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What is wcsncpy_s in C?

Talha Ashar

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The wcsncpy_s function in C copies a specified number of characters from a given wide string to another location.

If the number of characters to copy is less than the size of the destination string, the wcsncpy_s function stops after writing the null-terminating character. If there is no null-terminating character in the source string, the wcsncpy_s function automatically adds it to the destination string.

The process is illustrated below:

Note: A wide string is comprised of characters from the Unicode character set.

To use the wcsncpy_s function, you will need to include the <wchar.h> library in the program, as shown below:

#include <wchar.h>

The prototype of the wcsncpy_s function is shown below:

errno_t *wcsncpy_s(wchar_t *dest, rsize_t numElements, const wchar_t *src, rsize_t count);

Parameters

The wcsncpy_s function takes the following objects as parameters:

  • dest: the destination string
  • numElements: the size of the destination string
  • src: the source string
  • count: the maximum number of characters to copy from src to dest

Return value

The wcsncpy_s function copies the specified number of characters from the src string to the dest string and returns 00 if the operation is successful; otherwise, it returns a non-zero value and writes the null-terminating character to dest.

The wcsncpy_s function will raise an error in the following situations:

  • If either src or dest are null pointers.
  • If either numElements or count is 00.
  • If count is greater than the size of the destination string. We can avoid this by giving the count parameter a value of _TRUNCATE, in which case wcsncpy_s will only copy the number of characters that the destination string can safely hold.
  • If the src and dest strings overlap.

Example

The code below shows how the wcsncpy_s function works in C:

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
#include <wchar.h>
#include <string.h>
int main() {
// initializing strings
wchar_t src[] = L"Learning with Educative";
wchar_t dst[40];
// copying string
wcsncpy_s(dst, 40, src, wcslen(src));
return 0;
}

Explanation

First, two wide strings are initialized for the source and destination addresses. The ‘L’ identifier in line 8 informs the compiler that the Unicode character set is being used.

The wcsncpy_s function proceeds to copy the entire source string to the destination address in line 12. The argument 40 specifies that the destination string can hold a maximum of 4040 characters. The wcslen function finds the length of the source string so that the entire string is copied to dest. After the src string is copied, the wcsncpy_s function appends a null-terminating character to dest.

The GCC compiler does not support the function in the example above, so you will get an implicit declaration of function… error. Use the following variant to get the job done:

wchar_t* wcsncpy(wchar_t* dest, const wchar_t* src, size_t n);

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Talha Ashar
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Grokking Modern System Design Interview for Engineers & Managers

Ace your System Design Interview and take your career to the next level. Learn to handle the design of applications like Netflix, Quora, Facebook, Uber, and many more in a 45-min interview. Learn the RESHADED framework for architecting web-scale applications by determining requirements, constraints, and assumptions before diving into a step-by-step design process.

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