RtlUnicodeStringCat - NtDoc

#ifndef _NTSTRSAFE_H_INCLUDED_
#ifndef NTSTRSAFE_LIB_IMPL
#ifndef NTSTRSAFE_NO_UNICODE_STRING_FUNCTIONS

/*++

  NTSTATUS
  RtlUnicodeStringCat(
  _Inout_  PUNICODE_STRING     DestinationString,
  _In_     PCUNICODE_STRING    SourceString
  );

  Routine Description:

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

  This function returns an NTSTATUS value, and not a pointer.  It returns
  STATUS_SUCCESS if the string was concatenated without truncation, otherwise
  it will return a failure code. In failure cases as much of SourceString will be
  appended to DestinationString as possible.

Arguments:

DestinationString   -   pointer to the counted unicode destination string

SourceString        -   pointer to the counted unicode source string

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

DestinationString and pszSrc should not be NULL.  See RtlUnicodeStringCatEx
if you require the handling of NULL values.

Return Value:

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

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
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
RtlUnicodeStringCat(
        _Inout_ PUNICODE_STRING DestinationString,
        _In_ PCUNICODE_STRING SourceString)
{
    NTSTATUS status;
    wchar_t* pszDest;
    size_t cchDest;
    size_t cchDestLength;

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

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

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

        if (NT_SUCCESS(status))
        {
            size_t cchCopied = 0;

            status = RtlWideCharArrayCopyWorker(pszDest + cchDestLength,
                    cchDest - cchDestLength,
                    &cchCopied,
                    pszSrc,
                    cchSrcLength);

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

    return status;
}

#endif
#endif
#endif
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