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Groups > comp.lang.c > #101655

Re: Correct way of doing string manipulation functions

From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: Correct way of doing string manipulation functions
Date 2017-01-13 12:14 -0800
Organization None to speak of
Message-ID <lnpojqlnu6.fsf@kst-u.example.com> (permalink)
References (5 earlier) <d16bb2a3-d00e-4f02-b1ea-4f5faa11197e@googlegroups.com> <o598nh$3ue$1@dont-email.me> <2ed0e3b8-3330-4854-bb4a-207d2c764978@googlegroups.com> <o5b5he$jj2$1@dont-email.me> <71d645f1-065f-40fb-8130-010950fd5166@googlegroups.com>

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Malcolm McLean <malcolm.mclean5@btinternet.com> writes:
> On Friday, January 13, 2017 at 6:17:56 PM UTC, Richard Heathfield wrote:
>> On 13/01/17 17:45, Malcolm McLean wrote:
>> 
>> > Anything which implements its string copy function as a tight loop
>> > reading 4 or 8 bytes at a time.

Any string copy function that requires the string to be strictly aligned
does not work correctly on C strings.

>> >                                  Or anything which does runtime
>> > checks for valid UTF-8.
>> 
>> If that's a problem at all, it's already a problem for strings. Why 
>> would it be an /extra/ problem for substrings?
>> 
> The compiler can and usually will prevent string from being created
> with unaligned first addresses, on platforms where unaligned accesses
> are cheaper or easier to integrate with other code. It can't do
> anything if you take a suffix.

If I define an array object containing a string:

    const char arr[] = "hello";

then there's no guarantee that arr is strictly aligned.  It likely is,
but nothing in C requires it, and if you're writing code that assumes it
is, your code is broken.

> There does need to be a fix, but that fix might involve patching in
> some special code, or even prohibiting the C from passing an unaligned
> string at all. Part of the art of programming is to be nice to other
> people, by which I don't mean old ladies (except old ladies who also
> happen to be programmers).

Suppose you have a string "foobar.txt", and you want just the "txt"
suffix as a string:

    const char *filename = "foobar.txt";
    const char *suffix = filename + 7; // simplified for this example

`filename` *might* be strictly aligned.  If it is, `suffix` certainly
is not.

What's your solution?  Allocate another strictly aligned array
object, and copy the suffix to it -- using a copy that has to deal
with unaligned memory anyway?  In the name of efficiency?

