vswprintf() function in C/C++

Last Updated : 18 Sep, 2018

This vswprintf() function writes the wide string to the wide string buffer. A maximum of (len-1) wide characters are written to buffer which is followed by a null wide character. Syntax:

int vswprintf( wchar_t* ws, size_t len, const wchar_t* format, va_list arg )

Parameters: The function accepts four mandatory parameters which are described below:

ws: specifies the pointer to a given wide string buffer which will store the result
len: specifies the maximum length of wide characters written back to the buffer including the terminating null character
format: specifies the pointer to a null terminated wide string
arg: specifies the value identifying a variable arguments list

Note: All format specifiers have the same meaning as in printf therefore, %lc shall be used to write a wide character ( and not %c ), as well as %ls shall be used for wide strings ( and not %s ). Return value: The function returns two value as below:

On success, the vswprintf() function returns number of wide characters written excluding the terminating null wide character.
A negative number is returned on failure, including when the resulting string to be written to ws would be longer than n characters.

Below programs illustrate the above function: Program 1 :

CPP

// C++ program to illustrate the
// vswprintf() function
#include <bits/stdc++.h>
using namespace std;
 
// function to check the number
// of wide characters written
void find ( wchar_t* ws, size_t len, const wchar_t *format, ... )
{
    // hold the variable argument
    va_list arg;
     
    // A function that invokes va_start
    // shall also invoke va_end before it returns.
    va_start ( arg, format );
 
    vswprintf ( ws, len, format, arg );
 
    va_end ( arg );
}
 
// Driver code
int main ()
{
    // buffer with size 60 
    wchar_t ws[60];
    
    // initializing the string as latin characters
    wchar_t str[] = L"\u0025 \u0026 \u0027 \u0028 \u0029";
    
    // print the letters
    find(ws, 60, L"Some Latin letters : %ls\n", str);
    wprintf(L" %ls ", ws);
 
    return 0;
}

Output:

Some Latin letters : % & ' ( )

Program 2 :

CPP

// C++ program to illustrate the
// vswprintf() function
// When the size of the buffer is smaller
// than the total length of the string written 
#include <bits/stdc++.h>
using namespace std;

// function to check the number
// of wide characters written
void find ( wchar_t* ws, size_t len, const wchar_t *format, ... )
{
    // hold the variable argument
    va_list arg;
    
    // A function that invokes va_start
    // shall also invoke va_end before it returns.
    va_start ( arg, format );

    vswprintf ( ws, len, format, arg );

    va_end ( arg );
}

// Driver code
int main ()
{
    // initializing the string as english characters
    wchar_t str[] = L"Geek for geeks";

    // buffer with size 20 
    wchar_t ws[20];
    
        find(ws, 20, L"GFG is : %ls\n", str);
    wprintf(L"%ls", ws);

    return 0;
}

Output:

GFG is : Geek for g

Note: If the resulting string would be longer than n-1 wide characters, the remaining characters are discarded and not stored.

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