According to the C standard, is a negative integer literal interpreted as single literal, or operator and literal?

Question

How are negative integers interpreted by the C standard/compilers - as a single literal, or as a (unary) operator and a numeric literal?

For example, is -16 interpreted a -16 or -(16)?

Answer 1

C 2018 6.4.4.1 1 shows the grammar for integer constants. It says an integer-constant is one of:

• decimal-constant integer-suffix_opt
• octal-constant integer-suffix_opt
• hexadecimal-constant integer-suffix_opt

Since we are only interested in how these start, the integer-suffix does not concern us. The following grammar rules show:

• A decimal-constant starts with a nonzero-digit, which is of course one of 1, 2, 3, 4, 5, 6, 7, 8, or 9.
• An octal-constant starts with 0.
• A hexadecimal-constant starts with 0x or 0X.

Therefore, no integer-constant starts with - or +.

-16 is parsed as the unary - operator followed by the integer constant 16. This forms an integer constant expression as specified in C 2018 6.6 6, which says:

… An integer constant expression shall have integer type and shall only have operands that are integer constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, _Alignof expressions, and floating constants that are the immediate operands of casts…