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+<title>Apache 1.3 API notes - Apache HTTP Server</title>
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+<body id="manual-page"><div id="page-header">
+<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
+<p class="apache">Apache HTTP Server Version 2.0</p>
+<img alt="" src="../images/feather.gif" /></div>
+<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
+<div id="path">
+<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.0</a> > <a href="./">Developer Documentation</a></div><div id="page-content"><div id="preamble"><h1>Apache 1.3 API notes</h1>
+<div class="toplang">
+<p><span>Available Languages: </span><a href="../en/developer/API.html" title="English"> en </a></p>
+</div>
+
+ <div class="warning"><h3>Warning</h3>
+ <p>This document has not been updated to take into account changes made
+ in the 2.0 version of the Apache HTTP Server. Some of the information may
+ still be relevant, but please use it with care.</p>
+ </div>
+
+ <p>These are some notes on the Apache API and the data structures you have
+ to deal with, <em>etc.</em> They are not yet nearly complete, but hopefully,
+ they will help you get your bearings. Keep in mind that the API is still
+ subject to change as we gain experience with it. (See the TODO file for
+ what <em>might</em> be coming). However, it will be easy to adapt modules
+ to any changes that are made. (We have more modules to adapt than you
+ do).</p>
+
+ <p>A few notes on general pedagogical style here. In the interest of
+ conciseness, all structure declarations here are incomplete -- the real
+ ones have more slots that I'm not telling you about. For the most part,
+ these are reserved to one component of the server core or another, and
+ should be altered by modules with caution. However, in some cases, they
+ really are things I just haven't gotten around to yet. Welcome to the
+ bleeding edge.</p>
+
+ <p>Finally, here's an outline, to give you some bare idea of what's coming
+ up, and in what order:</p>
+
+ <ul>
+ <li>
+ <a href="#basics">Basic concepts.</a>
+
+ <ul>
+ <li><a href="#HMR">Handlers, Modules, and
+ Requests</a></li>
+
+ <li><a href="#moduletour">A brief tour of a
+ module</a></li>
+ </ul>
+ </li>
+
+ <li>
+ <a href="#handlers">How handlers work</a>
+
+ <ul>
+ <li><a href="#req_tour">A brief tour of the
+ <code>request_rec</code></a></li>
+
+ <li><a href="#req_orig">Where request_rec structures come
+ from</a></li>
+
+ <li><a href="#req_return">Handling requests, declining,
+ and returning error codes</a></li>
+
+ <li><a href="#resp_handlers">Special considerations for
+ response handlers</a></li>
+
+ <li><a href="#auth_handlers">Special considerations for
+ authentication handlers</a></li>
+
+ <li><a href="#log_handlers">Special considerations for
+ logging handlers</a></li>
+ </ul>
+ </li>
+
+ <li><a href="#pools">Resource allocation and resource
+ pools</a></li>
+
+ <li>
+ <a href="#config">Configuration, commands and the like</a>
+
+ <ul>
+ <li><a href="#per-dir">Per-directory configuration
+ structures</a></li>
+
+ <li><a href="#commands">Command handling</a></li>
+
+ <li><a href="#servconf">Side notes --- per-server
+ configuration, virtual servers, <em>etc</em>.</a></li>
+ </ul>
+ </li>
+ </ul>
+</div>
+<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#basics">Basic concepts</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#handlers">How handlers work</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#pools">Resource allocation and resource pools</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#config">Configuration, commands and the like</a></li>
+</ul></div>
+<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="basics" id="basics">Basic concepts</a></h2>
+ <p>We begin with an overview of the basic concepts behind the API, and how
+ they are manifested in the code.</p>
+
+ <h3><a name="HMR" id="HMR">Handlers, Modules, and Requests</a></h3>
+ <p>Apache breaks down request handling into a series of steps, more or
+ less the same way the Netscape server API does (although this API has a
+ few more stages than NetSite does, as hooks for stuff I thought might be
+ useful in the future). These are:</p>
+
+ <ul>
+ <li>URI -> Filename translation</li>
+ <li>Auth ID checking [is the user who they say they are?]</li>
+ <li>Auth access checking [is the user authorized <em>here</em>?]</li>
+ <li>Access checking other than auth</li>
+ <li>Determining MIME type of the object requested</li>
+ <li>`Fixups' -- there aren't any of these yet, but the phase is intended
+ as a hook for possible extensions like <code class="directive"><a href="../mod/mod_env.html#setenv">SetEnv</a></code>, which don't really fit well elsewhere.</li>
