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Groups > comp.lang.c > #66255

Re: trapping NaN to find uninitialised variables?

From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: trapping NaN to find uninitialised variables?
Date 2015-07-23 07:56 -0700
Organization None to speak of
Message-ID <lnmvymzzvl.fsf@kst-u.example.com> (permalink)
References <636d6775-affc-4f24-9394-f8b190c5174a@googlegroups.com>

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pedro1492@lycos.com writes:
> Open64 has flag -trapuv and gcc has -finit-real=nan
> The claim is that variables are created NaN, so if your program does
> not initialise them before use, then it bites you on the arse.
[...]

What is the context for that claim?

C never guarantees that floating-point objects are initialized
to NaN.

An FP object with static storage duration and no explicit
initialization is initialized to 0.0.  An object with automatic
storage duration, or allocated with malloc, is typically initialized
to whatever garbage was in that chunk of memory before it was
allocated.  An FP object allocated with calloc is initialized to
all-bits-zero, which is *probably* 0.0, but that's not guaranteed.

An implementation could make additional guarantees.  As for gcc, the
-finit-real=nan option is implemented only for Fortran, not for C.

-- 
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

trapping NaN to find uninitialised variables? pedro1492@lycos.com - 2015-07-23 01:42 -0700
  Re: trapping NaN to find uninitialised variables? Bartc <bc@freeuk.com> - 2015-07-23 10:35 +0100
  Re: trapping NaN to find uninitialised variables? Robert Wessel <robertwessel2@yahoo.com> - 2015-07-23 04:51 -0500
  Re: trapping NaN to find uninitialised variables? raltbos@xs4all.nl (Richard Bos) - 2015-07-23 11:03 +0000
  Re: trapping NaN to find uninitialised variables? Keith Thompson <kst-u@mib.org> - 2015-07-23 07:56 -0700
    Re: trapping NaN to find uninitialised variables? pedro1492@lycos.com - 2015-07-29 05:39 -0700
      Re: trapping NaN to find uninitialised variables? Keith Thompson <kst-u@mib.org> - 2015-07-29 08:58 -0700
  Re: trapping NaN to find uninitialised variables? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-23 10:39 -0700

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