#ifndef _NTINTSAFE_H_INCLUDED_
#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP | WINAPI_PARTITION_SYSTEM | WINAPI_PARTITION_GAMES)
/////////////////////////////////////////////////////////////////////////
//
// signed operations
//
// Strongly consider using unsigned numbers.
//
// Signed numbers are often used where unsigned numbers should be used.
// For example file sizes and array indices should always be unsigned.
// (File sizes should be 64bit integers; array indices should be size_t.)
// Subtracting a larger positive signed number from a smaller positive
// signed number with RtlIntSub will succeed, producing a negative number,
// that then must not be used as an array index (but can occasionally be
// used as a pointer index.) Similarly for adding a larger magnitude
// negative number to a smaller magnitude positive number.
//
// intsafe.h does not protect you from such errors. It tells you if your
// integer operations overflowed, not if you are doing the right thing
// with your non-overflowed integers.
//
// Likewise you can overflow a buffer with a non-overflowed unsigned index.
//
#if defined(ENABLE_INTSAFE_SIGNED_FUNCTIONS)

//=============================================================================
// Signed subtraction functions
//=============================================================================

//
// INT8 Subtraction
//
_Must_inspect_result_
__inline
NTSTATUS
RtlInt8Sub(
    _In_ INT8 i8Minuend,
    _In_ INT8 i8Subtrahend,
    _Out_ _Deref_out_range_(==, i8Minuend - i8Subtrahend) INT8* pi8Result
    )
{
    C_ASSERT(sizeof(LONG) > sizeof(INT8));
    return RtlLongToInt8(((LONG)i8Minuend) - ((LONG)i8Subtrahend), pi8Result);
}

#endif
#endif
#endif
View code on GitHub

// ntintsafe.h

NTSTATUS RtlInt8Sub(
  [in]  INT8 i8Minuend,
  [in]  INT8 i8Subtrahend,
  [out] INT8 *pi8Result
);
View the official Windows Driver Kit DDI reference

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Windows Driver Kit DDI reference (nf-ntintsafe-rtlint8sub)

RtlInt8Sub function

Description

Subtracts one value of type INT8 from another.

Parameters

`i8Minuend` [in]

The value from which i8Subtrahend is subtracted.

`i8Subtrahend` [in]

The value to subtract from i8Minuend.

`pi8Result` [out]

A pointer to the result. If the operation results in a value that overflows or underflows the capacity of the type, 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 arithmetic operations and perform validity checks with minimal impact on performance.

View the official Windows Driver Kit DDI reference