#ifndef _NTINTSAFE_H_INCLUDED_
#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP | WINAPI_PARTITION_SYSTEM | WINAPI_PARTITION_GAMES)

//
// ULONGLONG -> UINT8 conversion
//
_Must_inspect_result_
__inline
NTSTATUS
RtlULongLongToUInt8(
    _In_ ULONGLONG ullOperand,
    _Out_ _Deref_out_range_(==, ullOperand) UINT8* pu8Result)
{
    NTSTATUS status;

    if (ullOperand <= UINT8_MAX)
    {
        *pu8Result = (UINT8)ullOperand;
        status = STATUS_SUCCESS;
    }
    else
    {
        *pu8Result = UINT8_ERROR;
        status = STATUS_INTEGER_OVERFLOW;
    }

    return status;
}

#endif
#endif
View code on GitHub

// ntintsafe.h

NTSTATUS RtlULongLongToUInt8(
  [in]  ULONGLONG ullOperand,
  [out] UINT8     *pu8Result
);
View the official Windows Driver Kit DDI reference

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Windows Driver Kit DDI reference (nf-ntintsafe-rtlulonglongtouint8)

RtlULongLongToUInt8 function

Description

Converts a value of type ULONGLONG to a value of type UINT8.

Parameters

`ullOperand` [in]

The value to be converted.

`pu8Result` [out]

A pointer to the converted value. In the case where the conversion causes a truncation of the original value, the function returns STATUS_INTEGER_OVERFLOW and this parameter is not valid.

Return value

Returns STATUS_SUCCESS if the operation is successful.

See the implementation of this helper function in ntintsafe.h in the WDK for possible error return values.

Remarks

This is one of a set of inline functions designed to provide type conversions and perform validity checks with minimal impact on performance.

View the official Windows Driver Kit DDI reference