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
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)
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
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.