Hi Monks

This behaviour is there with C/C++ "printf" also. We need to know how the "print" statement (see coments below ) is handling variables & its data. Our code in question is

my $x = 0;
print $x, $x++;  # Print 10??
[download]

All the arguments passed to the "print" is kept in a stack! print will push arguments from right-to-left into the stack. Inthe above case value of $x goes into stack (corresponding to $x++).

After the first "push", $x is incremented and on second "push" new value of $x is pushed. Then it start poping to print. For sure it will pop 1 first and then 0. So it will print 10. A program to demo this can be

#!/usr/bin/perl

use strict;


my @PRINT_STACK;
my $x = 0;

push @PRINT_STACK, $x++;
print "PRINT_STACK => ( ",@PRINT_STACK, " )\n";

push @PRINT_STACK, $x;
print "PRINT_STACK =>( ",join(", ",@PRINT_STACK)," )\n";

print pop @PRINT_STACK;
print pop @PRINT_STACK;


exit();
[download]

Again funny facts :)

my $x = 0;
print $x,$x, $x++;  # Print 110??
[download]

This will print 110 for sure, because the value of $x is NOT changed after first push.

Then what would be

my $x = 0;
print $x,$x++, $x++;  # Print 210??
[download]

In case of "C" it will print 210 as expected. but the output you will get in perl is 201. If I make a table with more $x++ in print, it will be as follows (assuming that initial value of $x is 0 ).

Print order	O/P in C	O/P in Perl
$x	0	0
$x,$x++	10	10
$x,$x,$x++	110	110
$x,$x++,$x++	210	201
$x,$x++,$x++,$x++	3210	3012
$x,$x++,$x++,$x++,$x++	43210	40123
$x,$x++,$x++,$x++,$x++,$x++	543210	501234

"C" is working as I explained above. Why Perl is behaving differently when we have "$x,$x++,$x++" ? Answer is again stacks.

when there are more than one expressions in same statement, perl uses another stack for evaluating them. ie, it wll push 0 to stack first. Then increments $x to 1. Then again it pushes 1 to stack and increments it to 2. Now all operations are over. Now Perl with pop contents of the operation stack and push to print stack. Then push the last $x. So it will print "201" for

print $x,$x++,$x++

Following program will demonstrate the entire thing :)

#!/usr/bin/perl

use strict;


my @PRINT_STACK;
my @OP_STACK;

my $x = 0;
my $temp;

#print $x,$x++,$x++;

push @OP_STACK, $x++;                                     # push the l
+ast $x++ to the operator stack (pushes 0)
print "OP_STACK => ( ",@OP_STACK, " )\n";

push @OP_STACK, $x++;                                     # push the n
+ext $x++ operator to the operator stack (pushes 1)
print "OP_STACK => ( ",join(", ",@OP_STACK)," )\n";

# All Operations are over 

$temp = pop @OP_STACK;                                    #pop The ope
+rator stack and push it to print stack (pops 1)
push @PRINT_STACK, $temp;                                 # pushes 1
print "PRINT_STACK => ( ",join(", ",@PRINT_STACK)," )\n";

$temp = pop @OP_STACK;                                    #pop The ope
+rator stack and push it to print stack (pops 0)
push @PRINT_STACK, $temp;                                 #pushes 0
print "PRINT_STACK => ( ",join(", ",@PRINT_STACK)," )\n";

push @PRINT_STACK, $x;                                    # push the f
+irst $x to the operator stack (pushes 2)
print "PRINT_STACK => ( ",join(", ",@PRINT_STACK)," )\n";

print pop @PRINT_STACK;
print pop @PRINT_STACK;
print pop @PRINT_STACK;

exit();
[download]

Isn't this a nice hack for obfuscation ?

Comments

• This behaviour goes with all function calls, NOT only print. It is the implementation of a function call. Please try the code

  $x=0;
  &myprint($x, $x++);
  
  sub myprint{
      print join ",",@_;
  }
  [download]

  Try puting more $x++ for the function call &myprint

Cheers!

--VC

UPDATE : I am not sure about all C compilers. I got above behavior with "gcc on FC6 & Turbo C on Windows XP". Also for Perl I tested with "Active Perl On Windows XP & Perl on FC6". I was NOT trying to say Perl & C will behave alike, but I was trying to say that this IS there with most of the implementation of C (gcc on FC6 and turbo C on Windows XP) & Perl (Active Perl On Win XP and perl on FC6). SORRY if misleading.

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