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Groups > comp.lang.c > #70188

Re: printf format specifier changes

From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: printf format specifier changes
Date 2015-09-10 12:47 -0700
Organization None to speak of
Message-ID <lnegi6njjj.fsf@kst-u.example.com> (permalink)
References (1 earlier) <mnebi4$p4v$1@dont-email.me> <1fe6466f-8b9c-40e6-8ce3-a5209c08fa7a@googlegroups.com> <msh2au$psh$1@dont-email.me> <7d3eddec-100b-4362-a987-c365c5a414cf@googlegroups.com> <1b85e519-b814-4570-b61b-705aca90a1b4@googlegroups.com>

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supercat@casperkitty.com writes:
> On Sunday, September 6, 2015 at 6:21:49 AM UTC-5, Malcolm McLean wrote:
>> But safety isn't much of an issue either. A bug in a call to printf()
>> will usually be very quickly found because it's only one step away
>> from the faulty output.  The exception is where you pass a dynamic
>> format string and someone embeds the wrong conversion types in it,
>> often by falling to escape %.
>
> Because ANSI failed to specify a means by which a prototype can
> indicate whether a variadic function expects floating-point values to
> be passed as double or long double, a significant fraction--perhaps a
> majority--of code which uses "long double" values and prints them uses
> the wrong format string, and only works because the its platform made
> "long double" and "double" synonymous, and regards the undefined "%lf"
> as a synonym for and "%f".

ANSI (and ISO) specified that float arguments are promoted to double,
and double and long double arguments are passed as they are.  The rules
are unambiguous.

Would you want to make it impossible to pass both double and long double
arguments to the same variadic function?  For example, this is perfectly
legal:

    float f = 1.0;
    double d = 2.0;
    long double ld = 3.0;
    printf("%f %f %Lf\n", f, d, ld);

Certainly some code gets format strings wrong.  I'm skeptical that most
code does so.  (I'm not sure how much code even uses long double; double
is sufficient for most purposes.)

C99 made "%lf" synonymous with "%f".

I think making double and long double the same size is mostly a Windows
thing; gcc gives long double a greater size, range, and precision than
double.

[...]

> C was designed with the intention that all floating-point values
> passed to a variadic function should be coerced to the same format; if
> ANSI C had kept that tradition, code could be written to achieve
> optimal semantics (wasting minimal time on conversions) when used with
> nearly all compilers for platforms with and without FPUs.
> Unfortunately, the fact that "printf" requires code to distinguish
> between "double" and longer floating-point types, and does so in a way
> that much if not most code gets wrong, compelled many compiler vendors
> to omit from the language the type which is yielded by operations on
> floating point values.

Pre-ANSI C only had float and double.  ANSI added long double.
On some implementations, long double might be substantially less
efficient than double, so promoting double arguments to long double
could be inefficient.

Remembering to use "%f" for double and "%Lf" for long double should be
no more difficult than remembering to use "%d", "%ld", and "%lld" for
int, long, and long long.