+ <li>Actually sending a response back to the client.</li>
+ <li>Logging the request</li>
+ </ul>
+
+ <p>These phases are handled by looking at each of a succession of
+ <em>modules</em>, looking to see if each of them has a handler for the
+ phase, and attempting invoking it if so. The handler can typically do one
+ of three things:</p>
+
+ <ul>
+ <li><em>Handle</em> the request, and indicate that it has done so by
+ returning the magic constant <code>OK</code>.</li>
+
+ <li><em>Decline</em> to handle the request, by returning the magic integer
+ constant <code>DECLINED</code>. In this case, the server behaves in all
+ respects as if the handler simply hadn't been there.</li>
+
+ <li>Signal an error, by returning one of the HTTP error codes. This
+ terminates normal handling of the request, although an ErrorDocument may
+ be invoked to try to mop up, and it will be logged in any case.</li>
+ </ul>
+
+ <p>Most phases are terminated by the first module that handles them;
+ however, for logging, `fixups', and non-access authentication checking,
+ all handlers always run (barring an error). Also, the response phase is
+ unique in that modules may declare multiple handlers for it, via a
+ dispatch table keyed on the MIME type of the requested object. Modules may
+ declare a response-phase handler which can handle <em>any</em> request,
+ by giving it the key <code>*/*</code> (<em>i.e.</em>, a wildcard MIME type
+ specification). However, wildcard handlers are only invoked if the server
+ has already tried and failed to find a more specific response handler for
+ the MIME type of the requested object (either none existed, or they all
+ declined).</p>
+
+ <p>The handlers themselves are functions of one argument (a
+ <code>request_rec</code> structure. vide infra), which returns an integer,
+ as above.</p>
+
+
+ <h3><a name="moduletour" id="moduletour">A brief tour of a module</a></h3>
+ <p>At this point, we need to explain the structure of a module. Our
+ candidate will be one of the messier ones, the CGI module -- this handles
+ both CGI scripts and the <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code> config file command. It's actually a great deal
+ more complicated than most modules, but if we're going to have only one
+ example, it might as well be the one with its fingers in every place.</p>
+
+ <p>Let's begin with handlers. In order to handle the CGI scripts, the
+ module declares a response handler for them. Because of <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>, it also has handlers for the
+ name translation phase (to recognize <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>ed URIs), the type-checking phase (any
+ <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>ed request is typed
+ as a CGI script).</p>
+
+ <p>The module needs to maintain some per (virtual) server information,
+ namely, the <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>es in
+ effect; the module structure therefore contains pointers to a functions
+ which builds these structures, and to another which combines two of them
+ (in case the main server and a virtual server both have <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>es declared).</p>
+
+ <p>Finally, this module contains code to handle the <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code> command itself. This particular
+ module only declares one command, but there could be more, so modules have
+ <em>command tables</em> which declare their commands, and describe where
+ they are permitted, and how they are to be invoked.</p>
+
+ <p>A final note on the declared types of the arguments of some of these
+ commands: a <code>pool</code> is a pointer to a <em>resource pool</em>
+ structure; these are used by the server to keep track of the memory which
+ has been allocated, files opened, <em>etc.</em>, either to service a
+ particular request, or to handle the process of configuring itself. That
+ way, when the request is over (or, for the configuration pool, when the
+ server is restarting), the memory can be freed, and the files closed,
+ <em>en masse</em>, without anyone having to write explicit code to track
+ them all down and dispose of them. Also, a <code>cmd_parms</code>
+ structure contains various information about the config file being read,
+ and other status information, which is sometimes of use to the function
+ which processes a config-file command (such as <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>). With no further ado, the
+ module itself:</p>
+
+ <div class="example"><p><code>
+ /* Declarations of handlers. */<br />
+ <br />
+ int translate_scriptalias (request_rec *);<br />
