C: What does it mean for a double to be == -0?

Question

I was just using gdb to print out a value of a double and to my surprise it printed -0

What is a double of -0 value mean in C?

By the way when I checked it's equality with 0 it returned true: To do the comparison I just did the following

in gdb

> print some_double
-0
> print some_double == 0
1

Answer 1

Negative zero is a useful concept for numerical computing and is a valid floating point number in C.

Answer 2

This is a well known issue with printf. Since you are using a floating point number, there is no one representation possible in binary. And most of the times the binary representation is not perfect. So, 0 is stored as 0.00000000000000...0042 sometimes and as -0.000000000000000000000123. When printf prints the second one, you get the odd -0.

I don't really know how you are comparing against 0, just remember to add an epsilon when doing floating point comparisons to adjust for oddities. E.g: to test if two floats are equal never write a == b but fabs(a-b) < 1e-13 where 1e-13 is the epsilon (choose an exponent that suits you).

Answer 3

Actually reading the book "Writing great code, Volume 1" in Chapter 4 the author gives the answer quite clearly it is because of the use of 1's complement to represent signed float in IEEE FP Standard. He says:

The mantissa uses a one’s complement format rather than two’s complement. This means that the 24-bit value of the mantissa is simply an unsigned binary number, and the sign bit, in bit position 31, determines whether that value is positive or negative. One’s complement numbers have the unusual property that there are two representations for zero (with the sign bit set or clear). Generally, this is important only to the person designing the floating-point software or hardware system.

Emphasis mine