Question

I’m writing a perl implementation of a protocol whose specification is given for C/C++ but I don’t know much C.

What does the condition if ((flags & 0x400) != 0) mean? How do I do that in perl?

Is if ($flags == "\x400") correct?

Answer 1

& is a bitwise AND. See Bitwise operators.

So flags is being treated as a series of bits, and then you AND it with something which has exactly the bit set that you want to check. If the result is non-zero, then the bit is set, otherwise the bit you were looking at isn’t set. In particular, in your example 0x400 = 0100 0000 0000 is being used to check if the 11th bit is set (to 1) in flags.

Typically, you wouldn’t use 0x400 but a named constant, so it is clear what that bit represents.

So if ($flags == "\x400") isn’t correct. See Working with bits in Perl.

A common example of bit-masking can be seen in Linux file permissions.