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<modulesynopsis metafile="mod_isapi.xml.meta">

<name>mod_isapi</name>

<description>ISAPI Extensions within Apache for Windows</description>

<status>Base</status>

<sourcefile>mod_isapi.c</sourcefile>

<identifier>isapi_module</identifier>

<compatibility>Win32 only</compatibility>

<summary>

<p>This module implements the Internet Server extension API. It

allows Internet Server extensions (<em>e.g.</em> ISAPI .dll

modules) to be served by Apache for Windows, subject to the

noted restrictions.</p>

<p>ISAPI extension modules (.dll files) are written by third

parties. The Apache Group does not author these modules, so we

provide no support for them. Please contact the ISAPI's author

directly if you are experiencing problems running their ISAPI

extension. <strong>Please <em>do not</em> post such problems to

Apache's lists or bug reporting pages.</strong></p>

</summary>

<section id="usage"><title>Usage</title>

<p>In the server configuration file, use

the <directive module="mod_mime">AddHandler</directive> directive to

associate ISAPI files with the <code>isapi-isa</code> handler, and map

it to them with their file extensions. To enable any .dll file to be

processed as an ISAPI extension, edit the httpd.conf file and add the

following line:</p>

<example>

AddHandler isapi-isa .dll

</example>

<p>There is no capability within the Apache server to leave a

requested module loaded. However, you may preload and keep a

specific module loaded by using the following syntax in your

httpd.conf:</p>

<example>

ISAPICacheFile c:/WebWork/Scripts/ISAPI/mytest.dll

</example>

<p>Whether or not you have preloaded an ISAPI extension, all

ISAPI extensions are governed by the same permissions and

restrictions as CGI scripts. That is, <directive module="core"

>Options</directive> <code>ExecCGI</code> must be set for the

directory that contains the ISAPI .dll file.</p>

<p>Review the <a href="#notes">Additional Notes</a> and the <a

href="#journal">Programmer's Journal</a> for additional details

and clarification of the specific ISAPI support offered by

<module>mod_isapi</module>.</p>

</section>

<section id="notes"><title>Additional Notes</title>

<p>Apache's ISAPI implementation conforms to all of the ISAPI

2.0 specification, except for some "Microsoft-specific"

extensions dealing with asynchronous I/O. Apache's I/O model

does not allow asynchronous reading and writing in a manner

that the ISAPI could access. If an ISA tries to access

unsupported features, including async I/O, a message is placed

in the error log to help with debugging. Since these messages

can become a flood, the directive <code>ISAPILogNotSupported

Off</code> exists to quiet this noise.</p>

<p>Some servers, like Microsoft IIS, load the ISAPI extension

into the server and keep it loaded until memory usage is too

high, or unless configuration options are specified. Apache

currently loads and unloads the ISAPI extension each time it is

requested, unless the <directive module="mod_isapi"

>ISAPICacheFile</directive> directive is specified.

