strcspn - returns the index of the first character in str1 that matches any of the characters in str2 in C

//Declaration:  size_t strcspn(const char *str1, const char *str2); 
//Return:       returns the index of the first character in *str1 that matches any of the characters in *str2. 

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

  int main(void)
  {
    int len;
    len = strcspn("this is a test", "ab");
    printf("%d", len);

    return 0;
  }

         
/*
8*/