[Cfp-interest 1512] Re: Question about strfromd in glibc

[Jim Thomas]

The reason we have the strfrom functions is to support 754’s convertToDecimalCharacter and convertToHexCharacter operations. The alternative of just saying snprintf supports them has these drawbacks:

(1) snprintf is from <stdio.h> which need not be supported by freestanding implementations. Freestanding implementations (that define __STDC_IEC_60559_BFP__ or __STDC_IEC_60559_DFP__) do need to support the numeric conversion functions in <stdlib.h> which include the strfrom functions.

(2) We’d need to invent length modifiers for the interchange and extended types.

(3) We’d have to figure out how to support the convertTo*Character operations for non-arithmetic formats (which don’t have associated types), either by further extending snprintf or by some other mechanism.

(4) snprintf has a much more complicated (Swiss Army Knife) interface than 754’s description of the convertTo*Character operations would suggest.

- JIm Thomas

> On Feb 20, 2020, at 5:29 AM, Mike Cowlishaw <mfc at speleotrove.com> wrote:
> 
> Good to know!    I think I'll just stick to snprintf,all the same.  :-)
>  
> Mike
> 
>> From: cfp-interest-bounces at oakapple.net [mailto:cfp-interest-bounces at oakapple.net] On Behalf Of Rajan Bhakta
>> Sent: 20 February 2020 02:05
>> To: CFP
>> Subject: [Cfp-interest 1507] Fw: Re: Question about strfromd in glibc
>> 
>> So it seems glibc does do the right thing (null terminate) and the web page was wrong.
>> 
>> Regards,
>> 
>> Rajan Bhakta
>> z/OS XL C/C++ Compiler Technical Architect
>> ISO C Standards Representative for Canada, PL22.11 Chair (USA)
>> C Compiler Development
>> Contact: rbhakta at us.ibm.com, Rajan Bhakta/Houston/IBM
>> ----- Forwarded by Rajan Bhakta/Houston/IBM on 02/19/2020 08:04 PM -----
>> 
>> From:        Joseph Myers <joseph at codesourcery.com>
>> To:        Rajan Bhakta <rbhakta at us.ibm.com>
>> Date:        02/19/2020 04:20 PM
>> Subject:        [EXTERNAL] Re: Question about strfromd in glibc
>> 
>> 
>> 
>> On Wed, 19 Feb 2020, Rajan Bhakta wrote:
>> 
>> > Hi Joseph,
>> > 
>> > Looking at http://man7.org/linux/man-pages/man3/strfromd.3.html <http://man7.org/linux/man-pages/man3/strfromd.3.html>, it seems 
>> > to say you could end up with strings that are not null terminated. But 
>> > from the C standard draft (N2454 for example) it says that the strings are 
>> > always null terminated and only that you may get incomplete (truncation 
>> > from the right) if 'n' is too small (yet still null terminated). Was this 
>> > an intentional decision or am I understanding the documentation wrong?
>> 
>> The strfrom functions are equivalent to particular calls to snprintf, so 
>> they should always null-terminate their output (when a nonzero size is 
>> specified).  The manpages are not official glibc documentation.
>> 
>> -- 
>> Joseph S. Myers
>> joseph at codesourcery.com
>> 
>> 
>> 
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