Re: Updating pointer with result of sprintf

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: Updating pointer with result of sprintf
Date	2017-11-28 16:22 +0000
Organization	A noiseless patient Spider
Message-ID	<87fu8ytlt0.fsf@bsb.me.uk> (permalink)
References	<ovjd76$sbt$1@dont-email.me> <73dd9def-b2ac-49b6-9aa3-78c8350c86f1@googlegroups.com> <m2r2sibgnj.fsf@despina.home>

Show all headers | View raw

"Pascal J. Bourguignon" <pjb@informatimago.com> writes:

> "Rick C. Hodgin" <rick.c.hodgin@gmail.com> writes:
>
>> On Tuesday, November 28, 2017 at 5:19:35 AM UTC-5, Noob wrote:
>>> Hello,
>>> 
>>> Consider the following code:
>>> 
>>>   unsigned char src[32]; /* initialized somewhere */
>>>   char buf[65];
>>
>> I've never seen code which modifies buf.  I would change it to
>> something like this:
>>  
>>     for (i = 0; i < sizeof src && i < sizeof buf / 3; ++i)
>>       sprintf(buf + (i * 3), "%02x", src[i]);

(I think you mean 2 here.)

> One advantage of using indexing, is that it allows you to easily fill
> the buffer from right-to-left, when you want to dump readably values
> that are stored in little-endian order.  But using snprintf!
>
>          snprintf(buf + sizeof(buf) - 1 - 2*i, 2, "%02x", src[i]);

The expression is wrong (you need buf + size - 1 - 2*(i+1)) but snprintf
adds a null so the end result will not be as expected.  You can do a
little dance, saving the byte that the null will overwrite and putting
it back, but it's a bit messy.  You get no advantage from using snprintf
over plain sprintf here.

-- 
Ben.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar

Thread

Updating pointer with result of sprintf Noob <root@127.0.0.1> - 2017-11-28 11:19 +0100
  Re: Updating pointer with result of sprintf bartc <bc@freeuk.com> - 2017-11-28 11:02 +0000
  Re: Updating pointer with result of sprintf jacobnavia <jacob@jacob.remcomp.fr> - 2017-11-28 12:09 +0100
    Re: Updating pointer with result of sprintf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-11-28 11:23 +0000
    Re: Updating pointer with result of sprintf Keith Thompson <kst-u@mib.org> - 2017-11-28 08:58 -0800
  Re: Updating pointer with result of sprintf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-11-28 11:10 +0000
    Re: Updating pointer with result of sprintf Spiros Bousbouras <spibou@gmail.com> - 2017-11-28 16:51 +0000
      Re: Updating pointer with result of sprintf Keith Thompson <kst-u@mib.org> - 2017-11-28 09:24 -0800
      Re: Updating pointer with result of sprintf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-11-29 00:33 +0000
        Re: Updating pointer with result of sprintf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-11-29 00:36 +0000
        Re: Updating pointer with result of sprintf Spiros Bousbouras <spibou@gmail.com> - 2017-11-29 19:04 +0000
  Re: Updating pointer with result of sprintf "Pascal J. Bourguignon" <pjb@informatimago.com> - 2017-11-28 12:47 +0100
    Re: Updating pointer with result of sprintf "Pascal J. Bourguignon" <pjb@informatimago.com> - 2017-11-28 12:49 +0100
  Re: Updating pointer with result of sprintf scott@slp53.sl.home (Scott Lurndal) - 2017-11-28 14:19 +0000
    Re: Updating pointer with result of sprintf gazelle@shell.xmission.com (Kenny McCormack) - 2017-11-28 14:27 +0000
    Re: Updating pointer with result of sprintf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-11-28 14:35 +0000
  Re: Updating pointer with result of sprintf "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-11-28 06:31 -0800
    Re: Updating pointer with result of sprintf "Pascal J. Bourguignon" <pjb@informatimago.com> - 2017-11-28 15:51 +0100
      Re: Updating pointer with result of sprintf "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-11-28 07:23 -0800
        Re: Updating pointer with result of sprintf "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-11-28 08:16 -0800
          Re: Updating pointer with result of sprintf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-11-28 16:32 +0000
            Re: Updating pointer with result of sprintf "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-11-28 08:57 -0800
            Re: Updating pointer with result of sprintf "Pascal J. Bourguignon" <pjb@informatimago.com> - 2017-11-29 00:47 +0100
          Re: Updating pointer with result of sprintf "Pascal J. Bourguignon" <pjb@informatimago.com> - 2017-11-29 00:45 +0100
      Re: Updating pointer with result of sprintf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-11-28 16:22 +0000
        Re: Updating pointer with result of sprintf Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-11-28 16:25 +0000
        Re: Updating pointer with result of sprintf "Pascal J. Bourguignon" <pjb@informatimago.com> - 2017-11-29 00:48 +0100
  Re: Updating pointer with result of sprintf Noob <root@127.0.0.1> - 2017-11-29 10:07 +0100
    Re: Updating pointer with result of sprintf Barry Schwarz <schwarzb@dqel.com> - 2017-11-29 09:18 -0800

csiph-web