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Bitwise Operators - Perl

In this article, you will learn how to find out useful information about files in Perl. It is excerpted from chapter 11 of the book Learning Perl, Fourth Edition, written by Randal L. Schwartz, Tom Phoenix and brian d foy (O'Reilly; ISBN: 0596101058). Copyright © 2006 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

TABLE OF CONTENTS:
  1. File Tests in Perl
  2. File Test Operators
  3. The stat and lstat Functions
  4. The localtime Function
  5. Bitwise Operators
  6. Using the Special Underscore Filehandle
By: O'Reilly Media
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May 10, 2007

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When you need to work with numbers bit by bit, as when working with the mode bits returned by stat, you’ll need to use the bitwise operators. These operators perform binary math operations on values. The bitwise-and operator (&) reports which bits are set in the left argument and in the right argument. For example, the expression 10 & 12 has the value 8. The bitwise-and needs to have a one-bit in both operands to produce a one-bit in the result. That means that the logical-and operation on ten (which is1010in binary) and twelve (which is1100) gives eight (which is1000, with a one-bit only where the left operand has a one-bit and the right operand also has a one-bit). See Figure 11-1.


Figure 11-1.  Bitwise-and addition

The different bitwise operators and their meanings are shown in Table 11-2.

Table 11-2.  Bitwise operators

Expression Meaning
10 & 12 Bitwise-and; which bits are true in both operands (this gives8)
10 | 12 Bitwise-or; which bits are true in one operand or the other (this gives14)
10 ^ 12 Bitwise-xor; which bits are true in one operand or the other but not both (this gives6)
6 << 2

Bitwise shift left; shift the left operand the number of bits shown by the right operand, adding zero-bits

at the least-significant places (this gives24)
25 >> 2

Bitwise shift right; shift the left operand the number of bits shown by the right operand, discarding the

 least-significant bits (this gives6)
~ 10

Bitwise negation, also called unary bit complement; return the number with the opposite bit for each bit

 in the operand (this gives0xFFFFFFF5, but see the text)

So, here’s an example of some things you could do with the$modereturned bystat. The results of these bit manipulations could be useful withchmod, which you’ll see in Chapter 12:

  # $mode is the mode value returned from a stat of CONFIG
  warn "Hey, the configuration file is world-writable!\n"
   
if $mode & 0002;        # configuration security problem
  my $classical_mode = 0777 & $mode;
                            # mask off extra high-bits
  my $u_plus_x = $classical_mode | 0100;
                            # turn one bit on
  my $go_minus_r = $classical_mode & (~ 0044);                      # turn two bits off

Using Bitstrings

All of the bitwise operators can work with bitstrings, as well as with integers. If the operands are integers, the result will be an integer. (The integer will be at least a 32-bit integer but may be larger if your machine supports that. That is, if you have a 64-bit machine, ~10 may give the 64-bit result 0xFFFFFFFFFFFFFFF5, rather than the 32-bit result0xFFFFFFF5.)

But if any operand of a bitwise operator is a string, Perl will perform the operation on that bitstring. That is,"\xAA" | "\x55"will give the string"\xFF". Note that these values are single-byte strings and the result is a byte with all eight bits set. Bitstrings may be arbitrarily long.

This is one of the few places where Perl distinguishes between strings and numbers. See theperlopmanpage for more information on using bitwise operators on strings.



 
 
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