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

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: How to copy a string safely and efficiently in C?
Date	2016-09-01 23:08 -0700
Organization	None to speak of
Message-ID	<lninueal5q.fsf@kst-u.example.com> (permalink)
References	<0b30b0cc-f7c5-4928-8acb-56bf2364797b@googlegroups.com> <nqamcf$fni$1@reader2.panix.com>

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John Gordon <gordon@panix.com> writes:
> In <0b30b0cc-f7c5-4928-8acb-56bf2364797b@googlegroups.com> will hunt <glovepm@gmail.com> writes:
>
>> should I use strncpy or snprintf, and why?
>
> It seems like strncpy() would be the obvious choice.  snprintf() is
> meant for assembling an output string from one or more component
> parts, which doesn't seem like what you want.

Yes, strncpy() is the obvious choice.  It's also, most of the time, the
*wrong* choice.

I've written about strncpy() at some length here:

https://the-flat-trantor-society.blogspot.com/2012/03/no-strncpy-is-not-safer-strcpy.html

To summarize: The name implies that strncpy() is to strcpy() as
strncat() is to strcat(), i.e., that it's a "safer" version that will
not write past the end of the target array (as long as you correct
tell it how big the target array is).  The name is misleading.
In a very real sense, strncpy() isn't really a string function.
If the target array isn't big enough, it will not be null-terminated,
meaning that it will not contain a string.  Any further attempt to
treat it as a string can fail in arbitrarily bad ways.  (It also
pads the target with nulls when it *is* big enough, but that's just
a very minor inefficiency.)

This:

    target[0] = '\0';
    strncat(target, src, sizeof target);

is very close to what strncpy() probably *would* do if it really
were a "safer" strcpy().

But even that provides what may be only a false sense of security.

The fact that you're considering using strncpy or snprintf rather than
strcpy means that you're at least thinking about avoiding overflowing
the target array.  That's a very good thing.  But simply avoiding a
buffer overflow may not be enough.  You should ask yourself, if the
target array isn't big enough to hold a copy of the source, *what should
I do*?  If you use strncat() or snprintf(), that question is already
answered for you: the copy quietly truncates your data.

Maybe that's what you want.  But imagine, for example, that you're
copying a Unix command string:

    "rm -rf /home/username/unimportant_directory"

And suppose the target only has enough room to hold:

    "rm -rf /home/username"

and you then pass it to system().  (If you're not familiar with
Unix-like systems, the latter will attempt to remove your entire
home directory.)

Only you can decide, in the context of your program, what should
happen if a target array isn't big enough.  Sometimes quiet
truncation is the right choice, sometimes you can try to use a bigger
array, and sometimes you'd be better off terminating the program
with a fatal error message rather than continuing to operate on
incomplete data.

snprintf() has the advantage that it returns the number of
characters it *would* have copied if there were enough room, so
you can detect truncation.  (You still have to remember to check,
and to decide how to handle it.)  snprintf() was added in C99,
and in the past Microsoft's implementation didn't support it
well; I don't think that's a problem these days, but check your
implementation's documentation.

To summarize: Programming is hard.

-- 
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"

Thread

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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