strncpy_s, _strncpy_s_l, wcsncpy_s, _wcsncpy_s_l, _mbsncpy_s, _mbsncpy_s_l, _tcsncpy_s, _tcsncpy_s_l, _tcsnccpy_s, _tcsnccpy_s_l (2024)

Copies characters of one string to another. These versions of strncpy, _strncpy_l, wcsncpy, _wcsncpy_l, _mbsncpy, _mbsncpy_l have security enhancements, as described in Security features in the CRT.

For _tcsnccpy_s, _tcsnccpy_s_l, _tcsnccpy_s, and _tcsnccpy_s_l see Generic-text function mappings.

Syntax

errno_t strncpy_s( char *strDest, size_t numberOfElements, const char *strSource, size_t count);errno_t _strncpy_s_l( char *strDest, size_t numberOfElements, const char *strSource, size_t count, _locale_t locale);errno_t wcsncpy_s( wchar_t *strDest, size_t numberOfElements, const wchar_t *strSource, size_t count);errno_t _wcsncpy_s_l( wchar_t *strDest, size_t numberOfElements, const wchar_t *strSource, size_t count, _locale_t locale);errno_t _mbsncpy_s( unsigned char *strDest, size_t numberOfElements, const unsigned char *strSource, size_t count);errno_t _mbsncpy_s_l( unsigned char *strDest, size_t numberOfElements, const unsigned char *strSource, size_t count, _locale_t locale);template <size_t size>errno_t strncpy_s( char (&strDest)[size], const char *strSource, size_t count); // C++ onlytemplate <size_t size>errno_t _strncpy_s_l( char (&strDest)[size], const char *strSource, size_t count, _locale_t locale); // C++ onlytemplate <size_t size>errno_t wcsncpy_s( wchar_t (&strDest)[size], const wchar_t *strSource, size_t count); // C++ onlytemplate <size_t size>errno_t _wcsncpy_s_l( wchar_t (&strDest)[size], const wchar_t *strSource, size_t count, _locale_t locale); // C++ onlytemplate <size_t size>errno_t _mbsncpy_s( unsigned char (&strDest)[size], const unsigned char *strSource, size_t count); // C++ onlytemplate <size_t size>errno_t _mbsncpy_s_l( unsigned char (&strDest)[size], const unsigned char *strSource, size_t count, _locale_t locale); // C++ only

Parameters

strDest
Destination string.

numberOfElements
The size of the destination string, in characters.

strSource
Source string.

count
Number of characters to be copied, or _TRUNCATE.

locale
The locale to use.

Return value

Zero if successful, STRUNCATE if truncation occurred, otherwise an error code.

Error conditions

Remarks

These functions try to copy the first D characters of strSource to strDest, where D is the lesser of count and the length of strSource. If those D characters will fit within strDest (whose size is given as numberOfElements) and still leave room for a null terminator, then those characters are copied and a terminating null is appended; otherwise, strDest[0] is set to the null character and the invalid parameter handler is invoked, as described in Parameter validation.

There's an exception to the above paragraph. If count is _TRUNCATE, then as much of strSource as will fit into strDest is copied while still leaving room for the terminating null, which is always appended.

For example,

char dst[5];strncpy_s(dst, 5, "a long string", 5);

means that strncpy_s copies five characters into a 5-byte buffer. This copy would leave no space for the null terminator, so strncpy_s zeroes out the string, and calls the invalid parameter handler.

If truncation behavior is needed, use _TRUNCATE or (size - 1):

strncpy_s(dst, 5, "a long string", _TRUNCATE);strncpy_s(dst, 5, "a long string", 4);

Unlike strncpy, if count is greater than the length of strSource, the destination string is NOT padded with null characters up to length count.

The behavior of strncpy_s is undefined if the source and destination strings overlap.

If strDest or strSource is NULL, or numberOfElements is 0, the invalid parameter handler is invoked. If execution is allowed to continue, the function returns EINVAL and sets errno to EINVAL.

wcsncpy_s and _mbsncpy_s are wide-character and multibyte-character versions of strncpy_s. The arguments and return value of wcsncpy_s and mbsncpy_s do vary accordingly. These six functions behave identically otherwise.

The output value is affected by the setting of the LC_CTYPE category setting of the locale. For more information, see setlocale. 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++, using these functions is simplified by template overloads; the overloads can infer buffer length automatically (eliminating the need to specify a size argument) and they can automatically replace older, non-secure functions with their newer, secure counterparts. For more information, see Secure template overloads.

The debug library versions of these functions first fill the buffer with 0xFE. To disable this behavior, use _CrtSetDebugFillThreshold.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Generic-text function mappings

The function in the tchar.h column maps to the function in the other columns depending on the character set that is defined at compile time.

Requirements

For more compatibility information, see Compatibility.

