Any reason to assign -0?

Question

I am going through some old C code using Lint which stumbled upon this line:

int16_t max = -0;

The Lint message is that the "Constant expression evaluates to 0 in operation '-'".

Is there any reason why someone would use -0?

Answer 1

In the C specification (6.2.6.2 Integer types), it states the following (emphasis mine):

For signed integer types, the bits of the object representation shall be divided into three groups: value bits, padding bits, and the sign bit. There need not be any padding bits; there shall be exactly one sign bit. Each bit that is a value bit shall have the same value as the same bit in the object representation of the corresponding unsigned type (if there are M value bits in the signed type and N in the unsigned type, then M £ N). If the sign bit is zero, it shall not affect the resulting value. If the sign bit is one, the value shall be modified in one of the following ways:

• the corresponding value with sign bit 0 is negated (sign and magnitude);
• the sign bit has the value -(2N) (two’s complement);
• the sign bit has the value -(2N - 1) (one’s complement).

Which of these applies is implementation-defined, as is whether the value with sign bit 1 and all value bits zero (for the first two), or with sign bit and all value bits 1 (for one’s complement), is a trap representation or a normal value. In the case of sign and magnitude and one’s complement, if this representation is a normal value it is called a negative zero.

In other words, C supports three different representations for signed integers and two of them have the concept of signed zero, which makes a distinction between a positive and a negative zero.

So, my explanation is that perhaps the author of your code snippet was trying to produce a negative zero value. But, as pointed out in Jens Gustedt's answer, this expression cannot actually produce a negative zero, which means the author may have made a wrong assumption there.

Answer 2

No, I can't see any reason for this. Others have mentioned that it is possible to have platforms with "negative zero", but such a negative zero can never be produced by this expression, so this is useless.

The corresponding paragraph in the C standard is 6.2.6.2 p3, emphasis is mine:

If the implementation supports negative zeros, they shall be generated only by:

— the &, |, ^, ~, <<, and >> operators with operands that produce such a value;

— the +, -, *, /, and % operators where one operand is a negative zero and the result is zero;

— compound assignment operators based on the above cases.

To produce a negative zero on such a platform you could use ~INT_MAX, for example, but that would not be a zero for other representations, so the code wouldn't be very portable.