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Re: How to copy a string safely and efficiently in C?

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In article <87lgyvpxno.fsf@bsb.me.uk>,
Ben Bacarisse  <ben.usenet@bsb.me.uk> wrote:
> [snip]
>
>You might consider using
>
>  memcpy(thing->field, (char [sizeof thing->field]){"N/A"}, sizeof thing->field);
>
>since you get the padding and the size automatically.  It can always be
>turned into a macro to remove the repetition.

I was curious about this, as it struck me that a decent compiler
would likely generate the same code for both of these.  For strings
longer than the destination this was more or less true with GCC 5.4
and Clang.  The generated machine code from GCC was identical; in
clang, the compound literal truncates the string data in the text
section.

Interestingly, in the case where the source was NOT longer than
the destination buffer, clang always calls strncpy (though it does
optimize it as a tail call); the memcpy case is optimized to a couple
of load instructions, depending on the destination buffer size.

For non-overflow, GCC optimizes out the calls to both strncpy() and
memcpy().

>> rather than
>>
>>              strncpy(thing->field, "N/A", THING_FIELD_SIZE);.
>
>On advantage of memcpy + compound literal is that if you use a string
>literal that is too long, you will most likely get a message from the
>compiler.  The strncpy won't go wrong, of course, but a string longer
>than THING_FIELD_SIZE is almost certainly a programmer error you'd like
>to know about.

Indeed.  That's a useful diagnostic.  It also generates faster code
on some compilers.

>> Note that the former version of the code will break if the thing field
>> size grows, **and using a macro for the size wouldn't help**, while the
>> latter version will automatically adapt to any increase in field size.
>
>That's the advantage of using a compound literal.

	- Dan C.

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How to copy a string safely and efficiently in C? will hunt <glovepm@gmail.com> - 2016-09-01 18:50 -0700
  Re: How to copy a string safely and efficiently in C? John Gordon <gordon@panix.com> - 2016-09-02 01:58 +0000
    Re: How to copy a string safely and efficiently in C? will hunt <glovepm@gmail.com> - 2016-09-01 19:40 -0700
      Re: How to copy a string safely and efficiently in C? Geoff <geoff@invalid.invalid> - 2016-09-01 20:29 -0700
        Re: How to copy a string safely and efficiently in C? James Kuyper <jameskuyper@verizon.net> - 2016-09-02 12:29 -0400
      Re: How to copy a string safely and efficiently in C? asetofsymbols@gmail.com - 2016-09-12 13:09 -0700
        Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-12 15:30 -0700
    Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-01 23:08 -0700
    Re: How to copy a string safely and efficiently in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-04 23:59 -0700
  Re: How to copy a string safely and efficiently in C? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-09-01 21:12 -0600
    Re: How to copy a string safely and efficiently in C? <william@wilbur.25thandClement.com> - 2016-09-01 22:12 -0700
  Re: How to copy a string safely and efficiently in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-01 23:58 -0700
  Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-02 10:42 +0100
    Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-04 22:23 -0700
      Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-04 23:20 -0700
        Re: How to copy a string safely and efficiently in C? mark.bluemel@gmail.com - 2016-09-05 01:03 -0700
          Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-05 01:46 -0700
        Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-05 10:30 +0100
          Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-10 11:25 -0700
        Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-11 06:23 -0700
          Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-11 15:41 -0700
            Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-11 23:40 -0700
              Re: How to copy a string safely and efficiently in C? Ian Collins <ian-news@hotmail.com> - 2016-09-12 21:04 +1200
                Re: How to copy a string safely and efficiently in C? Melzzzzz <Melzzzzz@zzzzz.com> - 2016-09-12 09:21 +0000
                Re: How to copy a string safely and efficiently in C? Ian Collins <ian-news@hotmail.com> - 2016-09-12 21:33 +1200
                Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-12 10:41 +0100
                Re: How to copy a string safely and efficiently in C? Melzzzzz <Melzzzzz@zzzzz.com> - 2016-09-12 10:48 +0000
                Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-12 07:04 -0700
  Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-02 12:36 +0100
    Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-02 14:24 +0100
      Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-02 14:51 +0100
      Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-04 23:42 -0700
  Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-02 18:05 +0100
    Re: How to copy a string safely and efficiently in C? Ike Naar <ike@iceland.freeshell.org> - 2016-09-02 19:48 +0000
  Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 01:45 +0000
    Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 11:06 +0100
      Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:09 +0000
        Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 15:30 +0100
          Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:52 +0000
            Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:54 +0000
              Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 17:18 +0100
                Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 18:15 +0000
              Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-13 20:30 -0700
                Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-15 14:03 +0000
                Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-15 12:32 -0700
                Re: How to copy a string safely and efficiently in C? luser droog <luser.droog@gmail.com> - 2016-09-15 12:45 -0700
                Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-15 13:00 -0700
                Re: How to copy a string safely and efficiently in C? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-09-15 14:52 -0600
                Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 16:04 -0700
                Re: How to copy a string safely and efficiently in C? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-09-15 15:52 -0600
                Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 17:07 -0700
                Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 15:53 -0700
                Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-15 22:17 +0100
                Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 17:01 -0700
                Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-15 16:36 -0700
                Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 19:19 -0700
                Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-15 21:39 -0700
                Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-16 06:17 -0700
                Re: How to copy a string safely and efficiently in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-16 07:04 -0700
                Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-16 09:11 -0700
                Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-22 22:24 -0700
                Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-19 15:15 +0000
    Re: How to copy a string safely and efficiently in C? Richard Damon <Richard@Damon-Family.org> - 2016-09-13 08:19 -0400
      Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:07 +0000
        Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-13 07:57 -0700
          Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 17:33 +0100
            Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 18:13 +0000
            Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-13 11:30 -0700
              Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 20:03 +0000
                Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-13 13:38 -0700
                Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 21:39 +0000
              Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 23:26 +0100
            Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-13 16:14 -0700
    Re: How to copy a string safely and efficiently in C? BartC <bc@freeuk.com> - 2016-09-13 15:59 +0100
    Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-13 16:07 -0700
      Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-14 00:24 +0000

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