StringCchCatExA function (strsafe.h)

Concatenates one string to another string. The size of the destination buffer is provided to the function to ensure that it does not write past the end of this buffer.

StringCchCatEx adds to the functionality of StringCchCat by returning a pointer to the end of the destination string as well as the number of characters left unused in that string. Flags may also be passed to the function for additional control.

StringCchCatEx is a replacement for the following functions:


  [in, out]       STRSAFE_LPSTR  pszDest,
  [in]            size_t         cchDest,
  [in]            STRSAFE_LPCSTR pszSrc,
  [out, optional] STRSAFE_LPSTR  *ppszDestEnd,
  [out, optional] size_t         *pcchRemaining,
  [in]            DWORD          dwFlags


[in, out] pszDest


The destination buffer, which contains the string that is to be concatenated to pszSrc, and that will receive the entire resultant string. The string at pszSrc is added to the end of the string at pszDest.

[in] cchDest

Type: size_t

The size of the destination buffer, in characters. This value must equal the length of pszSrc plus the length of pszDest plus 1 to account for both strings and the terminating null character. The maximum number of characters allowed is STRSAFE_MAX_CCH.

[in] pszSrc


The source string that is to be concatenated to the end of pszDest. This string must be null-terminated.

[out, optional] ppszDestEnd


The address of a pointer to the end of pszDest. If ppszDestEnd is non-NULL and any data is appended to the destination buffer, this points to the terminating null character at the end of the string.

[out, optional] pcchRemaining

Type: size_t*

The number of unused characters in pszDest, including the terminating null character. If pcchRemaining is NULL, the count is not kept or returned.

[in] dwFlags


One or more of the following values.

Value Meaning
If the function succeeds, the low byte of dwFlags (0) is used to fill the uninitialized portion of pszDest following the terminating null character.
Treat NULL string pointers like empty strings (TEXT("")). This flag is useful for emulating functions such as lstrcpy.
If the function fails, the low byte of dwFlags (0) is used to fill the entire pszDest buffer, and the buffer is null-terminated. In the case of a STRSAFE_E_INSUFFICIENT_BUFFER failure, any pre-existing or truncated string in the destination buffer is overwritten.
If the function fails, pszDest is set to an empty string (TEXT("")). In the case of a STRSAFE_E_INSUFFICIENT_BUFFER failure, any pre-existing or truncated string in the destination buffer is overwritten.
If the function fails, pszDest is untouched. Nothing is added to the original contents.

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 code Description
Source data was present, the strings were fully concatenated without truncation, and the resultant destination buffer is null-terminated.
The value in cchDest is either 0 or larger than STRSAFE_MAX_CCH, or the destination buffer is already full.
The copy operation failed due to insufficient buffer space. Depending on the value of dwFlags, the destination buffer may contain 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.


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

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

Neither pszSrc nor pszDest should be NULL unless the STRSAFE_IGNORE_NULLS flag is specified, in which case both may be NULL. However, an error due to insufficient space may still be returned even though NULL values are ignored.

StringCchCatEx 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 Type String Literal Function
char "string" StringCchCatExA
TCHAR TEXT("string") StringCchCatEx
WCHAR L"string" StringCchCatExW


The strsafe.h header defines StringCchCatEx as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.


Minimum supported client Windows XP with SP2 [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 with SP1 [desktop apps | UWP apps]
Target Platform Windows
Header strsafe.h

See also