This is inefficient, but Apache's memory model makes this the

most effective method. Many ISAPI modules are subtly

incompatible with the Apache server, and unloading these

modules helps to ensure the stability of the server.</p>

<p>Also, remember that while Apache supports ISAPI Extensions,

it <strong>does not support ISAPI Filters</strong>. Support for

filters may be added at a later date, but no support is planned

at this time.</p>

</section>

<section id="journal"><title>Programmer's Journal</title>

<p>If you are programming Apache 2.0 <module>mod_isapi</module>

modules, you must limit your calls to <code>ServerSupportFunction</code>

to the following directives:</p>

<dl>

<dt><code>HSE_REQ_SEND_URL_REDIRECT_RESP</code></dt>

<dd>Redirect the user to another location.<br />

This must be a fully qualified URL (<em>e.g.</em>

<code>http://server/location</code>).</dd>

<dt><code>HSE_REQ_SEND_URL</code></dt>

<dd>Redirect the user to another location.<br />

This cannot be a fully qualified URL, you are not allowed to

pass the protocol or a server name (<em>e.g.</em> simply

<code>/location</code>).<br />

This redirection is handled by the server, not the

browser.<br />

<note type="warning"><title>Warning</title>

<p>In their recent documentation, Microsoft appears to have

abandoned the distinction between the two

<code>HSE_REQ_SEND_URL</code> functions. Apache continues to treat

them as two distinct functions with different requirements

and behaviors.</p>

</note></dd>

<dt><code>HSE_REQ_SEND_RESPONSE_HEADER</code></dt>

<dd>Apache accepts a response body following the header if it

follows the blank line (two consecutive newlines) in the

headers string argument. This body cannot contain NULLs,

since the headers argument is NULL terminated.</dd>

<dt><code>HSE_REQ_DONE_WITH_SESSION</code></dt>

<dd>Apache considers this a no-op, since the session will be

finished when the ISAPI returns from processing.</dd>

<dt><code>HSE_REQ_MAP_URL_TO_PATH</code></dt>

<dd>Apache will translate a virtual name to a physical

name.</dd>

<dt><code>HSE_APPEND_LOG_PARAMETER</code></dt>

<dd>

This logged message may be captured in any of the following

logs:

<ul>

<li>in the <code>\"%{isapi-parameter}n\"</code> component in a

<directive module="mod_log_config">CustomLog</directive>

directive</li>

<li>in the <code>%q</code> log component with the

<directive module="mod_isapi">ISAPIAppendLogToQuery</directive>

<code>On</code> directive</li>

<li>in the error log with the <directive module="mod_isapi"

>ISAPIAppendLogToErrors</directive> <code>On</code> directive</li>

</ul>

<p>The first option, the <code>%{isapi-parameter}n</code> component,

is always available and preferred.</p>

</dd>

<dt><code>HSE_REQ_IS_KEEP_CONN</code></dt>

<dd>Will return the negotiated Keep-Alive status.</dd>

<dt><code>HSE_REQ_SEND_RESPONSE_HEADER_EX</code></dt>

<dd>Will behave as documented, although the <code>fKeepConn</code>

flag is ignored.</dd>

<dt><code>HSE_REQ_IS_CONNECTED</code></dt>

<dd>Will report false if the request has been aborted.</dd>

</dl>

<p>Apache returns <code>FALSE</code> to any unsupported call to

<code>ServerSupportFunction</code>, and sets the

<code>GetLastError</code> value to

<code>ERROR_INVALID_PARAMETER</code>.</p>

<p><code>ReadClient</code> retrieves the request body exceeding the

initial buffer (defined by <directive module="mod_isapi"

>ISAPIReadAheadBuffer</directive>). Based on the

<directive>ISAPIReadAheadBuffer</directive> setting (number of bytes

to buffer prior to calling the ISAPI handler) shorter requests are sent

complete to the extension when it is invoked. If the request is

longer, the ISAPI extension must use <code>ReadClient</code> to

retrieve the remaining request body.</p>

<p><code>WriteClient</code> is supported, but only with the

<code>HSE_IO_SYNC</code> flag or no option flag (value of

<code>0</code>). Any other <code>WriteClient</code> request

will be rejected with a return value of <code>FALSE</code>, and a

<code>GetLastError</code> value of

<code>ERROR_INVALID_PARAMETER</code>.</p>

<p><code>GetServerVariable</code> is supported, although extended server

variables do not exist (as defined by other servers.) All the

usual Apache CGI environment variables are available from

<code>GetServerVariable</code>, as well as the <code>ALL_HTTP</code>

and <code>ALL_RAW</code> values.</p>

<p>Apache 2.0 <module>mod_isapi</module> supports additional

features introduced in later versions of the ISAPI specification,

as well as limited emulation of async I/O and the

<code>TransmitFile</code> semantics. Apache also supports preloading

ISAPI .dlls for performance, neither of which were not available under

Apache 1.3 <code>mod_isapi</code>.</p>

</section>

<directivesynopsis>

<name>ISAPICacheFile</name>

<description>ISAPI .dll files to be loaded at startup</description>

<syntax>ISAPICacheFile <var>file-path</var> [<var>file-path</var>]

