StringCbCopyN function

Copies the specified number of bytes from one string to another. The size of the destination buffer is provided to the function to ensure that it does not write past the end of this buffer.

StringCbCopyN is a replacement for the following functions:


HRESULT StringCbCopyN(
  _Out_ LPTSTR  pszDest,
  _In_  size_t  cbDest,
  _In_  LPCTSTR pszSrc,
  _In_  size_t  cbSrc


pszDest [out]


The destination buffer, which receives the copied characters.

cbDest [in]

Type: size_t

The size of pszDest, in bytes. This value must be large enough to hold the copied bytes (the size of pszSrc or the value of cbSrc, whichever is smaller) and also account for the terminating null character. The maximum number of characters allowed is STRSAFE_MAX_CCH * sizeof(TCHAR).

pszSrc [in]


The source string. This string must be null-terminated.

cbSrc [in]

Type: size_t

The maximum number of bytes to be copied from pszSrc to pszDest.

Return value


This function can return one of the following values. It is strongly recommended that you use the SUCCEEDED and FAILED macros to test the return value of this function.

Return codeDescription

Source data was present, the data was copied from pszSrc without truncation, and the resultant destination buffer is null-terminated.


The value in cbDest is either larger than STRSAFE_MAX_CCH * sizeof(TCHAR), or the destination buffer is already full.


The copy operation failed due to insufficient buffer space. The destination buffer contains a truncated, null-terminated version of the intended result. In situations where truncation is acceptable, this may not necessarily be seen as a failure condition.


Note that this function returns an HRESULT value, unlike the functions that it replaces.


StringCbCopyN provides additional processing for proper buffer handling in your code. Poor buffer handling is implicated in many security issues that involve buffer overruns. StringCbCopyN always null-terminates and never overflows a valid destination buffer, even if the contents of the source string change during the operation.

While this routine is meant as a replacement for strncpy, there are differences in behavior. If cbSrc is larger than the number of bytes in pszSrc, StringCbCopyN—unlike strncpy—does not continue to pad pszDest with null characters until cbSrc bytes have been copied.

Behavior is undefined if the strings pointed to by pszSrc and pszDest overlap.

Neither pszSrc nor pszDest should be NULL. See StringCbCopyNEx if you require the handling of null string pointer values.

StringCbCopyN can be used in its generic form, or in its more specific forms. The data type of the string determines the form of this function that you should use, as shown in the following table.

String Data TypeString LiteralFunction



Minimum supported client

Windows XP with SP2 [desktop apps | UWP apps]

Minimum supported server

Windows Server 2003 with SP1 [desktop apps | UWP apps]



Unicode and ANSI names

StringCbCopyNW (Unicode) and StringCbCopyNA (ANSI)

See also