What does '<<' mean in C?

Question

what does this mean?

#define WS_RECURSIVE    (1 << 0)

I understand that it will define WS_Recursive (1 << 0) but w

Answer 1

It's a bit shift. (1 << 1) is 2 and (1 << 2) is 4. (1 << 0) is 1, which is rather silly, but at least it's precomputed at compile time.

Answer 2

<< computes a bitwise shift to the left. Shifting 1 to the left by 0 bits simply leaves the result as 1.

I noticed also where you got your code from that there's also:

#define WS_RECURSIVE    (1 << 0)
#define WS_DEFAULT  WS_RECURSIVE
#define WS_FOLLOWLINK   (1 << 1)
#define WS_DOTFILES     (1 << 2)
#define WS_MATCHDIRS    (1 << 3)

That is a way of creating bit fields, where you OR (|) flags together, and AND them (&) to check if they're set.

Answer 3

The << operator shifts the left-hand value left by the (right-hand value) bits. Your example does nothing! 1 shifted 0 bits to the left is still 1. However, 1 << 1 is 2, 1 << 2 is 4, etc. Is WS_RECURSIVE a flag in a bitfield?

Answer 4

<< is the left shift operator. It is shifting the number 1 to the left 0 bits, which is equivalent to the number 1.

It is commonly used to create flags, numbers that can be combined together with | (bit or) and various operations can be applied to them, such as testing whether a flag is set, setting a flag, removing a flag, etc.

The reason that they can be combined together without interfering with each other is that each one is a power of two, and that is the reason for using 1 << x, because that yields powers of two:

1 << 0 == 2⁰ == 1 == binary 0001
1 << 1 == 2¹ == 2 == binary 0010
1 << 2 == 2² == 4 == binary 0100
1 << 3 == 2³ == 8 == binary 1000
etc

You can read about bit flags here: http://www.codeproject.com/KB/tips/Binary_Guide.aspx

Answer 5

This is a bit shifting to the left. So 1 << 0 is actually 1. It is usually used this way when you want to define some flags, each of them is one bit set, for example:

#define FLAG1 (1 << 0)
#define FLAG2 (1 << 1)
#define FLAG3 (1 << 2)
#define FLAG4 (1 << 3)