Path: csiph.com!feeder.erje.net!2.eu.feeder.erje.net!eternal-september.org!feeder.eternal-september.org!reader02.eternal-september.org!.POSTED!not-for-mail
From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Updating pointer with result of sprintf
Date: Tue, 28 Nov 2017 08:58:21 -0800
Organization: None to speak of
Lines: 37
Message-ID: <lnd142z6fm.fsf@kst-u.example.com>
References: <ovjd76$sbt$1@dont-email.me> <ovjg4f$gcn$1@dont-email.me>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Injection-Info: reader02.eternal-september.org; posting-host="0a617e3b507af251ad64dcde7ced884b"; logging-data="30381"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/Rtj2ow0q6sLTpofE3b1Pz"
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)
Cancel-Lock: sha1:ykmm8i+r0zOZSoCOKEh75mpXws4= sha1:jO8vURB8mKHDNPLSNbjm+o/APX8=
Xref: csiph.com comp.lang.c:123574

jacobnavia <jacob@jacob.remcomp.fr> writes:
> Le 28/11/2017 à 11:19, Noob a écrit :
>> buf += sprintf(buf, "%02x", src[i]);
>
> If sprintf fails, it usually returns -1 (EOF)

N1570 7.21.6.6p3:
    The sprintf function returns the number of characters written
    in the array, not counting the terminating null character,
    or a negative value if an encoding error occurred.

Don't assume that the error return is EOF.

> Why would it fail?
>
> Memory constraints, being unable to allocate some temporary space, for 
> instance.

The only documented failure condition for sprintf is an encoding error
(7.21.3p14).  In this particular case, you can be sure that an encoding
error cannot occur.  Any other failure would be the result of undefined
behavior.  It *might* be worth checking that the value returned by
sprintf() is non-negative, just in case the implementation handles other
errors cleanly, but if such an error occurs then something has gone
seriously wrong.

> That is very unlikely, but it *could* happen. In that case your pointer
> loses the last character written or it becomes invalid (supposing buf-1 
> is outside the bounds of buf).

Or buf-N for any arbitrary N.

-- 
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"