If you need a string to be aligned for some reason, sure, you'll
have to do that kind of thing.  But most of the time, you don't,
and imposing a general rule to allow for the rare cases where you
might need extra alignment is nonsense.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-10 15:43 -0800
  Re: Correct way of doing string manipulation functions "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-10 19:36 -0500
    Re: Correct way of doing string manipulation functions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-11 03:41 +0000
      Re: Correct way of doing string manipulation functions James Kuyper <jameskuyper@verizon.net> - 2017-01-11 07:19 -0500
    Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-11 14:36 -0800
  Correct way of doing string manipulation functions "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-01-10 16:41 -0800
    Re: Correct way of doing string manipulation functions "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-01-10 16:54 -0800
      Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-11 14:38 -0800
        Re: Correct way of doing string manipulation functions "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-01-11 18:24 -0800
    Re: Correct way of doing string manipulation functions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-11 03:38 +0000
      Re: Correct way of doing string manipulation functions "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-01-10 20:02 -0800
        Re: Correct way of doing string manipulation functions "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-01-10 20:28 -0800
        Re: Correct way of doing string manipulation functions Ian Collins <ian-news@hotmail.com> - 2017-01-11 18:19 +1300
          Re: Correct way of doing string manipulation functions "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-01-11 02:40 -0800
      Re: Correct way of doing string manipulation functions "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-01-10 20:18 -0800
    Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-11 14:37 -0800
      Re: Correct way of doing string manipulation functions "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-01-11 18:23 -0800
  Re: Correct way of doing string manipulation functions BartC <bc@freeuk.com> - 2017-01-11 01:12 +0000
    Re: Correct way of doing string manipulation functions BartC <bc@freeuk.com> - 2017-01-11 11:59 +0000
    Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-11 14:39 -0800
  Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-10 17:55 -0800
    Re: Correct way of doing string manipulation functions "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-01-10 18:21 -0800
    Re: Correct way of doing string manipulation functions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-11 03:21 +0000
      Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-11 07:33 -0800
        Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-11 07:43 -0800
        Re: Correct way of doing string manipulation functions BartC <bc@freeuk.com> - 2017-01-11 15:50 +0000
          Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-11 08:07 -0800
            Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-11 14:46 -0800
              Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-12 03:04 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-12 03:37 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-12 05:47 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-12 06:14 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-12 06:34 -0800
                Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-12 07:38 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-12 07:59 -0800
            Re: Correct way of doing string manipulation functions Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-14 22:24 -0800
              Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-15 03:30 -0800
                Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-15 03:52 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-15 04:23 -0800
      Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-11 14:42 -0800
  Re: Correct way of doing string manipulation functions Keith Thompson <kst-u@mib.org> - 2017-01-10 19:19 -0800
    Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-11 14:40 -0800
  Re: Correct way of doing string manipulation functions Barry Schwarz <schwarzb@dqel.com> - 2017-01-10 20:31 -0800
    Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-11 14:45 -0800
  Re: Correct way of doing string manipulation functions Gareth Owen <gwowen@gmail.com> - 2017-01-11 17:20 +0000
    Re: Correct way of doing string manipulation functions "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-11 12:38 -0500
      Strings as first class citizens (Was: Correct way of doing string manipulation functions) gazelle@shell.xmission.com (Kenny McCormack) - 2017-01-11 17:53 +0000
        Re: Strings as first class citizens (Was: Correct way of doing string manipulation functions) Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-11 12:46 -0800
          Re: Strings as first class citizens (Was: Correct way of doing string manipulation functions) fir <profesor.fir@gmail.com> - 2017-01-11 14:40 -0800
          Re: Strings as first class citizens (Was: Correct way of doing string manipulation functions) Robert Wessel <robertwessel2@yahoo.com> - 2017-01-11 18:12 -0600
      Re: Correct way of doing string manipulation functions supercat@casperkitty.com - 2017-01-11 09:56 -0800
    Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-11 14:48 -0800
  Re: Correct way of doing string manipulation functions Thiago Adams <thiago.adams@gmail.com> - 2017-01-11 12:09 -0800
    Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-11 15:00 -0800
  Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-11 15:11 -0800
    Re: Correct way of doing string manipulation functions "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-11 19:03 -0500
      Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-12 02:46 -0800
    Re: Correct way of doing string manipulation functions Barry Schwarz <schwarzb@dqel.com> - 2017-01-12 00:08 -0800
      Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-12 03:02 -0800
        Re: Correct way of doing string manipulation functions Richard Heathfield <rjh@cpax.org.uk> - 2017-01-12 11:38 +0000
          Re: Correct way of doing string manipulation functions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-12 13:51 +0000
            Re: Correct way of doing string manipulation functions Richard Heathfield <rjh@cpax.org.uk> - 2017-01-12 14:41 +0000
        Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-12 09:32 -0800
          Re: Correct way of doing string manipulation functions Richard Heathfield <rjh@cpax.org.uk> - 2017-01-12 18:15 +0000
          Re: Correct way of doing string manipulation functions Keith Thompson <kst-u@mib.org> - 2017-01-12 11:46 -0800
            Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-12 13:52 -0800
          Re: Correct way of doing string manipulation functions Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-12 16:50 -0800
            Re: Correct way of doing string manipulation functions Richard Heathfield <rjh@cpax.org.uk> - 2017-01-13 00:59 +0000
              Re: Correct way of doing string manipulation functions raltbos@xs4all.nl (Richard Bos) - 2017-01-13 08:59 +0000
                Re: Correct way of doing string manipulation functions Richard Heathfield <rjh@cpax.org.uk> - 2017-01-13 13:07 +0000
                Re: Correct way of doing string manipulation functions Keith Thompson <kst-u@mib.org> - 2017-01-13 10:11 -0800
                Re: Correct way of doing string manipulation functions BartC <bc@freeuk.com> - 2017-01-13 18:49 +0000
              Re: Correct way of doing string manipulation functions supercat@casperkitty.com - 2017-01-13 08:43 -0800
              Re: Correct way of doing string manipulation functions Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-13 09:45 -0800
                Re: Correct way of doing string manipulation functions Richard Heathfield <rjh@cpax.org.uk> - 2017-01-13 18:17 +0000
                Re: Correct way of doing string manipulation functions Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-13 10:30 -0800
                Re: Correct way of doing string manipulation functions Richard Heathfield <rjh@cpax.org.uk> - 2017-01-13 18:48 +0000
                Re: Correct way of doing string manipulation functions supercat@casperkitty.com - 2017-01-13 11:02 -0800
                Re: Correct way of doing string manipulation functions Richard Heathfield <rjh@cpax.org.uk> - 2017-01-13 19:15 +0000
                Re: Correct way of doing string manipulation functions supercat@casperkitty.com - 2017-01-13 12:18 -0800
                Re: Correct way of doing string manipulation functions Keith Thompson <kst-u@mib.org> - 2017-01-13 12:14 -0800
                Re: Correct way of doing string manipulation functions supercat@casperkitty.com - 2017-01-13 12:49 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-13 13:19 -0800
                Re: Correct way of doing string manipulation functions supercat@casperkitty.com - 2017-01-13 10:46 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-13 11:55 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-13 10:41 -0800
                Re: Correct way of doing string manipulation functions supercat@casperkitty.com - 2017-01-13 10:51 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-13 12:28 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-13 13:07 -0800
                Re: Correct way of doing string manipulation functions supercat@casperkitty.com - 2017-01-13 13:29 -0800
                Re: Correct way of doing string manipulation functions BartC <bc@freeuk.com> - 2017-01-13 21:45 +0000
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-13 13:59 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-13 15:08 -0800
                Re: Correct way of doing string manipulation functions supercat@casperkitty.com - 2017-01-13 14:13 -0800
                Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-13 13:47 -0800
            Re: Correct way of doing string manipulation functions fir <profesor.fir@gmail.com> - 2017-01-13 08:02 -0800
      Re: Correct way of doing string manipulation functions supercat@casperkitty.com - 2017-01-12 07:50 -0800
      Re: Correct way of doing string manipulation functions Keith Thompson <kst-u@mib.org> - 2017-01-12 08:37 -0800
        Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-12 09:41 -0800
    Re: Correct way of doing string manipulation functions Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-13 17:39 -0800
      Re: Correct way of doing string manipulation functions "Fernando D. Bozzo" <fdbozzo@gmail.com> - 2017-01-13 18:02 -0800

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