modiface.sgml

用来作为linux中SIP SERVER,完成VOIP网络电话中服务器的功能

<!-- $Id: modiface.sgml,v 1.4 2004/08/24 08:45:12 janakj Exp $ -->
<!DOCTYPE Book PUBLIC "-//OASIS//DTD DocBook V4.2//EN" [
<!ENTITY ser "SIP Express Router">
<!ENTITY moddir "/usr/lib/ser/modules">
]>

<book>
    <chapter>
	<title>Module Interface</title>
	<abstract>
	    <para>
		&ser; features modular architecture which allows us to split &ser;'s functionality
		across several modules. This approach gives us greater flexibility, only required
		set of functions can be loaded upon startup which minimizes the server's memory
		footprint. Modules can be also provided by 3rd party developers and distributed
		separately from the main server. Most of the functionality that &ser; provides is
		available through modules, the core itself contains only minimum set of functions
		that is essential for proper server's behavior or that is needed by all modules.
	    </para>
	    <para>
		This chapter provides detailed information on module interface of &ser;, which is
		used to pass information on available functions and parameters from the modules to
		the core.
	    </para>
	</abstract>
	<section>
	    <title>Shared Objects</title>
	    <abstract>
		<para>
		    First it would be good to know how &ser; loads and uses modules before we
		    describe the module interface in detail. This section gives a brief overview of
		    &ser;'s module subsystem.
		</para>
	    </abstract>
	    <para>
		&ser; modules are compiled as <quote>shared objects</quote>. A file containing a
		shared object has usually .so suffix. All modules (shared objects) will be stored in
		one directory after installation. For example <abbrev>tm</abbrev> module, which
		contains code essential for stateful processing, will be stored in file named
		<filename moreinfo="none">tm.so</filename>. By default these files are stored in
		<filename moreinfo="none">&moddir;</filename> directory.
	    </para>
	    <para>
		You can later load the modules using <command moreinfo="none">loadmodule</command>
		command in your configuration file. If you want to load previously mentioned
		<filename moreinfo="none">tm.so</filename> module, you can do it using <command
		moreinfo="none">loadmodule "&moddir;/tm.so"</command> in your configuration
		file. This command invokes dynamic linker provided by the operating system which
		opens <filename moreinfo="none">tm.so</filename> file, loads it into memory and
		resolves all symbol dependencies (a module might require symbols from the core, for
		example functions and variables).
	    </para>
	    <para>
		As the last step of the module loading the core tries to find variable named
		<varname>exports</varname>, which describes all functions and parameters provided by
		the module. These functions and parameters are later available to the server and can
		be used either in the configuration file or by other modules.
	    </para>
	</section>
	<section>
	    <title>Exporting Functions</title>
	    <abstract>
		<para>
		    Each module can provide zero or more functions, which can be used in the
		    configuration file or by other modules internally. This section gives a detailed
		    description of structure describing exported functions and passing this
		    information to the core through the module interface.
		</para>
	    </abstract>
	    <para>
		Each function exported by a module must be described by
		<structname>cmd_export_t</structname> structure. Structures describing all exported
		functions are arranged into an array and pointer to the array is then passed to the
		core. The last element of the array must contain 0 in all it's fields, this element
		serves as the mark telling the core that this is the very last element and it must
		stop scanning the array.
	    </para>
	    <para>
		Each exported function is described by the following structure:
	    </para>
	    <programlisting format="linespecific">
struct cmd_export_ {
	char* name;             /* null terminated command name */
	cmd_function function;  /* pointer to the corresponding function */
	int param_no;           /* number of parameters used by the function */
	fixup_function fixup;   /* pointer to the function called to "fix" the parameters */
	int flags;              /* Function flags */
};	   

