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

  /* This test sets up 2 strings, buffer and w_string. It then   */
  /* uses wcsspn() to calculate the maximum segment of w_string  */
  /* that consists entirely of characters 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, "abcedjklmabcjklabcdehjkl", STRING_SIZE)

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

  /* Using wcsspn - work out the largest string in w_string that */
  /* consists entirely of characters from buffer                 */

            result = wcsspn(w_string, buffer);
        printf("Longest segment found in w_string is: %d", result);

        }

    Running the example program produces the following result:

    Longest segment found in w_string is: 5