[php-maint] Bug#613982: printf generates scientific notation inaccurately

Ronan mr_platelet+jin6vr at fastmail.fm
Fri Feb 18 17:03:08 UTC 2011

Package: php5-cli
Version: 5.3.3-7
Severity: normal

printf, when outputting a number in scientific
notation, allows the precision to be specified.
However, the precision is capped at 40.  If a cap
has to be set at all, then it should be set at 52,
because this is the smallest value which allows all
IEEE double-precision floating-point numbers to be
output with perfect accuracy.

I've included a terminal transcript, in the file
sprintf-transcript, which shows a small PHP program
and its output.  The important point is that the
second number gives us 12 more bits of precision
than the first.

*** sprintf-transcript
Script started on Fri 18 Feb 2011 16:57:47 GMT
bash$ PS1='>>>>>>>>>>>> ' # make the prompt stand out more
>>>>>>>>>>>> cat demonstrate 
#!/usr/bin/php5 -n

  PHP will emit a warning about truncating
  the precision to 40 digits
printf("%.52e\n", 1 + pow(2, -52));

  this scale is large enough to allow the
  calculation below to be performed without
  any loss of precision

print bcadd(1, bcpow(2, -52)) . "\n";
>>>>>>>>>>>> php5 -n demonstrate 

Notice: printf(): Requested precision of 52 digits was truncated to PHP maximum of 40 digits in /home/ronan/work/php-bugs/sprintf/demonstrate on line 9
>>>>>>>>>>>> exit

Script done on Fri 18 Feb 2011 16:58:00 GMT

-- System Information:
Debian Release: 6.0
  APT prefers stable
  APT policy: (500, 'stable')
Architecture: i386 (i686)

Kernel: Linux 2.6.26-2-686 (SMP w/1 CPU core)
Locale: LANG=en_IE.UTF-8, LC_CTYPE=en_IE.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages php5-cli depends on:
ii  libbz2-1.0              1.0.5-6          high-quality block-sorting file co
ii  libc6                   2.11.2-10        Embedded GNU C Library: Shared lib
ii  libcomerr2              1.41.12-2        common error description library
ii  libdb4.8                4.8.30-2         Berkeley v4.8 Database Libraries [
ii  libgssapi-krb5-2        1.8.3+dfsg-4     MIT Kerberos runtime libraries - k
ii  libk5crypto3            1.8.3+dfsg-4     MIT Kerberos runtime libraries - C
ii  libkrb5-3               1.8.3+dfsg-4     MIT Kerberos runtime libraries
ii  libmagic1               5.04-5           File type determination library us
ii  libonig2                5.9.1-1          Oniguruma regular expressions libr
ii  libpcre3                8.02-1.1         Perl 5 Compatible Regular Expressi
ii  libqdbm14               1.8.77-4         QDBM Database Libraries [runtime]
ii  libssl0.9.8             0.9.8o-4squeeze1 SSL shared libraries
ii  libxml2                 2.7.8.dfsg-2     GNOME XML library
ii  mime-support            3.48-1           MIME files 'mime.types' & 'mailcap
ii  php5-common             5.3.3-7          Common files for packages built fr
ii  tzdata                  2010o-1          time zone and daylight-saving time
ii  ucf                     3.0025+nmu1      Update Configuration File: preserv
ii  zlib1g                  1: compression library - runtime

php5-cli recommends no packages.

Versions of packages php5-cli suggests:
ii  php-pear                      5.3.3-7    PEAR - PHP Extension and Applicati

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