typedef struct cmd_export_ cmd_export_t;
</programlisting>
	    <itemizedlist>
		<title>Meaning of the fileds:</title>
		<listitem>
		    <simpara><varname>char* name</varname><simpara> 
		    <simpara>
			This is the name under which the function will be visible to the
			core. Usually it is the same as the name of the corresponding function.
		    </simpara>
		</listitem>
		<listitem>
		    <simpara><varname>cmd_function function</varname></simpara> 
		    <para>cmd_function type is defined as follows: </para>
		    <programlisting format="linespecific">
typedef int (*cmd_function)(struct sip_msg*, char*, char*); 
</programlisting>
		    <simpara>
			The first parameter is a <acronym>SIP</acronym> message being processed, the
			other 2 parameters are given from the configuration file.
		    </simpara>
		    <note>
			<simpara>
			    From time to time you might need to export a function that has different
			    synopsis. This can happen if you export functions that are supposed to
			    be called by other modules only and must not be called from the
			    configuration script. In this case you will have to do type-casting
			    otherwise the compiler will complain and will not compile your module.
			</simpara>
			<simpara>
			    Simply put (cmd_function) just before the function name, for example
			    <function moreinfo="none">(cmd_function)my_function</function>.  Don't
			    use this unless you know what are you doing ! The server might crash if
			    you pass wrong parameters to the function later !
			</simpara>
		    </note>
		</listitem>
		<listitem>
		    <simpara><varname>int param_no</varname></simpara>
		    <simpara>
			Number of parameters of the function. It can be 0, 1 or 2. The function will
			be not visible from the configuration script if you use another value.
		    </simpara>
		</listitem>
		<listitem>
		    <simpara><varname>fixup_function fixup</varname></simpara>
		    <simpara>
			This is the function that will be used to <quote>fixup</quote> function
			parameters. Set this field to 0 if you don't need this.
		    </simpara>
		    <simpara>
			If you provide pointer to a fixup function in this field, the fixup function
			will be called for each occurrence of the exported function in the
			configuration script.
		    </simpara>
		    <simpara>
			The fixup function can be used to perform some operation on the function
			parameters. For example, if one of the parameters is a regular expression,
			you can use the fixup to compile the regular expression. The fixup functions
			are called only once - upon the server startup and so the regular expression
			will be compiled before the server starts processing messages. When the
			server calls the exported function to process a <acronym>SIP</acronym>
			message, the function will be given the already compiled regular expression
			and doesn't have to compile it again. This is a significant performance
			improvement.
		    </simpara>
		    <simpara>
			Fixup functions can also be used to convert string to integer. As you have
			might noticed, the exported functions accept up to 2 parameters of type
			char*. Because of that it is not possible to pass integer parameters from
			the script files directly. If you want to pass an integer as a parameter,
			you must pass it as string (i.e. enclosed in quotes).
		    </simpara>
		    <simpara>
			Fixup function can be used to convert the string back to integer. Such a
			conversion should happend only once because the string parameter doesn't
			change when the server is running. Fixup is therefore ideal place for the
			conversion, it will be converted upon the server startup before the server
			starts processing <acronym>SIP</acronym> messages. After the conversion the
			function will get directly the converted value. See existing modules for
			example of such a fixup function.
		    </simpara>
		</listitem>
		<listitem>
		    <simpara><varname>int flags</varname></simpara>
		    <simpara>
			Usage of each function can be restricted. You may want to write a function
			that can be used by other modules but cannot be called from the script. If
			you write a function that is supposed to process <acronym>SIP</acronym>
			requests only, you may want to restrict it so it will be never called for
			<acronym>SIP</acronym> replies and vice versa. That's what is flags field
			for.
		    </simpara>
		    <simpara>
			This field is OR value of different flags. Currently only REQUEST_ROUTE and
			REPLY_ROUTE flags are defined and used by the core. If you use REQUEST_ROUTE
			flag, then the function can be called from the main route block. If you use
			REPLY_ROUTE flag, then the function can be called from reply route blocks
			(More on this in the SER User's Guide). If this field is set to 0, then the
			function can be called internally (i.e. from other modules) only. If you
			want to make your function callable anywhere in the script, you can use
			REQUEST_ROUTE | REPLY_ROUTE.
		    </simpara>
		</listitem>
	    </itemizedlist>
	</section>
	<section>
	    <title>Exporting Parameters</title>
	    <abstract>
		<simpara>
		    Each module can provide zero or more parameters, which can affect the module's
		    behavior. This section gives a detailed description of structures describing
		    exported parameters and passing this information to the core through the module
		    interface.
		</simpara>
	    </abstract>
	    <simpara>
		Each parameter exported by a module must be described by
		<structname>param_export_t</structname> structure. Structures describing all
		exported parameters are arranged into an array and pointer to the array is then
		passed to the core. The last element of the array must contain 0 in all it's fields,
		this element serves as the mark telling the core that this is the very last element
		and it must stop scanning the array (This is same as in array of exported
		functions).
	    </simpara>
	    <simpara>
		Each exported parameter is described by the following structure:
	    </simpara>
	    <programlisting format="linespecific">