GNU bug report logs - #10782
zero padding does not work in case of strings

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Praveen A <pravi.a <at> gmail.com>
To: bug-coreutils <at> gnu.org
Subject: zero padding does not work in case of strings
Date: Sat, 11 Feb 2012 00:22:13 +0530

$ printf %04d 12
0012$ printf %4d 12
  12$ printf %04s 12
  12$ printf %4s 12
  12$

It would be very useful if it works for strings too (I found this when
trying to zero pad hex values in an ipv6 address). Now it requires an
additional sed find and replace to get the desired result.

Thanks
Praveen
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Message #12 received at 10782-done <at> debbugs.gnu.org (full text, mbox):

From: Eric Blake <eblake <at> redhat.com>
To: Praveen A <pravi.a <at> gmail.com>
Cc: 10782-done <at> debbugs.gnu.org
Subject: Re: bug#10782: zero padding does not work in case of strings
Date: Fri, 10 Feb 2012 12:29:53 -0700

[Message part 1 (text/plain, inline)]

tag 10782 notabug
thanks

On 02/10/2012 11:52 AM, Praveen A wrote:
> $ printf %04d 12
> 0012$ printf %4d 12
>   12$ printf %04s 12
>   12$ printf %4s 12
>   12$

Thanks for the report.  In printf(3), POSIX states the use of the 0
modifier with %s is undefined:

http://pubs.opengroup.org/onlinepubs/9699919799/functions/printf.html

"0    For d , i , o , u , x , X , a , A , e , E , f , F , g , and G
conversion specifiers.... For other conversions, the behavior is undefined."

Note that glibc silently ignores %04x, and gcc correctly warns about it
being fishy:

$ cat foo.c
#include <stdio.h>
int main() { printf("%04s\n", "hi"); return 0; }
$ gcc -o foo foo.c -Wall
foo.c: In function ‘main’:
foo.c:2:1: warning: '0' flag used with ‘%s’ gnu_printf format [-Wformat]
$ ./foo
  hi

> 
> It would be very useful if it works for strings too (I found this when
> trying to zero pad hex values in an ipv6 address). Now it requires an
> additional sed find and replace to get the desired result.

Note that you didn't even test coreutils - you tested the built-in
printf of your shell.  Coreutils intentionally errors out on this
undefined behavior, to make it obvious that there is no sane documented
way to implement it:

$ /usr/bin/printf %04s 12
/usr/bin/printf: %04s: invalid conversion specification

Implementing %04s in coreutils, but not also in glibc, bash, dash, ksh,
and so forth, all at the same time, would just cause user confusion due
to the inconsistencies; plus, you would have to wait for those fixes to
propagate into your machine before you could rely on it.  Rather, you
should patch your script to use %d or %x or other tricks in a manner
that works today:

# Use $(()) to convert hex to decimal, then 0-pad it back out to hex
$ printf %04x $((0x12))
0012

# given an arbitrary string in $1, add 0-padding if needed to extend
# it to at least 4 characters
$ f() {
  case ${#1} in
    0|1|2|3) printf %0$((4-${#1}))d%s 0 "$1" ;;
    *) printf %s "$1" ;;
  esac
}
$ f 12
0012

As such, I'm closing this bug report as something we won't change.

-- 
Eric Blake   eblake <at> redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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