uint64t

uint64_t is a fixed-width unsigned integer type defined in the C standard library and the C++ standard library. It is intended to represent integers exactly 64 bits wide. The name often appears misspelled as “uint64t,” but the standard type is uint64_t (with an underscore). In C, it is declared in stdint.h; in C++, it is available via cstdint.

The type provides an unsigned range from 0 to 2^64 − 1, which is 0 to 18446744073709551615. Because it is a fixed-width type, its size is guaranteed to be 64 bits on implementations that provide it. On many platforms it is implemented as unsigned long long or unsigned long, but the exact underlying type is not part of the standard requirement; the important property is the 64-bit width.

Using uint64_t promotes portability in code that relies on a specific width for storage, arithmetic, or binary protocols. For constants, the standard provides macros such as UINT64_MAX for the maximum value and UINT64_C(n) to define 64-bit constants in a portable way. To access the type, include stdint.h (C) or cstdint (C++). If a compiler or environment does not provide an exact 64-bit unsigned type, uint64_t may be absent; in such cases alternative fixed-width or larger-width types from the same family, such as uint_fast64_t or uint_least64_t, may be used.

Related types include int64_t (signed 64-bit), as well as their width-defined variants like int64_t, uint_least64_t, and uint_fast64_t. uint64_t is widely used for large counters, file sizes, timestamps, serialization, and cryptographic computations where a precise 64-bit width is required.