In `double x = 9.0;`, `9.0` is a literal: source text that specifies a fixed value. Literals also include integers such as `0` and `3`, character literals such as `'a'`, and string literals such as `"hello"`. Unlike an identifier, a literal spells out a value rather than naming a program entity. section 2.1 section 5.3
A literal’s spelling determines how its value is read:
- `123` is decimal. A leading `0` makes `077` octal, with value 63. `0xFFFF` is hexadecimal. The binary form `0b1010`, with value 10, was introduced in C23. section 5.3
- `1.7E-13` uses a decimal exponent: `mEe` means `m × 10^e`. `0x1.7aP-13` uses a hexadecimal fraction and a power of two; its exponent is written in decimal. section 5.3
- `'a'` is an integer character literal representing a character code. In `'\n'`, the backslash introduces a special character. A string literal such as `"hello"` specifies text, also with special backslash notation. section 5.3
TAKEAWAY: A numeral’s value does not determine its type by itself. Spelling and suffixes matter: `1` is signed in the illustrated case, while `1U` is unsigned. Integer suffixes include `L`, `LL`, and `U`; floating-point examples include `0.2F`, `0.2`, and `0.2L`. String literals are the exception to the rule that literals specify numbers. section 5.3
Table 5.3 Examples for literals and their types, under the supposition that signed and unsigned have the commonly used representation with 32 bits
| Literal 𝑥 | Value | Type | Value of -𝑥 |
| 2'147'483'647 | +2,147,483,647 | signed | -2,147,483,647 |
| 2'147'483'648 | +2,147,483,648 | signed long | -2,147,483,648 |
| 4'294'967'295 | +4,294,967,295 | signed long | -4,294,967,295 |
0x7FFF'FFFF |
+2,147,483,647 | signed | -2,147,483,647 |
0x8000'0000 |
+2,147,483,648 | unsigned | +2,147,483,648 |
0xFFFF'FFFF |
+4,294,967,295 | unsigned | +1 |
| 1 | +1 | signed | -1 |
| 1U | +1 | unsigned | +4,294,967,295 |
Compare equal-valued decimal and hexadecimal literals under the table’s stated 32-bit representation assumption. Notice that `0x8000'0000` is unsigned there, so applying `-` does not produce a negative value. section 5.3
Literal syntax is not string-conversion syntax
`strtoul("0b1", nullptr, 2)` encounters different accepted formats across standard versions: the result is 0 under C17 and 1 under C23. The call processes characters in a string literal; it does not ask the compiler to interpret `0b1` as a numeric literal. Likewise, C23 digit separators in number literals have no portable counterpart in these conversion functions. TAKEAWAY: Do not use string-conversion functions to find the boundaries of, or scan text originating from, number literals. section 8.5.1
