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/**@MODULEPAGE "url" - URL Module
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@section url_meta Module Meta Information

The Sofia @b url module contains macros and functions for using URL
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datatype #url_t, parsing and printing URLs. 
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@CONTACT Pekka Pessi <Pekka.Pessi@nokia.com>

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@STATUS @SofiaSIP Core library
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@LICENSE LGPL

@section url_syntax Using URL Library

The URL library provides URL datatype and helper functions related to it. 
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There is URL parser, which separates the URL components to the #url_t
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structure.

@note 
Please note that we use terms URL and URI interchangeable.

The formal URI syntax is defined in the 
<a href="http://www.ietf.org/rfc/rfc2396.txt">RFC2396</a>.

The URLs consist of a subset of printable ASCII (ECMA-5) characters. The
subset excludes space and characters commonly used as @e delimiters in
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text-based protocols, such as <b> < > # \% </b>and<b> " </b> (double
quote), and so called @e unwise characters whose positions are reserved for
national extensions in ECMA-5. In US-ASCII, those characters are:
<code><b>
{ } | \ ^ [ ] `
</b></code>
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There are also nine characters that can have special syntactic meaning in
some parts of the URI. These @e reserved characters are used to separate
syntactical parts of the URLs from each other. The reserved characters are
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as follows: <b> : @ / ; ? & = + </b>and<b> $</b>.
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The URL library understands two alternative URL syntaxes. First, the
basic syntax used by, e.g., @b ftp:, @b http: and @b rtsp: URLs:

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<i>scheme</i> ":" ["//" [ <i>user</i> [":" <i>password</i> ] "@"] 
<i>host</i> [":" <i>port</i> ] ] 
      ["/" <i>path</i> ] ["?" <i>query</i> ] ["#" <i>fragment</i> ]
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Alternatively, the syntax used by @b mailto:, @b sip:, @b im:, @b tel,
and @b pres: URLs:

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<i>scheme</i> ":" [ [ <i>user</i> [":" <i>password</i> ] "@"] 
<i>host</i> [":" <i>port</i> ] ] [";" <i>params</i> ] ["?" <i>query</i> ]
["#" <i>fragment</i> ]
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Note that also "*" is considered to be a valid URL (with type #url_any).

For example: \n
@code
http://example.org:7100/cgi-bin/query?key=90786
ftp://user:pass\@ftp.example.com/pub/
sip:user:pass\@example.com;user=ip
tel:+358718008000
@endcode
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@subsection url_parsing Converting a String to #url_t
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The function url_make() converts a string to a freshly allocated #url_t
structure. The URL components are split into parts as shown above.
The hex encoding using \% is removed if the encoded character can
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syntactically be part of the field. For instance, "%41" is decoded as
"A" in the user part, but "%40" (@) is left as is. (This is called
canonization of the URL fields.)

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The function url_format() is provided for generating the URL with
printf()-like formatting.

For example, when we make the url from the string below
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@code
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sip:joe%2Euser@example%2Ecom;method=%4D%45%53%53%41%47%45?body=CANNED%20MSG
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@endcode
the components are NUL-terminated, canonized and assigned to the structure
as follows:
@code
 url_type = url_sip
 url_root = 0 
 url_scheme = "sip"
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 url_user = "joe.user"
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 url_password = NULL
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 url_host = "example.com"
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 url_port = NULL
 url_path = NULL
 url_params = "method=MESSAGE"
 url_headers = "body=CANNED%20MSG"
 url_fragment = NULL
@endcode

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You can use the function url_param() and url_have_param() to access
particular parameters from @ref url_t::url_params "url->url_params" string.
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@subsection url_parsing Converting a #url_t structure to string
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The function url_as_string() converts contents of #url_t structure to a
newly allocated string.
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@subsection url_reference Functions and Macros in URL Module

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The include file <url.h> contains the types, function and macros of URL
module. The functions and macros are listed here for the reference, too.
The most important functions and macros for manipulating URLs are here:
@code
url_t *url_make(su_home_t *h, char const *str);
url_t *url_format(su_home_t *h, char const *fmt, ...);
char *url_as_string(su_home_t *home, url_t const *url);
url_t *url_hdup(su_home_t *h, url_t const *src);
int url_sanitize(url_t *u);
char const *url_scheme(enum url_type_e type);

#define URL_INIT_AS(type) 
void url_init(url_t *url, enum url_type_e type);

int url_cmp(url_t const *a, url_t const *b);
int url_cmp_all(url_t const *a, url_t const *b);

isize_t url_param(char const *params, char const *tag,
		  char value[], isize_t vlen);
int url_has_param(url_t const *url, char const *name);
isize_t url_have_param(char const *params, char const *tag);
int url_param_add(su_home_t *h, url_t *url, char const *param);
@endcode

There are functions for handling %-encoding used in URLs:
@code
int url_reserved_p(char const *s);
char *url_escape(char *d, char const *s, char const reserved[]);
int url_esclen(char const *s, char const reserved[]);
char *url_unescape(char *d, char const *s);
@endcode

There are a few function and macros helping resolving URLs:
@code
char const *url_port_default(enum url_type_e url_type);
char const *url_tport_default(enum url_type_e url_type);
char const *url_port(url_t const *u);
#define URL_PORT(u) 
@endcode
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In addition to the basic URL structure, #url_t, the library interface
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provides an union type #url_string_t for passing unparsed strings instead
of parsed URLs as function arguments:
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@code
#define URL_STRING_P(u) ((u) && *((url_string_t*)(u))->us_str != 0)
#define URL_IS_STRING(u) ((u) && *((url_string_t*)(u))->us_str != 0)
int url_string_p(url_string_t const * url);
int url_is_string(url_string_t const * url);
#define URL_STRING_MAKE(s) 
@endcode

There are a macros for printf()-like formatting of URLs:
@code
#define URL_PRINT_FORMAT
#define URL_PRINT_ARGS(u) 
@endcode

These functions calculate MD5 digest of URL or contribute contents of the
URL to MD5 sum:
@code
void url_update(struct su_md5_t *md5, url_t const *url);
void url_digest(void *hash, int hsize, url_t const *, char const *key);
@endcode

SIP or SIPS URIs have some parameters that control transport of the request. 
In some cases, they should be detected and removed:
@code
int url_have_transport(url_t const *u);
int url_strip_transport(url_t *u);
@endcode

Finally, there are functions used as building blocks for protocol parsers
using URLs:
@code
int url_d(url_t *url, char *s);
isize_t url_len(url_t const * url);
issize_t url_e(char buffer[], isize_t n, url_t const *url);
#define URL_E(buf, end, url)
isize_t url_xtra(url_t const * url);
issize_t url_dup(char *, isize_t , url_t *dst, url_t const *src);
#define URL_DUP(buf, end, dst, src) 
@endcode
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*/