#include <stdlib.h>
        #include <stdio.h>
        #include <wchar.h>
        #include <string.h>

        /* This test sets up 2 strings, buffer and w_string, and */
        /* then uses wcscspn() to calculate the maximum segment  */
        /* of w_string, which consists entirely of characters    */
        /* NOT from buffer.                                      */

        #define BUFF_SIZE 20
        #define STRING_SIZE 50

        main()
        {
            wchar_t buffer[BUFF_SIZE];
            wchar_t w_string[STRING_SIZE];
            size_t result;

            /* Initialize the buffer */

            if (mbstowcs(buffer, "abcdefg", BUFF_SIZE) == (size_t)-1) {

                perror("mbstowcs");
                exit(EXIT_FAILURE);
            }

            /* Initialize the string */

            if (mbstowcs(w_string, "jklmabcjklabcdehjklmno", STRING_SIZE)

                perror("mbstowcs");
                exit(EXIT_FAILURE);
            }

        /* Using wcscspn - work out the largest string in w_string */
        /* which consists entirely of characters NOT from buffer   */

            result = wcscspn(w_string, buffer);
            printf("Longest segment NOT found in w_string is: %d", result);

        }

    Running the example program produces the following result:

    Longest segment NOT found in w_string is: 4