[FIXED] Why can't a hexadecimal integer constant be negative?

Issue

This table shows type is the first type in which the value can fit. Examining 0xFFFFFFF0 with gdb shows it to be an unsigned type:

(gdb) ptype 0xFFFFFFF0
type = unsigned int

But 0xFFFFFFF0 is -16 when interpreted as a signed integer so why can't it fit in the int type? Doing the same with a decimal input shows the desired type:

ptype -16
type = int

I also wanted to ask about this sentence in the same link above:

There are no negative integer constants. Expressions such as -1 apply the unary minus operator to the value represented by the constant, which may involve implicit type conversions.

So -1 = 0 -1 = -1, AKA a negative integer? I don't understand this sentence.

Solution

There are no negative integer constants in C.

The grammar for integer constants is:

• A decimal constant is a sequence of decimal digits not starting with 0 and with an optional suffix (u and/or l or ll in either order with any cases [uppercase or lowercase]).
• An octal constant is a sequence of octal digits starting with 0 and an optional suffix.
• A hexadecimal constant is 0x followed by a sequence of hexadecimal digits, with any cases.

None of those have a minus sign. All integers constants in C are numerals with nonnegative values. Where a minus sign appears with a constant in C code, as in -34, it is two separate tokens: a unary negation operator followed by a constant.

The goal of selecting a type for a constant is to select a type that can represent its value. As a hexadecimal numeral, the value of FFFFFFF0₁₆ is 4,294,967,280. If 0xFFFFFFF0 were made a 32-bit int, the value of that int would be −16 in two’s complement or some other values in one’s complement or sign-and-magnitude. It would not be 4,294,967,280. So a 32-bit int cannot represent the value of this numeral, so the compiler does not use a 32-bit int for the type.

There are also no negative floating-point constants. Enumeration constants and character constants can be negative in some circumstances.

Answered By - Eric Postpischil

Answer Checked By - Terry (PHPFixing Volunteer)