Trying to use the strtok function to make sub strings and store into arrays

I am wanting to take in an input from a string, then go through each line and parse the line into sub strings by splitting using spaces. I have a while loop nested in a while loop to try and get this work. When I print the char* in the loop I get the expected output, but once exiting the loop and then printing different position I get unexpected values.

while ((getline(&line, &len, fp)) != -1){
    //line is an array of characters
    //piece is a char pointer that stores the sub strings of a line
    //where a string is broken into sub strings by a space
    char *piece = strtok(line, " ");
    while(piece != NULL){
      tokens[j] = piece;
      printf("%s\n", tokens[j]);
      piece = strtok(NULL, " ");
      j++;
    }
  }
  printf("%s\n", tokens[0]);

>Solution :

You either need to not overwrite line (per @AdrianMole) for instance by realloc(line, new_larger_size) so it can hold your entire input, or copy each token and subsequently free the memory allocated with for example strdup():

tokens[j] = strdup(piece);
...
// cleanup: assumes last tokens[i] is NULL.  If tokens itself is heap allocated you need to free it too
for(int i = 0; tokens[i]; i++) free(tokens[j]);

I answered a somewhat similar question yesterday: How to return 2d char array (char double pointer) in C?