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RtlStringCbCatN function

The RtlStringCbCatNW and RtlStringCbCatNA functions concatenate two byte-counted strings while limiting the size of the appended string.


  _Inout_  LPTSTR pszDest,
  _In_     size_t cbDest,
  _In_     LPCTSTR pszSrc,
  _In_     size_t cbMaxAppend


pszDest [in, out]

Pointer to a buffer which, on input, contains a null-terminated string to which pszSrc will be concatenated. On output, this is the destination buffer that contains the entire resultant string. The string at pszSrc, up to cbMaxAppend bytes, is added to the end of the string at pszDest and terminated with a null character.

cbDest [in]

The size of the destination buffer, in bytes. The buffer must be large enough to include both strings and the terminating null character.

For Unicode strings, the maximum number of bytes is NTSTRSAFE_MAX_CCH * sizeof(WCHAR).

For ANSI strings, the maximum number of bytes is NTSTRSAFE_MAX_CCH * sizeof(char).

pszSrc [in]

Pointer to a null-terminated string. This string will be concatenated to the end of the string that is contained in the buffer at pszDest.

cbMaxAppend [in]

The maximum number of bytes to append to the string that is contained in the buffer at pszDest.

Return value

The function returns one of the NTSTATUS values that are listed in the following table. For information about how to test NTSTATUS values, see Using NTSTATUS Values.

Return codeDescription

This success status means source data was present, the strings were concatenated without truncation, and the resultant destination buffer is null-terminated.


This warning status means the concatenation operation did not complete due to insufficient buffer space. The destination buffer contains a truncated, null-terminated version of the intended result.


This error status means the function received an invalid input parameter. For more information, see the following paragraph.

The function returns the STATUS_INVALID_PARAMETER value when:

  • The value in cbDest is larger than the maximum buffer size.
  • The destination buffer was already full.
  • A NULL pointer was present.
  • The destination buffer's length was zero, but a nonzero length source string was present.



RtlStringCbCatNW and RtlStringCbCatNA should be used instead of the following functions:

  • strncat

  • wcsncat

The size, in bytes, of the destination buffer is provided to RtlStringCbCatNW and RtlStringCbCatNA to ensure that they do not write past the end of the buffer.

Use RtlStringCbCatNW to handle Unicode strings and RtlStringCbCatNA to handle ANSI strings. The form you use depends on your data, as shown in the following table

String data typeString literalFunction








If pszSrc and pszDest point to overlapping strings, the behavior of the function is undefined.

Neither pszSrc nor pszDest can be NULL. If you need to handle NULL string pointer values, use RtlStringCbCatNEx.

For more information about the safe string functions, see Using Safe String Functions.



Available in Windows XP with Service Pack 1 (SP1) and later versions of Windows.


Ntstrsafe.h (include Ntstrsafe.h)





Unicode and ANSI names

RtlStringCbCatNW (Unicode) and RtlStringCbCatNA (ANSI)

See also




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