Portably extracting data from a bytestring

[James S. Singleton]

Let S be a pointer to a bytestring of length L. I would like to extract 4
bytes from S at the location p = S + d, with 0 < d < L - 4, and store them
into an unsigned int. I am looking for suggestions on how to do this

1) In portable ANSI C.
2) As efficiently as possible.
3) Taking full account of the potential data alignment and
endianness issues that this action must tackle.

[Ben Pfaff]

Christopher Benson-Manica <at***@nospam.c yberspace.org> writes:

What, exactly, is the difference between "normative" and
"informativ e"?

Normative text is part of the standard.
Informative text, like footnotes, examples, and some appendices,
are not part of the standard. They are for information only.
--
Go not to Usenet for counsel, for they will say both no and yes.

[Tim Rentsch]

Christopher Benson-Manica <at***@nospam.c yberspace.org> writes:

Tim Rentsch <tx*@alumnus.ca ltech.edu> wrote:

This sentence has disappeared from the Standard by now. There is
however a similar statement in an informative annex.

What, exactly, is the difference between "normative" and
"informativ e"? IIUC, "informativ e" is not strictly "standard" - does
that mean that there is no "normative" text specifying how
implementations should deal with union member access?

Taken from ISO/IEC Directives part 3:

3.4

normative elements

those elements setting out the provisions to which it is
necessary to conform in order to be able to claim compliance
with the standard

A "normative element" must be observed in order to conform to the
standard in question. Any "informativ e" text is supposed to be
right (and presumably useful), but it does not by itself impose
requirements on whatever is being defined in the standard. Both
normative text and informative text are part of a standard, but
only normative text imposes requirements that must be observed.

There is normative text that gives requirements for accessing
union members, but that text is sprinkled through the rest of the
C Standard. So it isn't easy to tell if the logical consequences
of those requirements imply implementation defined behavior.

[Tim Rentsch]

Ben Pfaff <bl*@cs.stanfor d.edu> writes:

Christopher Benson-Manica <at***@nospam.c yberspace.org> writes:

What, exactly, is the difference between "normative" and
"informativ e"?

Normative text is part of the standard.
Informative text, like footnotes, examples, and some appendices,
are not part of the standard. They are for information only.

Not exactly. Both normative elements and informative elements
are part of a standard, but only normative elements give
provisions that must be observed in order to claim conformance
(to whatever it is that's being standardized).

(I admit it's a minor distinction; I thought some people
might appreciate the clarification.)

[Christopher Benson-Manica]

Tim Rentsch <tx*@alumnus.ca ltech.edu> wrote:

(I admit it's a minor distinction; I thought some people
might appreciate the clarification.)

I did - thank you.

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[pete]

Skarmander wrote:

James S. Singleton wrote:

Let S be a pointer to a bytestring of length L.
I would like to extract 4
bytes from S at the location p = S + d, with 0 < d < L - 4,
and store them
into an unsigned int. I am looking for suggestions on how to do this

1) In portable ANSI C.

Impossible if I take you literally, since an unsigned int isn't
guaranteed to be any bigger than 16 bits, and 4 bytes will be 32 bits
(since we're presumably talking about 8-bit bytes, not "C bytes" which
can be larger).

"portable ANSI C" means "C bytes"

You are correct in that it is impossible.

Code doesn't have to be portable to be useful.

Pretending that code is portable when it isn't, is wrong.

--
pete

[Skarmander]

pete wrote:
<snip>

Code doesn't have to be portable to be useful.

Pretending that code is portable when it isn't, is wrong.

"Portable modulo X" is a meaningful concept. That is, "portable across
all machines where an int is 32 bits" is meaningful. Whether it's
acceptable depends, and nobody could advertise it as "100% portable ISO
C", but it's not "pretending ". Just as long as you don't call it
"portable" without qualification.

S.

[Jordan Abel]

On 2005-10-27, Skarmander <in*****@dontma ilme.com> wrote:

pete wrote:
<snip>

Code doesn't have to be portable to be useful.

Pretending that code is portable when it isn't, is wrong.

"Portable modulo X" is a meaningful concept. That is, "portable across
all machines where an int is 32 bits" is meaningful. Whether it's
acceptable depends, and nobody could advertise it as "100% portable ISO
C", but it's not "pretending ". Just as long as you don't call it
"portable" without qualification.

And all that's needed in this case is to use long instead of int.

[pete]

Jordan Abel wrote:

On 2005-10-27, Skarmander <in*****@dontma ilme.com> wrote:

pete wrote:
<snip>

Code doesn't have to be portable to be useful.

Pretending that code is portable when it isn't, is wrong.

"Portable modulo X" is a meaningful concept.
That is, "portable across
all machines where an int is 32 bits" is meaningful. Whether it's
acceptable depends,
and nobody could advertise it as "100% portable ISO
C", but it's not "pretending ". Just as long as you don't call it
"portable" without qualification.

OK.
And all that's needed in this case is to use long instead of int.
Qulaification is still required for that.

For this specification:
I would like to extract 4 bytes and store them into an unsigned int. 1) In portable ANSI C.

sizeof(long) can be less than 4 if CHAR_BIT is greater than 8.

--
pete