.html
> >>
> >> --
> >>      -Vance
> >
> > _______________________________________________
> > erlang-questions mailing list
> > erlang-questions@REDACTED
> > http://erlang.org/mailman/listinfo/erlang-questions
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20181207/d81e1cb7/attachment.htm>