Data::Dumper and printf

Trihedralguy has asked for the wisdom of the Perl Monks concerning the following question:

I was wondering why the interaction between Data::Dumper and printf is different between an array and an array as ref. See Example:


#!/usr/local/bin/perl
use strict;
use Data::Dumper;


my @array = (1, 2, 3, 4);

printf "As Array:\n";
printf Dumper(@array);
printf "As Reference to Array:\n";
printf Dumper(\@array);
[download]

"print" doesnt seem to have any problems, "printf" doesnt seem to return the proper results...But I want to know why!

Comment on Data::Dumper and printf Download Code

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Re: Data::Dumper and printf by kennethk (Abbot) on Nov 20, 2012 at 22:52 UTC
The issue is that printf expects a format string as the first argument (thus the 'f'). When you pass the array reference to `Dumper`, it returns a single string, which `printf` interprets as its format string. When you pass `Dumper` the array, it returns an array of strings and so `printf` thinks the first argument is the format, and thus only prints that. #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.	[reply] [d/l] [select]
Re^2: Data::Dumper and printf by Anonymous Monk on Nov 21, 2012 at 05:50 UTC
And using printf like that forms a class of vulnerabilities known as format string vulnerabilities, in C and other languages that pass to the libc function directly. The interpreted languages generally don't and are safe from this. It causes a denial of service (program crash) most of the time.	[reply]
Re^2: Data::Dumper and printf by Trihedralguy (Pilgrim) on Nov 21, 2012 at 00:05 UTC
That is what I was thinking, but I wasn't completely sure that this was the answer to the question. One of my peers asked me why this was working the way it was and I wasn't 100% sure what the correct answer was. Thank you for your explanation.	[reply]
Re^3: Data::Dumper and printf by LanX (Saint) on Nov 21, 2012 at 17:05 UTC
one confusing thing is that printf and sprintf have different prototypes! `DB<108> prototype "CORE::sprintf" => "\$\@" DB<109> prototype "CORE::printf" => undef` [download] so `sprintf @array` will evaluate the array in scalar context, such that now the number of elements will be interpreted as format string... Cheers Rolf	[reply] [d/l] [select]
Re: Data::Dumper and printf by ww (Archbishop) on Nov 20, 2012 at 22:51 UTC
For starters, `printf` is NOT a slide-in substitute for `print.` C:\>perldoc -f printf printf FILEHANDLE FORMAT, LIST printf FILEHANDLE printf FORMAT, LIST printf Equivalent to "print FILEHANDLE sprintf(FORMAT, LIST)", ex +cept that $\ (the output record separator) is not appended. The + first argument of the list will be interpreted as the "printf" f +ormat. See "sprintf" for an explanation of the format argument. I +f you omit the LIST, $_ is used; to use FILEHANDLE without a LIS +T, you must use a real filehandle like "FH", not an indirect one +like $fh. If "use locale" is in effect and POSIX::setlocale() h +as been called, the character used for the decimal separator +in formatted floating-point numbers is affected by the LC_NUM +ERIC locale setting. See perllocale and POSIX. Don't fall into the trap of using a "printf" when a simple "print" would do. The "print" is more efficient and less e +rror prone. [download]	[reply] [d/l] [select]