Re: pack() returns an unusable string

Parameters can't contain the NUL character. Your so-called working code breaks if you change 2.4 to 2.5 (which generates at least one a NUL byte).

I recommend that you pass the hex of the packed string. You could also use base64, JSON, etc, etc, etc.

Furthermore, you have a code-injection bug. "'$p'" is not an appropriate way to generate a Perl string literal. $p could easily contain byte 0x27 (single quote). A trailing byte 0x5C (backslash) would also be problematic.

I recommend that you pass the value as an argument.

use strict;
use warnings;
my $template = 'd<';
my $nv = 2.5;
my $p = pack $template, $nv;

system $^X, '-wle', 'print $ARGV[0]', unpack('H*', $p);
[download]

Seeking work! You can reach me at ikegami@adaelis.com

Comment on Re: pack() returns an unusable string Select or Download Code

Replies are listed 'Best First'.
Re^2: pack() returns an unusable string by syphilis (Archbishop) on May 29, 2021 at 08:53 UTC
I recommend that you pass the value as an argument What I was wanting to do was to pass the actual string as the argument. It didn't really occur to me that doing that would be such a problem. I thought that people would be able to show me that this could be achieved with little fuss. (I was wrong ;-) What I was interested in doing is easily demonstrated in the following script, where I've used the code that you supplied, but have replaced the last line with 2 lines: `use strict; use warnings; my $template = 'D<'; my $nv = 2.4; my $p = pack $template, $nv; # I'm running this script on perl-5.34.0 # with nvtype of 'long double'. # I want to see the result of perl-5.34.0 # with nvtype of 'double' unpacking $p : my $perl = "C:/perl-5.34.0/bin/MSWin32-x64-multi-thread/perl.exe"; system $perl, '-wle', 'print unpack("H", $p)';` [download] If that DWIMmed, it would output : `9a999999999999990040000000000000` [download] but, of course, all that it emits is the warning: `Use of uninitialized value $_ in print at -e line 1.` [download] Sure, I could take a leaf out of your book and alter my last line to: `system $perl, '-wle', 'print $ARGV[0]', unpack('H', $p);` [download] But that `unpack('H*', $p)` is being evaluated by the 'long double' build of perl. I wanted to see what happened when it was evaluated by my 'double' build of perl-5.34.0. I've since accomplished this by other means. Cheers, Rob	[reply] [d/l] [select]
Re^3: pack() returns an unusable string by ikegami (Patriarch) on May 29, 2021 at 23:19 UTC
`unpack "H"` hasn't changed, so what you're saying makes no sense. Encode the string in the parent (say using `unpack "H"`), then recreate the original in the child if so you desire (using `pack "H*"`). Seeking work! You can reach me at ikegami@adaelis.com	[reply] [d/l] [select]
Re^4: pack() returns an unusable string by syphilis (Archbishop) on May 30, 2021 at 08:46 UTC
unpack "H" hasn't changed, so what you're saying makes no sense.* Well - you apparently knew that, but I didn't. I was fairly certain it would be the case and that it should be the case, but I wanted to verify it with an actual example, and your approach does not provide that verification. (Admittedly, it is pretty hard to see how anything else could be the case ;-) I was also wanting to do the same thing, but replacing "H" with "D" - in order to verify that rounding would be performed as expected. (It is ... so long as one's expectation is correct.) Wrt perl-5.34.0, unlike `unpack("H", $string)` we see that the value returned by `unpack("D", $string)` will often vary between 'double' and 'long double' builds, for the very same $string. I don't doubt that you regard running those checks as also being something that "makes no sense". But having a "D" template on a perl whose nvtype is 'double' is a little disconcerting to me, and I just want to make sure that its behaviour is clear in my mind. Cheers, Rob	[reply] [d/l] [select]
Re^5: pack() returns an unusable string by ikegami (Patriarch) on May 31, 2021 at 17:58 UTC
Re^6: pack() returns an unusable string by syphilis (Archbishop) on May 31, 2021 at 23:31 UTC
Some notes below your chosen depth have not been shown here
Re^5: pack() returns an unusable string by ikegami (Patriarch) on May 31, 2021 at 01:10 UTC


Just another Perl shrine
	PerlMonks