+ int type_scriptalias (request_rec *);<br />
+ int cgi_handler (request_rec *);<br />
+ <br />
+ /* Subsidiary dispatch table for response-phase <br />
+ * handlers, by MIME type */<br />
+ <br />
+ handler_rec cgi_handlers[] = {<br />
+ <span class="indent">
+ { "application/x-httpd-cgi", cgi_handler },<br />
+ { NULL }<br />
+ </span>
+ };<br />
+ <br />
+ /* Declarations of routines to manipulate the <br />
+ * module's configuration info. Note that these are<br />
+ * returned, and passed in, as void *'s; the server<br />
+ * core keeps track of them, but it doesn't, and can't,<br />
+ * know their internal structure.<br />
+ */<br />
+ <br />
+ void *make_cgi_server_config (pool *);<br />
+ void *merge_cgi_server_config (pool *, void *, void *);<br />
+ <br />
+ /* Declarations of routines to handle config-file commands */<br />
+ <br />
+ extern char *script_alias(cmd_parms *, void *per_dir_config, char *fake,
+ char *real);<br />
+ <br />
+ command_rec cgi_cmds[] = {<br />
+ <span class="indent">
+ { "ScriptAlias", script_alias, NULL, RSRC_CONF, TAKE2,<br />
+ <span class="indent">"a fakename and a realname"},<br /></span>
+ { NULL }<br />
+ </span>
+ };<br />
+ <br />
+ module cgi_module = {
+</code></p><pre> STANDARD_MODULE_STUFF,
+ NULL, /* initializer */
+ NULL, /* dir config creator */
+ NULL, /* dir merger */
+ make_cgi_server_config, /* server config */
+ merge_cgi_server_config, /* merge server config */
+ cgi_cmds, /* command table */
+ cgi_handlers, /* handlers */
+ translate_scriptalias, /* filename translation */
+ NULL, /* check_user_id */
+ NULL, /* check auth */
+ NULL, /* check access */
+ type_scriptalias, /* type_checker */
+ NULL, /* fixups */
+ NULL, /* logger */
+ NULL /* header parser */
+};</pre></div>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="handlers" id="handlers">How handlers work</a></h2>
+ <p>The sole argument to handlers is a <code>request_rec</code> structure.
+ This structure describes a particular request which has been made to the
+ server, on behalf of a client. In most cases, each connection to the
+ client generates only one <code>request_rec</code> structure.</p>
+
+ <h3><a name="req_tour" id="req_tour">A brief tour of the request_rec</a></h3>
+ <p>The <code>request_rec</code> contains pointers to a resource pool
+ which will be cleared when the server is finished handling the request;
+ to structures containing per-server and per-connection information, and
+ most importantly, information on the request itself.</p>
+
+ <p>The most important such information is a small set of character strings
+ describing attributes of the object being requested, including its URI,
+ filename, content-type and content-encoding (these being filled in by the
+ translation and type-check handlers which handle the request,
+ respectively).</p>
+
+ <p>Other commonly used data items are tables giving the MIME headers on
+ the client's original request, MIME headers to be sent back with the
+ response (which modules can add to at will), and environment variables for
+ any subprocesses which are spawned off in the course of servicing the
+ request. These tables are manipulated using the <code>ap_table_get</code>
+ and <code>ap_table_set</code> routines.</p>
+
+ <div class="note">
+ <p>Note that the <code>Content-type</code> header value <em>cannot</em>
+ be set by module content-handlers using the <code>ap_table_*()</code>
+ routines. Rather, it is set by pointing the <code>content_type</code>
+ field in the <code>request_rec</code> structure to an appropriate
+ string. <em>e.g.</em>,</p>
+ <div class="example"><p><code>
+ r->content_type = "text/html";
+ </code></p></div>
+ </div>
+
+ <p>Finally, there are pointers to two data structures which, in turn,
+ point to per-module configuration structures. Specifically, these hold
+ pointers to the data structures which the module has built to describe
+ the way it has been configured to operate in a given directory (via
+ <code>.htaccess</code> files or <code class="directive"><a href="../mod/core.html#directory"><Directory></a></code> sections), for private data it has built in the
+ course of servicing the request (so modules' handlers for one phase can
+ pass `notes' to their handlers for other phases). There is another such
+ configuration vector in the <code>server_rec</code> data structure pointed
+ to by the <code>request_rec</code>, which contains per (virtual) server
+ configuration data.</p>
+
+ <p>Here is an abridged declaration, giving the fields most commonly
+ used:</p>
+
+ <div class="example"><p><code>
+ struct request_rec {<br />
+ <br />
+ pool *pool;<br />
+ conn_rec *connection;<br />
+ server_rec *server;<br />
