Why Do Excessively Large Integer Literals in C and C Sometimes Not Default to `int`?

Integer Literals: Not Always Int by Default

While iterating in a loop, excessively large integer literals can lead to unexpected issues. Normally, literals without suffixes are interpreted as int. However, this assumption may not hold for very large values.

Overflow and Promotion

When an integer literal exceeds the range of int (typically 32-bit), undefined behavior (UB) can result. However, in some cases, the compiler may automatically promote the literal to a larger type, such as long int (which is usually 64-bit).

Standard Specifications

Both C and C standards regulate the promotion of integer literals to appropriate types.

• C: Section 6.4.4.1 of the C99 standard states that the literal type is the first listed in which its value can be represented (i.e., int, long int, long long int) or an extended integer type if necessary.
• C : Section [lex.icon] ¶2 of the C 11 standard similarly specifies promotion to long int or long long int as needed.

Example

Consider the integer literal 10000000000. In a C environment, this would be interpreted as a long int. However, if the compiler used is a 32-bit compiler, this could result in a compilation error due to the literal being too large for the available integer types.

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