A use case of strtok_r (instead of strtok) to split a string str wrt ifs characters.

string_splitting.c

char **ifs_split(const char *str, const char *ifs)
{
    size_t        index = 0;
    size_t        size = 1;
    char          *segment = NULL;
    char          *start = NULL;
    char          *rest = NULL;
    char          **tab = NULL; // Thou shall free thee.

    start = dupstr(str);
    tab = calloc(1, sizeof (char *));
    if (NULL == tab)
        return NULL;

    segment = strtok_r(start, ifs, &rest);
    do
        tab = append_str(segment, tab, &index, &size);
    while (NULL != (segment = strtok_r(rest, ifs, &rest)));

    free(start);
    return tab; // Is NULL-terminated.
}

/* dupstr is my strdup. If you don't see what append_str() does */
char **append_str(const char *str,   char   **tab,
                  size_t     *index, size_t *size)
{
    char **tmp = tab;

    if (*index + 1 >= *size)
    {
        tmp = realloc(tab, (*size *= 2) * sizeof (char *));
        if (NULL == tmp)
        {
            free(tab);
            return NULL;
        }
    }

    tmp[*index] = dupstr(str);
    tmp[++(*index)] = NULL;

    return tmp;
}