wcsncpy, wcsncpy_s

Defined in header `<wchar.h>`
	(1)
`wchar_t* wcsncpy( wchar_t* dest, const wchar_t* src, size_t count );`		(since C95) (until C99)
`wchar_t wcsncpy( wchar_t restrict dest, const wchar_t *restrict src, size_t count );`		(since C99)
`errno_t wcsncpy_s( wchar_t restrict dest, rsize_t destsz, const wchar_t restrict src, rsize_t count);`	(2)	(since C11)

1) Copies at most count characters of the wide string pointed to by src (including the terminating null wide character) to wide character array pointed to by dest.

If count is reached before the entire string src was copied, the resulting wide character array is not null-terminated.

If, after copying the terminating null wide character from src, count is not reached, additional null wide characters are written to dest until the total of count characters have been written.

If the strings overlap, the behavior is undefined.

2) Same as (1), except that the function does not continue writing zeroes into the destination array to pad up to count, it stops after writing the terminating null character (if there was no null in the source, it writes one at dest[count] and then stops). Also, the following errors are detected at runtime and call the currently installed constraint handler function:

src or dest is a null pointer
destsz or count is zero or greater than RSIZE_MAX/sizeof(wchar_t)
count is greater or equal destsz, but destsz is less or equal wcsnlen_s(src, count), in other words, truncation would occur
overlap would occur between the source and the destination strings

As with all bounds-checked functions, wcsncpy_s only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <wchar.h>.

Parameters

dest	-	pointer to the wide character array to copy to
src	-	pointer to the wide string to copy from
count	-	maximum number of wide characters to copy
destsz	-	the size of the destination buffer

Return value

1) returns a copy of dest

2) returns zero on success, returns non-zero on error. Also, on error, writes L'\0' to dest[0] (unless dest is a null pointer or destsz is zero or greater than RSIZE_MAX/sizeof(wchar_t)) and may clobber the rest of the destination array with unspecified values.

Notes

In typical usage, count is the number of elements in the destination array.

Although truncation to fit the destination buffer is a security risk and therefore a runtime constraints violation for wcsncpy_s, it is possible to get the truncating behavior by specifying count equal to the size of the destination array minus one: it will copy the first count wide characters and append the null wide terminator as always: wcsncpy_s(dst, sizeof dst / sizeof *dst, src, (sizeof dst / sizeof *dst)-1);

Example

#include <stdio.h>
#include <wchar.h>
#include <locale.h>
 
int main(void)
{
    const wchar_t src[] = L"わゐ";
    wchar_t dest[6] = {L'あ', L'い', L'う', L'え', L'お'};
 
    wcsncpy(dest, src, 4); // this will copy わゐ and repeat L'\0' two times
 
    puts("The contents of dest are: ");
    setlocale(LC_ALL, "en_US.utf8");
 
    const long dest_size = sizeof dest / sizeof *dest;
    for(wchar_t* p = dest; p-dest != dest_size; ++p) {
        *p ? printf("%lc ", *p)
           : printf("\\0 ");
    }
}

Possible output:

The contents of dest are: 
わ ゐ \0 \0 お \0

References

C17 standard (ISO/IEC 9899:2018):

7.29.4.2.2 The wcsncpy function (p: 314)
K.3.9.2.1.2 The wcsncpy_s function (p: 464)

C11 standard (ISO/IEC 9899:2011):

7.29.4.2.2 The wcsncpy function (p: 431)
K.3.9.2.1.2 The wcsncpy_s function (p: 640-641)

C99 standard (ISO/IEC 9899:1999):

7.24.4.2.2 The wcsncpy function (p: 377)

wcscpywcscpy_s (C95)(C11)	copies one wide string to another (function)
wmemcpywmemcpy_s (C95)(C11)	copies a certain amount of wide characters between two non-overlapping arrays (function)
strncpystrncpy_s (C11)	copies a certain amount of characters from one string to another (function)
C++ documentation for `wcsncpy`

Docs

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