How does this function parse strings? I want to understand the theory

October 15, 2022

In this code I do not quite understand the lines
s += strspn (s, delim);
and
end = s + strcspn (s, delim);
As far as I know strspn returns the index of first occurrence of a delimiter in string.
Therefore if s is the string, then the delimiter is at index strcspn (s, delim)-1.
So, the command s += strspn (s, delim) would move the string pointer to the index after the delimiter.
Which means if I have a string s = "Abc,dsf.eerr" and delim = " ,.\n" and then if I execute this line, s += strspn (s, delim); then does that mean *s = ‘d’?
What is the significance of this line end = s + strcspn (s, delim);.
I need to implement strtok where the delimiter is not deleted but returned as a parameter. I just need to understand how this code works to do so. This might be a very basic question but still thanks for your help.

char *
__strtok_r (char *s, const char *delim, char **save_ptr)
{
  char *end;
  if (s == NULL)
    s = *save_ptr;
  if (*s == '\0')
    {
      *save_ptr = s;
      return NULL;
    }
  /* Scan leading delimiters.  */
  s += strspn (s, delim);
  if (*s == '\0')
    {
      *save_ptr = s;
      return NULL;
    }
  /* Find the end of the token.  */
  end = s + strcspn (s, delim);
  if (*end == '\0')
    {
      *save_ptr = end;
      return s;
    }
  /* Terminate the token and make *SAVE_PTR point past it.  */
  *end = '\0';
  *save_ptr = end + 1;
  return s;
}

>Solution :

Imagine it like this:

char *str = ",.,.,.DEF,.,.,.GAH";
char *s = str;
char *delim = ",.";

Now, you want to get to field DEF. So what you do:

To find the start of the field, you have to advance as long as there are delimiters:

s += strspn(s, delim)
strspn(s, delim) = strlen(",.,.,.") = 6
So *s = D

To find the end of the field from there, you have to advance as long as there are no delimiters:

end = s + strcspn(s, delim)
strcspn(s, delim) = strlen("DEF") = 3
So: *end = ','

Which means if I have a string s = "Abc,dsf.eerr" and delim = " ,.\n" and then if I execute this line, s += strspn (s, delim); then does that mean *s = ‘d’?

No, strspn("Abc,dsf.eerr", " ,.\n") = 0. A is not in " ,.\n".