7.18.1.3 Fastest minimum-width integer types

1

Each of the following types designates an integer type that is usually fastest[1] to operate with among all integer types that have at least the specified width.

2

The typedef name int_fastN_t designates the fastest signed integer type with a width of at least N . The typedef name uint_fastN_t designates the fastest unsigned integer type with a width of at least N .

3

The following types are required:

int_fast8_t                                 uint_fast8_t
int_fast16_t                                uint_fast16_t
int_fast32_t                                uint_fast32_t
int_fast64_t                                uint_fast64_t

All other types of this form are optional.

Footnotes

[1]

The designated type is not guaranteed to be fastest for all purposes; if the implementation has no clear grounds for choosing one type over another, it will simply pick some integer type satisfying the signedness and width requirements.