...</syntax>

<contextlist><context>server config</context><context>virtual host</context>

</contextlist>

<usage>

<p>Specifies a space-separated list of file names to be loaded

when the Apache server is launched, and remain loaded until the

server is shut down. This directive may be repeated for every

ISAPI .dll file desired. The full path name of each file should

be specified. If the path name is not absolute, it will be treated

relative to <directive module="core">ServerRoot</directive>.</p>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>ISAPIReadAheadBuffer</name>

<description>Size of the Read Ahead Buffer sent to ISAPI

extensions</description>

<syntax>ISAPIReadAheadBuffer <var>size</var></syntax>

<default>ISAPIReadAheadBuffer 49152</default>

<contextlist><context>server config</context><context>virtual host</context>

<context>directory</context><context>.htaccess</context></contextlist>

<override>FileInfo</override>

<usage>

<p>Defines the maximum size of the Read Ahead Buffer sent to

ISAPI extensions when they are initially invoked. All remaining

data must be retrieved using the <code>ReadClient</code> callback; some

ISAPI extensions may not support the <code>ReadClient</code> function.

Refer questions to the ISAPI extension's author.</p>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>ISAPILogNotSupported</name>

<description>Log unsupported feature requests from ISAPI

extensions</description>

<syntax>ISAPILogNotSupported on|off</syntax>

<default>ISAPILogNotSupported off</default>

<contextlist><context>server config</context><context>virtual host</context>

<context>directory</context><context>.htaccess</context></contextlist>

<override>FileInfo</override>

<usage>

<p>Logs all requests for unsupported features from ISAPI

extensions in the server error log. This may help administrators

to track down problems. Once set to on and all desired ISAPI modules

are functioning, it should be set back to off.</p>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>ISAPIAppendLogToErrors</name>

<description>Record <code>HSE_APPEND_LOG_PARAMETER</code> requests from

ISAPI extensions to the error log</description>

<syntax>ISAPIAppendLogToErrors on|off</syntax>

<default>ISAPIAppendLogToErrors off</default>

<contextlist><context>server config</context><context>virtual host</context>

<context>directory</context><context>.htaccess</context></contextlist>

<override>FileInfo</override>

<usage>

<p>Record <code>HSE_APPEND_LOG_PARAMETER</code> requests from ISAPI

extensions to the server error log.</p>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>ISAPIAppendLogToQuery</name>

<description>Record <code>HSE_APPEND_LOG_PARAMETER</code> requests from

ISAPI extensions to the query field</description>

<syntax>ISAPIAppendLogToQuery on|off</syntax>

<default>ISAPIAppendLogToQuery on</default>

<contextlist><context>server config</context><context>virtual host</context>

<context>directory</context><context>.htaccess</context></contextlist>

<override>FileInfo</override>

<usage>

<p>Record <code>HSE_APPEND_LOG_PARAMETER</code> requests from ISAPI

extensions to the query field (appended to the <directive

module="mod_log_config">CustomLog</directive> <code>%q</code>

component).</p>

</usage>

</directivesynopsis>

<directivesynopsis>

<name>ISAPIFakeAsync</name>

<description>Fake asynchronous support for ISAPI callbacks</description>

<syntax>ISAPIFakeAsync on|off</syntax>

<default>ISAPIFakeAsync off</default>

<contextlist><context>server config</context><context>virtual host</context>

<context>directory</context><context>.htaccess</context></contextlist>

<override>FileInfo</override>

<usage>

<p>While set to on, asynchronous support for ISAPI callbacks is

simulated.</p>

</usage>

</directivesynopsis>

</modulesynopsis>