I keep getting the error segmentation fault (core dumped) when I run my progam.

#include<stdio.h>
#include<stdlib.h>
    int nextword(char *str);

    int main(void)
    {
      char str[] = "Hello! Today is a beautiful day!!\t\n";
      int i = nextword(str);
       while(i != -1)
         {
          printf("%s\n",&(str[i]));
          i = nextword(NULL);
          }
      return 0;
      }

    int nextword(char *str)
    {
      // create two static variables - these stay around across calls
      static char *s;
      static int nextindex;
      int thisindex;
      // reset the static variables
      if (str != NULL)
        {
          s = str;
          thisindex = 0;
          // TODO:  advance this index past any leading spaces
          while (s[thisindex]=='\n' || s[thisindex]=='\t' || s[thisindex]==' ' )
        thisindex++;

        }
      else
        {
          // set the return value to be the nextindex
          thisindex = nextindex;
        }
      // if we aren't done with the string...
      if (thisindex != -1)
        {
          nextindex = thisindex;
          // TODO: two things
          // 1: place a '\0' after the current word
          // 2: advance nextindex to the beginning
          // of the next word
          while (s[nextindex] != ' ' && s[nextindex] != '\0')
        nextindex++;

          str[nextindex] = '\0';
          nextindex++;
        }
      return thisindex;
    }

The goal of the program is to print each word in the string str[] to the console on a new line. I am a beginning programmer and this is an assignment so I must use this type of format (no string library allowed). I just would like to know where I went wrong and how I can fix it.