+ <br />
+ /* What object is being requested */<br />
+ <br />
+ char *uri;<br />
+ char *filename;<br />
+ char *path_info;
+</code></p><pre>char *args; /* QUERY_ARGS, if any */
+struct stat finfo; /* Set by server core;
+ * st_mode set to zero if no such file */</pre><p><code>
+ char *content_type;<br />
+ char *content_encoding;<br />
+ <br />
+ /* MIME header environments, in and out. Also, <br />
+ * an array containing environment variables to<br />
+ * be passed to subprocesses, so people can write<br />
+ * modules to add to that environment.<br />
+ *<br />
+ * The difference between headers_out and <br />
+ * err_headers_out is that the latter are printed <br />
+ * even on error, and persist across internal<br />
+ * redirects (so the headers printed for <br />
+ * <code class="directive"><a href="../mod/core.html#errordocument">ErrorDocument</a></code> handlers will have
+ them).<br />
+ */<br />
+ <br />
+ table *headers_in;<br />
+ table *headers_out;<br />
+ table *err_headers_out;<br />
+ table *subprocess_env;<br />
+ <br />
+ /* Info about the request itself... */<br />
+ <br />
+</code></p><pre>int header_only; /* HEAD request, as opposed to GET */
+char *protocol; /* Protocol, as given to us, or HTTP/0.9 */
+char *method; /* GET, HEAD, POST, <em>etc.</em> */
+int method_number; /* M_GET, M_POST, <em>etc.</em> */
+
+</pre><p><code>
+ /* Info for logging */<br />
+ <br />
+ char *the_request;<br />
+ int bytes_sent;<br />
+ <br />
+ /* A flag which modules can set, to indicate that<br />
+ * the data being returned is volatile, and clients<br />
+ * should be told not to cache it.<br />
+ */<br />
+ <br />
+ int no_cache;<br />
+ <br />
+ /* Various other config info which may change<br />
+ * with .htaccess files<br />
+ * These are config vectors, with one void*<br />
+ * pointer for each module (the thing pointed<br />
+ * to being the module's business).<br />
+ */<br />
+ <br />
+</code></p><pre>void *per_dir_config; /* Options set in config files, <em>etc.</em> */
+void *request_config; /* Notes on *this* request */
+
+</pre><p><code>
+ };
+ </code></p></div>
+
+
+ <h3><a name="req_orig" id="req_orig">Where request_rec structures come from</a></h3>
+ <p>Most <code>request_rec</code> structures are built by reading an HTTP
+ request from a client, and filling in the fields. However, there are a
+ few exceptions:</p>
+
+ <ul>
+ <li>If the request is to an imagemap, a type map (<em>i.e.</em>, a
+ <code>*.var</code> file), or a CGI script which returned a local
+ `Location:', then the resource which the user requested is going to be
+ ultimately located by some URI other than what the client originally
+ supplied. In this case, the server does an <em>internal redirect</em>,
+ constructing a new <code>request_rec</code> for the new URI, and
+ processing it almost exactly as if the client had requested the new URI
+ directly.</li>
+
+ <li>If some handler signaled an error, and an <code>ErrorDocument</code>
+ is in scope, the same internal redirect machinery comes into play.</li>
+
+ <li><p>Finally, a handler occasionally needs to investigate `what would
+ happen if' some other request were run. For instance, the directory
+ indexing module needs to know what MIME type would be assigned to a
+ request for each directory entry, in order to figure out what icon to
+ use.</p>
+
+ <p>Such handlers can construct a <em>sub-request</em>, using the
+ functions <code>ap_sub_req_lookup_file</code>,
+ <code>ap_sub_req_lookup_uri</code>, and <code>ap_sub_req_method_uri</code>;
+ these construct a new <code>request_rec</code> structure and processes it
+ as you would expect, up to but not including the point of actually sending
+ a response. (These functions skip over the access checks if the
+ sub-request is for a file in the same directory as the original
+ request).</p>
+
+ <p>(Server-side includes work by building sub-requests and then actually
+ invoking the response handler for them, via the function
+ <code>ap_run_sub_req</code>).</p>
+ </li>
+ </ul>
+
+
+ <h3><a name="req_return" id="req_return">Handling requests, declining, and returning
+ error codes</a></h3>
+ <p>As discussed above, each handler, when invoked to handle a particular
+ <code>request_rec</code>, has to return an <code>int</code> to indicate
+ what happened. That can either be</p>
+
+ <ul>
+ <li><code>OK</code> -- the request was handled successfully. This may or
+ may not terminate the phase.</li>
+
+ <li><code>DECLINED</code> -- no erroneous condition exists, but the module
+ declines to handle the phase; the server tries to find another.</li>
+
+ <li>an HTTP error code, which aborts handling of the request.</li>
+ </ul>
+
