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RtlStringCbPrintfEx function

The RtlStringCbPrintfExW and RtlStringCbPrintfExA functions create a byte-counted text string, with formatting that is based on supplied formatting information.


NTSTATUS RtlStringCbPrintfEx(
  _Out_opt_ LPTSTR  pszDest,
  _In_      size_t  cbDest,
  _Out_opt_ LPTSTR  *ppszDestEnd,
  _Out_opt_ size_t  *pcbRemaining,
  _In_      DWORD   dwFlags,
  _In_opt_  LPCTSTR pszFormat,


pszDest [out, optional]

A pointer to a caller-supplied buffer that receives a formatted, null-terminated string. The function creates this string from both the formatting string that is supplied by pszFormat and the function's argument list. The pszDest pointer can be NULL, but only if STRSAFE_IGNORE_NULLS is set in dwFlags.

cbDest [in]

The size of the destination buffer, in bytes. The buffer must be large enough to contain the formatted string plus 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).

If pszDest is NULL, cbDest must be zero.

ppszDestEnd [out, optional]

If the caller supplies a non-NULL address pointer then, after the operation completes, the function loads that address with a pointer to the destination buffer's resulting null string terminator.

pcbRemaining [out, optional]

If the caller supplies a non-NULL address pointer, the function loads the address with the number of unused bytes that are in the buffer pointed to by pszDest, including bytes used for the terminating null character.

dwFlags [in]

One or more flags and, optionally, a fill byte. The flags are defined as follows:


If set and the function succeeds, the low byte of dwFlags is used to fill the portion of the destination buffer that follows the terminating null character.


If set, either pszDest or pszSrc, or both, can be NULL. NULLpszSrc pointers are treated like empty strings (TEXT("")), which can be copied. NULLpszDest pointers cannot receive nonempty strings.


If set and the function fails, the low byte of dwFlags is used to fill the entire destination buffer, and the buffer is null-terminated. This operation overwrites any preexisting buffer contents.


If set and the function fails, the destination buffer is set to an empty string (TEXT("")). This operation overwrites any preexisting buffer contents.


If set and the function returns STATUS_BUFFER_OVERFLOW, the contents of the destination buffer are not modified.


pszFormat [in, optional]

A pointer to a null-terminated text string that contains printf-styled formatting directives. The pszFormat pointer can be NULL, but only if STRSAFE_IGNORE_NULLS is set in dwFlags.


A list of arguments that are interpreted by the function, based on formatting directives contained in the pszFormat string.

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 output string was created without truncation, and the resultant destination buffer is null-terminated.


This warning status means the operation did not complete due to insufficient space in the destination buffer. If STRSAFE_NO_TRUNCATION is set in dwFlags, the destination buffer is not modified. If the flag is not set, the destination buffer contains a truncated version of the created 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:

  • An invalid flag was specified.
  • The value in cbDest is larger than the maximum buffer size.
  • The destination buffer was already full.
  • A NULL pointer was present without the STRSAFE_IGNORE_NULLS flag.
  • The destination buffer pointer was NULL, but the buffer size was not zero.
  • The destination buffer pointer was NULL, or its length was zero, but a nonzero length source string was present.



RtlStringCbPrintfExW and RtlStringCbPrintfExA should be used instead of the following functions:

  • sprintf

  • swprintf

  • _snprintf

  • _snwprintf

All of these functions accept a format string and a list of arguments, interpret them, and create a formatted string. The size, in bytes, of the destination buffer is provided to RtlStringCbPrintfExW and RtlStringCbPrintfExA to ensure that they do not write past the end of the buffer.

RtlStringCbPrintfExW and RtlStringCbPrintfExA add to the functionality of RtlStringCbPrintf by returning a pointer to the end of the destination string, as well as the number of bytes left unused in that string. Flags can be passed to the function for additional control.

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

String data typeString literalFunction








If pszDest and pszFormat point to overlapping strings or if any argument strings overlap, the behavior of the function is undefined.

Neither pszFormat nor pszDest can be NULL unless the STRSAFE_IGNORE_NULLS flag is set, in which case either or both can be NULL. If pszDest is NULL, pszFormat must either be NULL or point to an empty string.

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


Target platform



Available starting with Windows XP with Service Pack 1 (SP1).


Ntstrsafe.h (include Ntstrsafe.h)





Unicode and ANSI names

RtlStringCbPrintfExW (Unicode) and RtlStringCbPrintfExA (ANSI)

See also




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