Thread: How exactly is strspn and strscpn used in spliting up a string here

So I found a bit of code over in stackoverflow that I found a bit difficult to understand. What it does is it splits a string based on a delimiter and then stores the rest in a pointer while returning the part that was split.

Here it is

Code:

char *strtok_r(char *str, const char *delim, char **nextp)
{
    char *ret = NULL;


    if (str == NULL)
    {
        str = *nextp;
    }


    str += strspn(str, delim); // This part I don't understand


    if (*str == '\0')
    {
        return NULL;
    }


    ret = str;


    str += strcspn(str, delim); // and this part


    if (*str)
    {
        *str++ = '\0';
    }


    *nextp = str;


    return ret;
}

what strspn does is returns the length of the initial portion of str1 containing only characters that appear in str2 and strcspn does the opposite. The function's return type is size_t and not something like char *. So how is it that at the end of this function it is able to get the delimited part of the string based on the length that it got from strspn and strscpn?