strncat, _strncat_l, wcsncat, wcsncat_l, _mbsncat _mbsncat_l

Article
12/21/2012

Append characters of a string. More secure versions of these functions are available, see strncat_s, _strncat_s_l, wcsncat_s, _wcsncat_s_l, _mbsncat_s, _mbsncat_s_l .

Important

_mbsncat and _mbsncat_l cannot be used in applications that execute in the Windows Runtime. For more information, see CRT functions not supported with /ZW.

char *strncat(
   char *strDest,
   const char *strSource,
   size_t count 
);
wchar_t *wcsncat(
   wchar_t *strDest,
   const wchar_t *strSource,
   size_t count 
);
unsigned char *_mbsncat(
   unsigned char *strDest,
   const unsigned char *strSource,
   size_t count
);
unsigned char *_mbsncat_l(
   unsigned char *strDest,
   const unsigned char *strSource,
   size_t count,
   _locale_t locale
);
template <size_t size>
char *strncat(
   char (&strDest)[size],
   const char *strSource,
   size_t count 
); // C++ only
template <size_t size>
wchar_t *wcsncat(
   wchar_t (&strDest)[size],
   const wchar_t *strSource,
   size_t count 
); // C++ only
template <size_t size>
unsigned char *_mbsncat(
   unsigned char (&strDest)[size],
   const unsigned char *strSource,
   size_t count
); // C++ only
template <size_t size>
unsigned char *_mbsncat_l(
   unsigned char (&strDest)[size],
   const unsigned char *strSource,
   size_t count,
   _locale_t locale
); // C++ only

Parameters

strDest
Null-terminated destination string.
strSource
Null-terminated source string.
count
Number of characters to append.
locale
Locale to use.

Return Value

Returns a pointer to the destination string. No return value is reserved to indicate an error.

Remarks

The strncat function appends, at most, the first count characters of strSource to strDest. The initial character of strSource overwrites the terminating null character of strDest. If a null character appears in strSource before count characters are appended, strncat appends all characters from strSource, up to the null character. If count is greater than the length of strSource, the length of strSource is used in place of count. The all cases, the resulting string is terminated with a null character. If copying takes place between strings that overlap, the behavior is undefined.

Security Note
strncat does not check for sufficient space in strDest; it is therefore a potential cause of buffer overruns. Keep in mind that count limits the number of characters appended; it is not a limit on the size of strDest. See the example below. For more information, see Avoiding Buffer Overruns.

wcsncat and _mbsncat are wide-character and multibyte-character versions of strncat. The string arguments and return value of wcsncat are wide-character strings; those of _mbsncat are multibyte-character strings. These three functions behave identically otherwise.

The output value is affected by the setting of the LC_CTYPE category setting of the locale; see setlocale for more information. The versions of these functions without the _l suffix use the current locale for this locale-dependent behavior; the versions with the _l suffix are identical except that they use the locale parameter passed in instead. For more information, see Locale.

In C++, these functions have template overloads. For more information, see Secure Template Overloads.

Generic-Text Routine Mappings

TCHAR.H routine	_UNICODE & _MBCS not defined	_MBCS defined	_UNICODE defined
_tcsncat	strncat	_mbsnbcat	wcsncat
_tcsncat_l	_strncat_l	_mbsnbcat_l	_wcsncat_l

Note

_strncat_l and _wcsncat_l have no locale dependence and are not meant to be called directly. They are provided for internal use by _tcsncat_l.

Requirements

Routine	Required header
strncat	<string.h>
wcsncat	<string.h> or <wchar.h>
_mbsncat	<mbstring.h>
_mbsncat_l	<mbstring.h>

For additional compatibility information, see Compatibility in the Introduction.

Example

// crt_strncat.c
// Use strcat and strncat to append to a string.
#include <stdlib.h>

#define MAXSTRINGLEN 39

char string[MAXSTRINGLEN+1];
// or char *string = malloc(MAXSTRINGLEN+1);

void BadAppend( char suffix[], int n )
{
   strncat( string, suffix, n );
}

void GoodAppend( char suffix[], size_t n )
{
   strncat( string, suffix, __min( n, MAXSTRINGLEN-strlen(string)) );
}

int main( void )
{
   string[0] = '\0';
   printf( "string can hold up to %d characters\n", MAXSTRINGLEN );

   strcpy( string, "This is the initial string!" );
   // concatenate up to 20 characters...
   BadAppend( "Extra text to add to the string...", 20 );
   printf( "After BadAppend :  %s (%d chars)\n", string, strlen(string) );

   strcpy( string, "This is the initial string!" );
   // concatenate up to 20 characters...
   GoodAppend( "Extra text to add to the string...", 20 );
   printf( "After GoodAppend:  %s (%d chars)\n", string, strlen(string) );
}

Output

string can hold up to 39 characters
After BadAppend :  This is the initial string!Extra text to add to (47 chars)
After GoodAppend:  This is the initial string!Extra text t (39 chars)

Note that BadAppend caused a buffer overrun.

.NET Framework Equivalent

System::String::Concat

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