Example: Copy chars to a buffer

// crt_strncpy_s_1.cpp// compile with: /MTd// these #defines enable secure template overloads// (see last part of Examples() below)#define _CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES 1#define _CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES_COUNT 1#include <stdio.h>#include <stdlib.h>#include <string.h>#include <crtdbg.h> // For _CrtSetReportMode#include <errno.h>// This example uses a 10-byte destination buffer.errno_t strncpy_s_tester( const char * src, int count ){ char dest[10]; printf( "\n" ); if ( count == _TRUNCATE ) printf( "Copying '%s' to %d-byte buffer dest with truncation semantics\n", src, _countof(dest) ); else printf( "Copying %d chars of '%s' to %d-byte buffer dest\n", count, src, _countof(dest) ); errno_t err = strncpy_s( dest, _countof(dest), src, count ); printf( " new contents of dest: '%s'\n", dest ); return err;}void Examples(){ strncpy_s_tester( "howdy", 4 ); strncpy_s_tester( "howdy", 5 ); strncpy_s_tester( "howdy", 6 ); printf( "\nDestination buffer too small:\n" ); strncpy_s_tester( "Hi there!!", 10 ); printf( "\nTruncation examples:\n" ); errno_t err = strncpy_s_tester( "How do you do?", _TRUNCATE ); printf( " truncation %s occur\n", err == STRUNCATE ? "did" : "did not" ); err = strncpy_s_tester( "Howdy.", _TRUNCATE ); printf( " truncation %s occur\n", err == STRUNCATE ? "did" : "did not" ); printf( "\nSecure template overload example:\n" ); char dest[10]; strncpy( dest, "very very very long", 15 ); // With secure template overloads enabled (see #defines at // top of file), the preceding line is replaced by // strncpy_s( dest, _countof(dest), "very very very long", 15 ); // Instead of causing a buffer overrun, strncpy_s invokes // the invalid parameter handler. // If secure template overloads were disabled, strncpy would // copy 15 characters and overrun the dest buffer. printf( " new contents of dest: '%s'\n", dest );}void myInvalidParameterHandler( const wchar_t* expression, const wchar_t* function, const wchar_t* file, unsigned int line, uintptr_t pReserved){ wprintf(L"Invalid parameter handler invoked: %s\n", expression);}int main( void ){ _invalid_parameter_handler oldHandler, newHandler; newHandler = myInvalidParameterHandler; oldHandler = _set_invalid_parameter_handler(newHandler); // Disable the message box for assertions. _CrtSetReportMode(_CRT_ASSERT, 0); Examples();}

Copying 4 chars of 'howdy' to 10-byte buffer dest new contents of dest: 'howd'Copying 5 chars of 'howdy' to 10-byte buffer dest new contents of dest: 'howdy'Copying 6 chars of 'howdy' to 10-byte buffer dest new contents of dest: 'howdy'Destination buffer too small:Copying 10 chars of 'Hi there!!' to 10-byte buffer destInvalid parameter handler invoked: (L"Buffer is too small" && 0) new contents of dest: ''Truncation examples:Copying 'How do you do?' to 10-byte buffer dest with truncation semantics new contents of dest: 'How do yo' truncation did occurCopying 'Howdy.' to 10-byte buffer dest with truncation semantics new contents of dest: 'Howdy.' truncation did not occurSecure template overload example:Invalid parameter handler invoked: (L"Buffer is too small" && 0) new contents of dest: ''

Example: strncpy and strncpy_s

// crt_strncpy_s_2.c// contrasts strncpy and strncpy_s#include <stdio.h>#include <stdlib.h>int main( void ){ char a[20] = "test"; char s[20]; // simple strncpy usage: strcpy_s( s, 20, "dogs like cats" ); printf( "Original string:\n '%s'\n", s ); // Here we can't use strncpy_s since we don't // want null termination strncpy( s, "mice", 4 ); printf( "After strncpy (no null-termination):\n '%s'\n", s ); strncpy( s+5, "love", 4 ); printf( "After strncpy into middle of string:\n '%s'\n", s ); // If we use strncpy_s, the string is terminated strncpy_s( s, _countof(s), "mice", 4 ); printf( "After strncpy_s (with null-termination):\n '%s'\n", s );}

Original string: 'dogs like cats'After strncpy (no null-termination): 'mice like cats'After strncpy into middle of string: 'mice love cats'After strncpy_s (with null-termination): 'mice'

See also

String manipulation
Locale
Interpretation of multibyte-character sequences
_mbsnbcpy, _mbsnbcpy_l
strcat_s, wcscat_s, _mbscat_s
strcmp, wcscmp, _mbscmp
strcpy_s, wcscpy_s, _mbscpy_s
strncat_s, _strncat_s_l, wcsncat_s, _wcsncat_s_l, _mbsncat_s, _mbsncat_s_l
strncmp, wcsncmp, _mbsncmp, _mbsncmp_l
_strnicmp, _wcsnicmp, _mbsnicmp, _strnicmp_l, _wcsnicmp_l, _mbsnicmp_l
strrchr, wcsrchr, _mbsrchr, _mbsrchr_l
_strset, _strset_l, _wcsset, _wcsset_l, _mbsset, _mbsset_l
strspn, wcsspn, _mbsspn, _mbsspn_l