RtlStringCbCopyN function

The RtlStringCbCopyNW and RtlStringCbCopyNA functions copy a byte-counted string to a buffer while limiting the size of the copied string.


NTSTATUS RtlStringCbCopyN(
  _Out_ LPTSTR  pszDest,
  _In_  size_t  cbDest,
  _In_  LPCTSTR pszSrc,
  _In_  size_t  cbSrc


pszDest [out]

A pointer to a caller-supplied buffer that receives the copied string. The string at pszSrc, up to cbSrc bytes, is copied to the buffer at pszDest and terminated with a null character.

cbDest [in]

The size, in bytes, of the destination buffer. The buffer must be large enough for both the string 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]

A pointer to a caller-supplied, null-terminated string.

cbSrc [in]

The maximum number of bytes to copy from pszSrc to 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 string was copied without truncation, and the resultant destination buffer is null-terminated.


This warning status means the copy operation did not complete due to insufficient space in the destination buffer. The destination buffer contains a truncated version of the copied string.


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.



RtlStringCbCopyNW and RtlStringCbCopyNA should be used instead of strncpy. However, these functions differ in behavior. If cbSrc is larger than the number of bytes in pszSrc, the RtlStringCbCopyN functions—unlike strncpy—do not fill pszDest with null characters until cbSrc bytes have been copied.

RtlStringCbCopyN copies a given number of bytes from a source string. The size, in bytes, of the destination buffer is provided to the function to ensure that RtlStringCbCopyN does not write past the end of this buffer.

Use RtlStringCbCopyNW to handle Unicode strings and RtlStringCbCopyNA 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, see RtlStringCbCopyNEx.

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


Target platform



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


Ntstrsafe.h (include Ntstrsafe.h)





Unicode and ANSI names

RtlStringCbCopyNW (Unicode) and RtlStringCbCopyNA (ANSI)

See also




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