RtlUnicodeStringCchCatStringNEx - NtDoc

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

/*++

  NTSTATUS
  RtlUnicodeStringCchCatStringNEx(
  _Inout_ PUNICODE_STRING DestinationString   OPTIONAL,
  _In_    LPCTSTR         pszSrc              OPTIONAL,
  _In_    size_t          cchToAppend,
  _Out_opt_ PUNICODE_STRING RemainingString     OPTIONAL,
  _In_    DWORD           dwFlags
  );

  Routine Description:

  This routine is a safer version of the C built-in function 'strncat', with
  some additional parameters and for PUNICODE_STRINGs. In addition to the
  functionality provided by RtlUnicodeStringCchCatStringN, this routine
  also returns a PUNICODE_STRING which points to the end of the destination
  string. The flags parameter allows additional controls.

Arguments:

DestinationString   -   pointer to the counted unicode destination string

pszSrc              -   source string

cchToAppend         -   maximum number of characters to append

RemainingString     -   if RemainingString is non-null, the function will format
the pointer with the remaining buffer and number of
bytes left in the destination string

dwFlags             -   controls some details of the string copy:

STRSAFE_FILL_BEHIND
if the function succeeds, the low byte of dwFlags will be
used to fill the uninitialize part of destination buffer

STRSAFE_IGNORE_NULLS
do not fault if DestinationString is null and treat NULL pszSrc like
empty strings (L""). This flag is useful for emulating
functions like lstrcpy

STRSAFE_FILL_ON_FAILURE
if the function fails, the low byte of dwFlags will be
used to fill all of the destination buffer. This will
overwrite any truncated string returned when the failure is
STATUS_BUFFER_OVERFLOW

STRSAFE_ZERO_LENGTH_ON_FAILURE
if the function fails, the destination Length will be set
to zero. This will overwrite any truncated string
returned when the failure is STATUS_BUFFER_OVERFLOW.

STRSAFE_NO_TRUNCATION
if the function returns STATUS_BUFFER_OVERFLOW, pszDest
will not contain a truncated string, it will remain unchanged.

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

DestinationString and pszSrc should not be NULL unless the STRSAFE_IGNORE_NULLS flag
is specified.  If STRSAFE_IGNORE_NULLS is passed, both DestinationString and pszSrc
may be NULL.  An error may still be returned even though NULLS are ignored
due to insufficient space.

Return Value:

STATUS_SUCCESS -   if all of pszSrc or the first cchToAppend characters were
concatenated to DestinationString

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
RtlUnicodeStringCchCatStringNEx(
        _Inout_ PUNICODE_STRING DestinationString,
        _In_ NTSTRSAFE_PCWSTR pszSrc,
        _In_ size_t cchToAppend,
        _Out_opt_ PUNICODE_STRING RemainingString,
        _In_ DWORD dwFlags)
{
    NTSTATUS status;
    wchar_t* pszDest;
    size_t cchDest;
    size_t cchDestLength;

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

    if (NT_SUCCESS(status))
    {
        wchar_t* pszDestEnd = pszDest + cchDestLength;
        size_t cchRemaining = cchDest - cchDestLength;
        size_t cchNewDestLength = cchDestLength;

        status = RtlStringExValidateSrcW(&pszSrc, &cchToAppend, NTSTRSAFE_UNICODE_STRING_MAX_CCH, dwFlags);

        if (NT_SUCCESS(status))
        {
            if (dwFlags & (~STRSAFE_UNICODE_STRING_VALID_FLAGS))
            {
                status = STATUS_INVALID_PARAMETER;
            }
            else if (cchRemaining == 0)
            {
                // only fail if there was actually src data to append
                if ((cchToAppend != 0) && (*pszSrc != L'\0'))
                {
                    if (pszDest == NULL)
                    {
                        status = STATUS_INVALID_PARAMETER;
                    }
                    else
                    {
                        status = STATUS_BUFFER_OVERFLOW;
                    }
                }
            }
            else
            {
                size_t cchCopied = 0;

                status = RtlWideCharArrayCopyStringWorker(pszDestEnd,
                        cchRemaining,
                        &cchCopied,
                        pszSrc,
                        cchToAppend);

                pszDestEnd = pszDestEnd + cchCopied;
                cchRemaining = cchRemaining - cchCopied;

                cchNewDestLength = cchDestLength + cchCopied;

                if (NT_SUCCESS(status)              &&
                        (dwFlags & STRSAFE_FILL_BEHIND) &&
                        (cchRemaining != 0))
                {
                    // handle the STRSAFE_FILL_BEHIND flag
                    RtlUnicodeStringExHandleFill(pszDestEnd, cchRemaining, dwFlags);
                }
            }
        }

        if (!NT_SUCCESS(status)                                                                                      &&
                (dwFlags & (STRSAFE_NO_TRUNCATION | STRSAFE_FILL_ON_FAILURE | STRSAFE_ZERO_LENGTH_ON_FAILURE))  &&
                (cchDest != 0))
        {
            // handle the STRSAFE_NO_TRUNCATION, STRSAFE_FILL_ON_FAILURE, and STRSAFE_ZERO_LENGTH_ON_FAILURE flags
            RtlUnicodeStringExHandleOtherFlags(pszDest,
                    cchDest,
                    cchDestLength,
                    &cchNewDestLength,
                    &pszDestEnd,
                    &cchRemaining,
                    dwFlags);
        }

        if (DestinationString)
        {
            // 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));
        }

        if (NT_SUCCESS(status) || (status == STATUS_BUFFER_OVERFLOW))
        {
            if (RemainingString)
            {
                RemainingString->Length = 0;
                // safe to multiply cchRemaining * sizeof(wchar_t) since cchRemaining < NTSTRSAFE_UNICODE_STRING_MAX_CCH and sizeof(wchar_t) is 2
                RemainingString->MaximumLength = (USHORT)(cchRemaining * sizeof(wchar_t));
                RemainingString->Buffer = pszDestEnd;
            }
        }
    }

    return status;
}

#endif
#endif
#endif
#endif
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