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Re: Pointer goes to null

From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.c
Subject Re: Pointer goes to null
Date 2020-01-15 18:36 +0000
Organization A noiseless patient Spider
Message-ID <87lfq8ftxs.fsf@bsb.me.uk> (permalink)
References (1 earlier) <256s0fdum686t6hiv4e9akek6b0pb4h63q@4ax.com> <b4055b29-eba5-40fa-805d-31f4fcc3f587@googlegroups.com> <qv2erl$jp2$1@dont-email.me> <qv2fm6$n8m$1@dont-email.me> <1bab6894-09d3-41e5-8a79-f068db00b0d8@googlegroups.com>

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m.nasim85@gmail.com writes:

> Lew, acctually, I cannot accept either
>>   blk_cnt = ((data_bitlen)/256) + ((data_bitlen%256)/(data_bitlen%256)) 
> or
>>   blk_cnt = ((data_bitlen/256) + ((data_bitlen%256) != 0)) 
>
> 1st form may divide by zero. (you mentioned this)
> 2nd form may have unpredictable behavior as ((data_bitlen%256) != 0)
> returns TRUE ,which is only guaranteed not to be zero. it may be 1
> (which you intended) or -1.

No.  All of C's equality and comparison operators are defined to give
either 0 or 1.  No other value is possible.  Some of C's /functions/
don't behave this way (strcmp, isascii etc.), but that another matter.

But Lew's second suggestion does not do what your original code did.
Your code had the odd exception that the block count should be 1 when
the bit length was zero.  If you just want the number of whole or part
256-bit blocks, the usual way is to write

  blk_cnt = (data_bitlen + 255) / 256;

but that does not give blk_cnt == 1 when data_bitlen == 0.  Do you
really want that?

-- 
Ben.

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Thread

Pointer goes to null m.nasim85@gmail.com - 2020-01-02 01:34 -0800
  Re: Pointer goes to null Jorgen Grahn <grahn+nntp@snipabacken.se> - 2020-01-02 12:47 +0000
  Re: Pointer goes to null Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2020-01-02 08:55 -0500
    Re: Pointer goes to null Bart <bc@freeuk.com> - 2020-01-02 18:49 +0000
      UB awaits... (Was: Pointer goes to null) gazelle@shell.xmission.com (Kenny McCormack) - 2020-01-02 19:53 +0000
        Re: UB awaits... (Was: Pointer goes to null) David Brown <david.brown@hesbynett.no> - 2020-01-02 23:34 +0100
          Re: UB awaits... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-03 01:43 +0000
            Re: UB awaits... David Brown <david.brown@hesbynett.no> - 2020-01-03 09:50 +0100
              Re: UB awaits... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-03 14:30 +0000
                Re: UB awaits... David Brown <david.brown@hesbynett.no> - 2020-01-03 17:27 +0100
  Re: Pointer goes to null Bart <bc@freeuk.com> - 2020-01-02 14:07 +0000
    Re: Pointer goes to null m.nasim85@gmail.com - 2020-01-02 07:29 -0800
      Re: Pointer goes to null Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-01-02 10:12 -0800
  Re: Pointer goes to null jak <nospam@please.ty> - 2020-01-02 16:46 +0100
  Re: Pointer goes to null Barry Schwarz <schwarzb@delq.com> - 2020-01-02 08:50 -0800
    Re: Pointer goes to null m.nasim85@gmail.com - 2020-01-03 16:20 -0800
      Re: Pointer goes to null Barry Schwarz <schwarzb@delq.com> - 2020-01-03 22:16 -0800
        Re: Pointer goes to null m.nasim85@gmail.com - 2020-01-07 03:21 -0800
      Re: Pointer goes to null Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2020-01-07 12:24 -0500
        Re: Pointer goes to null Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2020-01-07 12:38 -0500
          Re: Pointer goes to null m.nasim85@gmail.com - 2020-01-15 09:36 -0800
            Re: Pointer goes to null Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-15 18:36 +0000
              Re: Pointer goes to null Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2020-01-15 17:31 -0500
                Re: Pointer goes to null Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-16 02:20 +0000
                Re: Pointer goes to null m.nasim85@gmail.com - 2020-01-17 14:51 -0800
                Re: Pointer goes to null Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-17 23:32 +0000
              Re: Pointer goes to null m.nasim85@gmail.com - 2020-01-17 14:47 -0800
                Re: Pointer goes to null Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-17 23:25 +0000
            Re: Pointer goes to null James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-15 10:49 -0800
              Re: Pointer goes to null m.nasim85@gmail.com - 2020-01-17 14:49 -0800
        Re: Pointer goes to null Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-01-07 09:42 -0800
          Re: Pointer goes to null m.nasim85@gmail.com - 2020-01-15 09:40 -0800
  Re: Pointer goes to null James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-02 22:38 -0500

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