-- 
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

printf format specifier changes "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-06 09:04 -0700
  Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-06 17:50 +0100
    Re: printf format specifier changes "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-06 11:34 -0700
      Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-06 19:47 +0100
        Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-06 19:53 +0100
          Re: printf format specifier changes "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-06 11:57 -0700
      Re: printf format specifier changes "James Harris" <james.harris.1@gmail.com> - 2015-09-06 10:53 +0100
        Re: printf format specifier changes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-09-06 04:21 -0700
          Re: printf format specifier changes "James Harris" <james.harris.1@gmail.com> - 2015-09-07 10:40 +0100
            Re: printf format specifier changes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-09-07 03:07 -0700
              Re: printf format specifier changes "James Harris" <james.harris.1@gmail.com> - 2015-09-07 15:09 +0100
            Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-09-07 12:03 +0100
              Re: printf format specifier changes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-09-07 12:46 +0100
                Re: printf format specifier changes "James Harris" <james.harris.1@gmail.com> - 2015-09-07 15:24 +0100
              Re: printf format specifier changes "James Harris" <james.harris.1@gmail.com> - 2015-09-07 15:16 +0100
                Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-09-07 17:09 +0100
                Re: printf format specifier changes "James Harris" <james.harris.1@gmail.com> - 2015-09-07 17:47 +0100
                Re: printf format specifier changes Richard Damon <Richard@Damon-Family.org> - 2015-09-07 13:29 -0400
                Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-09-08 12:17 +0100
                Re: printf format specifier changes Robert Wessel <robertwessel2@yahoo.com> - 2015-09-08 10:13 -0500
                Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-09-08 17:09 +0100
                Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-09-08 10:08 -0700
                Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-09-08 19:11 +0100
                Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-09-08 12:24 -0700
                Re: printf format specifier changes Robert Wessel <robertwessel2@yahoo.com> - 2015-09-08 14:56 -0500
                Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-09-08 22:18 +0100
                Re: printf format specifier changes Ian Collins <ian-news@hotmail.com> - 2015-09-09 09:29 +1200
                Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-09-08 22:58 +0100
                Re: printf format specifier changes Ian Collins <ian-news@hotmail.com> - 2015-09-09 10:32 +1200
                Re: printf format specifier changes supercat@casperkitty.com - 2015-09-08 15:57 -0700
                Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-09-08 15:12 -0700
                Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-09-08 23:45 +0100
                Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-09-08 16:36 -0700
                Re: printf format specifier changes Robert Wessel <robertwessel2@yahoo.com> - 2015-09-08 19:17 -0500
                Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-09-09 09:56 +0100
                Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-09-09 11:33 -0700
                Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-09-09 20:35 +0100
                Re: printf format specifier changes Robert Wessel <robertwessel2@yahoo.com> - 2015-09-08 14:51 -0500
                Re: printf format specifier changes David Brown <david.brown@hesbynett.no> - 2015-09-08 22:41 +0200
              Re: printf format specifier changes Richard Damon <Richard@Damon-Family.org> - 2015-09-07 12:03 -0400
          Re: printf format specifier changes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-09-10 06:33 +0000
            Re: printf format specifier changes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-09-10 02:38 -0700
          Re: printf format specifier changes supercat@casperkitty.com - 2015-09-10 07:01 -0700
            Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-09-10 12:47 -0700
              Re: printf format specifier changes supercat@casperkitty.com - 2015-09-10 14:08 -0700
                Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-09-10 15:17 -0700
                Re: printf format specifier changes supercat@casperkitty.com - 2015-09-10 16:02 -0700
                Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-09-10 17:31 -0700
                Re: printf format specifier changes supercat@casperkitty.com - 2015-09-11 15:43 -0700
                Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-09-11 16:23 -0700
                Re: printf format specifier changes Robert Wessel <robertwessel2@yahoo.com> - 2015-09-11 23:33 -0500
                Re: printf format specifier changes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-11 21:44 -0700
                Re: printf format specifier changes supercat@casperkitty.com - 2015-09-12 09:02 -0700
                Re: printf format specifier changes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-26 15:00 -0700
                Re: printf format specifier changes supercat@casperkitty.com - 2015-09-27 11:21 -0700
                Re: printf format specifier changes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-29 10:10 -0700
        Re: printf format specifier changes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-09-06 13:41 +0100
          Re: printf format specifier changes Robert Wessel <robertwessel2@yahoo.com> - 2015-09-07 00:23 -0500
          Re: printf format specifier changes "James Harris" <james.harris.1@gmail.com> - 2015-09-07 11:31 +0100
            Re: printf format specifier changes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-09-07 03:54 -0700
    Re: printf format specifier changes Richard Heathfield <rjh@cpax.org.uk> - 2015-07-06 19:44 +0100
      Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-06 22:14 +0100
        Re: printf format specifier changes Robert Wessel <robertwessel2@yahoo.com> - 2015-07-06 22:43 -0500
        Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-07-06 23:01 -0700
          Re: printf format specifier changes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-07 06:43 +0000
          Re: printf format specifier changes Phil Carmody <pc+usenet@asdf.org> - 2015-07-09 10:52 +0300
            Re: printf format specifier changes Richard Heathfield <rjh@cpax.org.uk> - 2015-07-09 09:26 +0100
              Re: printf format specifier changes Phil Carmody <pc+usenet@asdf.org> - 2015-07-12 12:56 +0300
                Re: printf format specifier changes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-12 06:12 -0700
        Re: printf format specifier changes Rosario19 <Ros@invalid.invalid> - 2015-07-07 10:22 +0200
          Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-07 10:10 +0100
