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Re: Interpret this C code

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Philip Lantz <prl@canterey.us> writes:

> Ben Bacarisse wrote:
>
>> > # define c2ln(c,l1,l2,n) { \
>> >                         c+=n; \
>> >                         l1=l2=0; \
>> >                         switch (n) { \
>> >                         case 8: l2 =((unsigned long)(*(--(c))))<<24; \
>> >                         case 7: l2|=((unsigned long)(*(--(c))))<<16; \
>> >                         case 6: l2|=((unsigned long)(*(--(c))))<< 8; \
>> >                         case 5: l2|=((unsigned long)(*(--(c))));     \
>> >                         case 4: l1 =((unsigned long)(*(--(c))))<<24; \
>> >                         case 3: l1|=((unsigned long)(*(--(c))))<<16; \
>> >                         case 2: l1|=((unsigned long)(*(--(c))))<< 8; \
>> >                         case 1: l1|=((unsigned long)(*(--(c))));     \
>> >                                 } \
>> >                         }
>> 
>> A thing to note is that, in C, the switch statement "falls through", so
>> here the switch simply determines where to start.  It can, in effect,
>> read a big-endian integer of any size from 1 to 8 bytes.  (It has a
>> small technical flaw, in that it is not permitted to decrement a pointer
>> that points to the start of an object, and this this macro may do that.
>> ...)
>
> Might it? It appears to me that c always ends up pointing to the same place
> that it started.* Am I missing something?

No, you are right.  I was jumping to conclusions.  The prefix operator
is the correct way here.

<snip>
-- 
Ben.

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Interpret this C code blackhat401@gmail.com - 2017-03-28 17:50 -0700
  Re: Interpret this C code Jorgen Grahn <grahn+nntp@snipabacken.se> - 2017-03-29 05:43 +0000
  Re: Interpret this C code Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-29 10:50 +0100
    Re: Interpret this C code Philip Lantz <prl@canterey.us> - 2017-04-03 00:03 -0700
      Re: Interpret this C code Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-03 21:04 +0100
  Re: Interpret this C code praveen.bochetty@gmail.com - 2017-03-29 17:24 -0700

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