# URI Management

    /*
    **	(c) COPYRIGHT MIT 1995.
    **	Please first read the full copyright statement in the file COPYRIGH.
    */

This module contains code to parse URIs and various related things such as:

- Parse a URI for tokens
- Canonicalization of URIs
- Search a URI for illigal characters in order to prevent security holes

This module is implemented by HTParse.c, and it is a part of the W3C Sample Code Library.

    #ifndef HTPARSE_H
    #define HTPARSE_H

    #include "HTEscape.h"

## Parsing URIs

These functions can be used to get information in a URI.

### Parse a URI relative to another URI

This returns those parts of a name which are given (and requested) substituting bits from the related name where necessary. The `aName` argument is the (possibly relative) URI to be parsed, the `relatedName` is the URI which the `aName` is to be parsed relative to. Passing an empty string means that the `aName` is an absolute URI. The following are flag bits which may be OR'ed together to form a number to give the 'wanted' argument to HTParse. As an example we have the URL: "`http://www.w3.org/pub/WWW/TheProject.html#news`"

    #define PARSE_ACCESS		16		/* Access scheme, e.g. "HTTP" */
    #define PARSE_HOST		 8		/* Host name, e.g. "www.w3.org" */
    #define PARSE_PATH		 4		/* URL Path, e.g. "pub/WWW/TheProject.html" */
    #define PARSE_ANCHOR		 2		/* Fragment identifier, e.g. "news" */
    #define PARSE_PUNCTUATION	 1		/* Include delimiters, e.g, "/" and ":" */
    #define PARSE_ALL		31

where the format of a URI is as follows: "`ACCESS :// HOST / PATH # ANCHOR`"

`PUNCTUATION` means any delimiter like '/', ':', '#' between the tokens above. The string returned by the function must be freed by the caller.

    extern char * HTParse  (const char * aName, const char * relatedName,
    			int wanted);

### Create a Relative (Partial) URI

This function creates and returns a string which gives an expression of one address as related to another. Where there is no relation, an absolute address is retured.

On entry,
  Both names must be absolute, fully qualified names of nodes (no anchor bits)
On exit,
  The return result points to a newly allocated name which, if parsed by HTParse relative to relatedName, will yield aName. The caller is responsible for freeing the resulting name later.

    extern char * HTRelative (const char * aName, const char *relatedName);

## Is a URL Relative or Absolute?

Search the URL and determine whether it is a relative or absolute URL. We check to see if there is a ":" before any "/", "?", and "#". If this is the case then we say it is absolute. Otherwise we say it is relative.

    extern BOOL HTURL_isAbsolute (const char * url);

## URL Canonicalization

Canonicalization of URIs is a difficult job, but it saves a lot of down loads and double entries in the cache if we do a good job. A URI is allowed to contain the seqeunce xxx/../ which may be replaced by "" , and the seqeunce "/./" which may be replaced by "/". Simplification helps us recognize duplicate URIs. Thus, the following transformations are done:

- /etc/junk/../fred becomes /etc/fred
- /etc/junk/./fred becomes /etc/junk/fred

but we should NOT change

- http://fred.xxx.edu/../.. or
- ../../albert.html

In the same manner, the following prefixed are preserved:

- ./<etc>
- //<etc>

In order to avoid empty URIs the following URIs become:

- /fred/.. becomes /fred/..
- /fred/././.. becomes /fred/..
- /fred/.././junk/.././ becomes /fred/..

If more than one set of `://' is found (several proxies in cascade) then only the part after the last `://' is simplified.

    extern char *HTSimplify (char **filename);

## Prevent Security Holes

In many telnet like protocols, it can be very dangerous to allow a full ASCII character set to be in a URI. Therefore we have to strip them out. `HTCleanTelnetString()` makes sure that the given string doesn't contain characters that could cause security holes, such as newlines in ftp, gopher, news or telnet URLs; more specifically: allows everything between hexadesimal ASCII 20-7E, and also A0-FE, inclusive.

`str`
  the string that is *modified* if necessary. The string will be truncated at the first illegal character that is encountered.
returns
  YES, if the string was modified. NO, otherwise.

    extern BOOL HTCleanTelnetString (char * str);

    #endif	/* HTPARSE_H */