      Re: printf format specifier changes Philip Lantz <prl@canterey.us> - 2015-07-07 00:54 -0700
    Re: printf format specifier changes Nobody <nobody@nowhere.invalid> - 2015-07-07 15:08 +0100
      Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-07 16:12 +0100
        Re: printf format specifier changes raltbos@xs4all.nl (Richard Bos) - 2015-07-09 10:34 +0000
          Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-09 12:25 +0100
            Re: printf format specifier changes Phil Carmody <pc+usenet@asdf.org> - 2015-07-12 13:02 +0300
              Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-12 11:23 +0100
    Re: printf format specifier changes <william@wilbur.25thandClement.com> - 2015-07-07 11:50 -0700
      Re: printf format specifier changes "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-07 12:14 -0700
    Re: printf format specifier changes Les Cargill <lcargill99@comcast.com> - 2015-07-07 21:52 -0500
      Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-07-07 20:42 -0700
        Re: printf format specifier changes Phil Carmody <pc+usenet@asdf.org> - 2015-07-09 11:12 +0300
          Re: printf format specifier changes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-09 05:03 -0700
            Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-09 13:10 -0500
          Re: printf format specifier changes James Kuyper <jameskuyper@verizon.net> - 2015-07-09 08:58 -0400
          Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-07-09 08:10 -0700
            Re: printf format specifier changes Phil Carmody <pc+usenet@asdf.org> - 2015-07-12 12:58 +0300
              Re: printf format specifier changes James Kuyper <jameskuyper@verizon.net> - 2015-07-12 12:57 -0400
              Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-07-12 11:14 -0700
                Re: printf format specifier changes gazelle@shell.xmission.com (Kenny McCormack) - 2015-07-12 18:26 +0000
      Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-08 13:09 +0100
      Re: printf format specifier changes James Kuyper <jameskuyper@verizon.net> - 2015-07-08 08:10 -0400
        Re: printf format specifier changes Les Cargill <lcargill99@comcast.com> - 2015-07-08 08:12 -0500
          Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-07-08 08:38 -0700
          Re: printf format specifier changes James Kuyper <jameskuyper@verizon.net> - 2015-07-08 12:00 -0400
            Re: printf format specifier changes Les Cargill <lcargill99@comcast.com> - 2015-07-08 17:20 -0500
      Re: printf format specifier changes Phil Carmody <pc+usenet@asdf.org> - 2015-07-09 11:08 +0300
    Re: printf format specifier changes Phil Carmody <pc+usenet@asdf.org> - 2015-07-08 20:30 +0300
      Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-08 18:56 +0100
        Re: printf format specifier changes gordonb.lmwiv@burditt.org (Gordon Burditt) - 2015-07-09 11:09 -0500
          Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-09 14:05 -0500
      Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-07-08 11:27 -0700
        Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-08 15:07 -0500
  Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-06 23:39 -0500
    Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-07 10:19 +0100
      Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-07 07:57 -0500
        Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-07 14:15 +0100
          Re: printf format specifier changes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-07 14:46 +0100
          Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-07 09:37 -0500
        Re: printf format specifier changes Phil Carmody <pc+usenet@asdf.org> - 2015-07-09 11:24 +0300
          Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-09 09:55 -0500
            Re: printf format specifier changes <william@wilbur.25thandClement.com> - 2015-07-09 11:53 -0700
              Re: printf format specifier changes <william@wilbur.25thandClement.com> - 2015-07-09 13:48 -0700
              Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-09 17:01 -0500
    Re: printf format specifier changes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-07 10:59 +0100
      Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-07 07:48 -0500
        Re: printf format specifier changes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-07 14:30 +0100
          Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-07 10:05 -0500
            Re: printf format specifier changes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-07 23:16 +0100
              Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-07 20:29 -0500
              Re: printf format specifier changes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-08 00:43 -0700
              Re: printf format specifier changes Robert Wessel <robertwessel2@yahoo.com> - 2015-07-09 02:29 -0500
                Re: printf format specifier changes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-09 10:34 +0100
                Re: printf format specifier changes BGB <cr88192@hotmail.com> - 2015-07-09 09:35 -0500
          Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-07-07 08:37 -0700
            Re: printf format specifier changes "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-07 08:45 -0700
            Re: printf format specifier changes supercat@casperkitty.com - 2015-09-07 08:29 -0700
          Re: printf format specifier changes <william@wilbur.25thandClement.com> - 2015-07-07 12:12 -0700
  Re: printf format specifier changes pooja deshpande <namitadeshpande25@gmail.com> - 2015-07-09 09:29 -0700
    Re: printf format specifier changes "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-09 09:34 -0700
      Re: printf format specifier changes "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-09 09:59 -0700
        Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-09 22:33 +0100
          Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-07-09 15:17 -0700
            Re: printf format specifier changes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-10 00:30 +0000
              Re: printf format specifier changes Reinhardt Behm <rbehm@hushmail.com> - 2015-07-10 09:17 +0800
      Re: printf format specifier changes David Kleinecke <dkleinecke@gmail.com> - 2015-07-09 10:02 -0700
        Re: printf format specifier changes "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-09 10:19 -0700
      Re: printf format specifier changes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-09 21:50 +0100
        Re: printf format specifier changes Keith Thompson <kst-u@mib.org> - 2015-07-09 15:05 -0700
          Re: printf format specifier changes "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-09 15:09 -0700
          Re: printf format specifier changes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-10 00:28 +0100
            Re: printf format specifier changes raltbos@xs4all.nl (Richard Bos) - 2015-07-13 20:39 +0000
        Re: printf format specifier changes "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-09 15:07 -0700
          Re: printf format specifier changes Bartc <bc@freeuk.com> - 2015-07-09 23:16 +0100
            Re: printf format specifier changes "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-09 15:25 -0700

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