Implementation

ft_substr

ft_substr - return a new substring of string *s that begin at s[start] and have at most len of length.

If start doesn't exists in *s, we return an error. The error is a memory block of a single byte that is set to NULL, returned by ft_calloc. See that new_str_len = ft_strlen(s + start), so if len < new_str_len we have to alocate only len bytes. Otherwise, mean that len > new_str_len and we have to allocate new_str_len. Alocate new_str_len to copy the string s[start] mean duplicate the string s[start], so I call ft_strdup(s + start).

char	*ft_substr(char const *s, unsigned int start, size_t len)
{
	char	*new_str;
	size_t	new_str_len;
	size_t	s_len;

	if (!s)
		return (NULL);
	s_len = ft_strlen(s);
	if (start > s_len)
		return ((char *)ft_calloc(1, sizeof(char)));
	new_str_len = ft_strlen(s + start);
	if (len < new_str_len)
	{
		new_str = ft_calloc(len + 1, sizeof(char));
		if (!new_str)
			return (NULL);
		ft_strlcpy(new_str, s + start, len + 1);
		return (new_str);
	}
	return (ft_strdup(s + start));
}