RtlUnicodeStringCchCopyN - NtDoc

#ifndef _NTSTRSAFE_H_INCLUDED_
#ifndef NTSTRSAFE_LIB_IMPL
#ifndef NTSTRSAFE_NO_UNICODE_STRING_FUNCTIONS
#ifndef NTSTRSAFE_NO_CCH_FUNCTIONS

/*++

  NTSTATUS
  RtlUnicodeStringCchCopyN(
  _Inout_ PUNICODE_STRING     DestinationString,
  _In_  PCUNICODE_STRING    SourceString,
  _In_  size_t              cchToCopy
  );

  Routine Description:

  This routine is a safer version of the C built-in function 'strncpy' for
  PUNICODE_STRINGs.

  This function returns an NTSTATUS value, and not a pointer.  It returns
  STATUS_SUCCESS if the entire string or the first cchToCopy characters were
  copied without truncation, otherwise it will return a failure code. In
  failure cases as much of SourceString will be copied to DestinationString as possible.

Arguments:

DestinationString   -   pointer to the counted unicode destination string

SourceString        -   pointer to the counted unicode source string

cchToCopy           -   maximum number of characters to copy from source string,
not including the null terminator.

Notes:
Behavior is undefined if source and destination strings overlap.

DestinationString and SourceString should not be NULL. See RtlUnicodeStringCchCopyNEx
if you require the handling of NULL values.

Return Value:

STATUS_SUCCESS -   if there was source data and it was all copied

failure        -   the operation did not succeed

STATUS_BUFFER_OVERFLOW
Note: This status has the severity class Warning - IRPs completed with this
status do have their data copied back to user mode
-   this return value is an indication that the copy
operation failed due to insufficient space. When this
error occurs, the destination buffer is modified to
contain a truncated version of the ideal result. This is
useful for situations where truncation is ok

It is strongly recommended to use the NT_SUCCESS() macro to test the
return value of this function.

--*/

NTSTRSAFEDDI
RtlUnicodeStringCchCopyN(
        _Inout_ PUNICODE_STRING DestinationString,
        _In_ PCUNICODE_STRING SourceString,
        _In_ size_t cchToCopy)
{
    NTSTATUS status;
    wchar_t* pszDest;
    size_t cchDest;

    status = RtlUnicodeStringValidateDestWorker(DestinationString,
            &pszDest,
            &cchDest,
            NULL,
            NTSTRSAFE_UNICODE_STRING_MAX_CCH,
            0);

    if (NT_SUCCESS(status))
    {
        wchar_t* pszSrc;
        size_t cchSrcLength;
        size_t cchNewDestLength = 0;

        status = RtlUnicodeStringValidateSrcWorker(SourceString,
                &pszSrc,
                &cchSrcLength,
                NTSTRSAFE_UNICODE_STRING_MAX_CCH,
                0);

        if (NT_SUCCESS(status))
        {
            if (cchToCopy > NTSTRSAFE_UNICODE_STRING_MAX_CCH)
            {
                status = STATUS_INVALID_PARAMETER;
            }
            else
            {
                if (cchSrcLength < cchToCopy)
                {
                    cchToCopy = cchSrcLength;
                }

                status = RtlWideCharArrayCopyWorker(pszDest,
                        cchDest,
                        &cchNewDestLength,
                        pszSrc,
                        cchToCopy);
            }
        }

        // safe to multiply cchNewDestLength * sizeof(wchar_t) since cchDest < NTSTRSAFE_UNICODE_STRING_MAX_CCH and sizeof(wchar_t) is 2
        DestinationString->Length = (USHORT)(cchNewDestLength * sizeof(wchar_t));
    }

    return status;
}

#endif
#endif
#endif
#endif
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