+ <p>Note that if the error code returned is <code>REDIRECT</code>, then
+ the module should put a <code>Location</code> in the request's
+ <code>headers_out</code>, to indicate where the client should be
+ redirected <em>to</em>.</p>
+
+
+ <h3><a name="resp_handlers" id="resp_handlers">Special considerations for response
+ handlers</a></h3>
+ <p>Handlers for most phases do their work by simply setting a few fields
+ in the <code>request_rec</code> structure (or, in the case of access
+ checkers, simply by returning the correct error code). However, response
+ handlers have to actually send a request back to the client.</p>
+
+ <p>They should begin by sending an HTTP response header, using the
+ function <code>ap_send_http_header</code>. (You don't have to do anything
+ special to skip sending the header for HTTP/0.9 requests; the function
+ figures out on its own that it shouldn't do anything). If the request is
+ marked <code>header_only</code>, that's all they should do; they should
+ return after that, without attempting any further output.</p>
+
+ <p>Otherwise, they should produce a request body which responds to the
+ client as appropriate. The primitives for this are <code>ap_rputc</code>
+ and <code>ap_rprintf</code>, for internally generated output, and
+ <code>ap_send_fd</code>, to copy the contents of some <code>FILE *</code>
+ straight to the client.</p>
+
+ <p>At this point, you should more or less understand the following piece
+ of code, which is the handler which handles <code>GET</code> requests
+ which have no more specific handler; it also shows how conditional
+ <code>GET</code>s can be handled, if it's desirable to do so in a
+ particular response handler -- <code>ap_set_last_modified</code> checks
+ against the <code>If-modified-since</code> value supplied by the client,
+ if any, and returns an appropriate code (which will, if nonzero, be
+ USE_LOCAL_COPY). No similar considerations apply for
+ <code>ap_set_content_length</code>, but it returns an error code for
+ symmetry.</p>
+
+ <div class="example"><p><code>
+ int default_handler (request_rec *r)<br />
+ {<br />
+ <span class="indent">
+ int errstatus;<br />
+ FILE *f;<br />
+ <br />
+ if (r->method_number != M_GET) return DECLINED;<br />
+ if (r->finfo.st_mode == 0) return NOT_FOUND;<br />
+ <br />
+ if ((errstatus = ap_set_content_length (r, r->finfo.st_size))<br />
+ ||
+ (errstatus = ap_set_last_modified (r, r->finfo.st_mtime)))<br />
+ return errstatus;<br />
+ <br />
+ f = fopen (r->filename, "r");<br />
+ <br />
+ if (f == NULL) {<br />
+ <span class="indent">
+ log_reason("file permissions deny server access", r->filename, r);<br />
+ return FORBIDDEN;<br />
+ </span>
+ }<br />
+ <br />
+ register_timeout ("send", r);<br />
+ ap_send_http_header (r);<br />
+ <br />
+ if (!r->header_only) send_fd (f, r);<br />
+ ap_pfclose (r->pool, f);<br />
+ return OK;<br />
+ </span>
+ }
+ </code></p></div>
+
+ <p>Finally, if all of this is too much of a challenge, there are a few
+ ways out of it. First off, as shown above, a response handler which has
+ not yet produced any output can simply return an error code, in which
+ case the server will automatically produce an error response. Secondly,
+ it can punt to some other handler by invoking
+ <code>ap_internal_redirect</code>, which is how the internal redirection
+ machinery discussed above is invoked. A response handler which has
+ internally redirected should always return <code>OK</code>.</p>
+
+ <p>(Invoking <code>ap_internal_redirect</code> from handlers which are
+ <em>not</em> response handlers will lead to serious confusion).</p>
+
+
+ <h3><a name="auth_handlers" id="auth_handlers">Special considerations for authentication
+ handlers</a></h3>
+ <p>Stuff that should be discussed here in detail:</p>
+
+ <ul>
+ <li>Authentication-phase handlers not invoked unless auth is
+ configured for the directory.</li>
+
+ <li>Common auth configuration stored in the core per-dir
+ configuration; it has accessors <code>ap_auth_type</code>,
+ <code>ap_auth_name</code>, and <code>ap_requires</code>.</li>
+
+ <li>Common routines, to handle the protocol end of things, at
+ least for HTTP basic authentication
+ (<code>ap_get_basic_auth_pw</code>, which sets the
+ <code>connection->user</code> structure field
+ automatically, and <code>ap_note_basic_auth_failure</code>,
+ which arranges for the proper <code>WWW-Authenticate:</code>
+ header to be sent back).</li>
+ </ul>
+
+
+ <h3><a name="log_handlers" id="log_handlers">Special considerations for logging
+ handlers</a></h3>
+ <p>When a request has internally redirected, there is the question of
+ what to log. Apache handles this by bundling the entire chain of redirects
+ into a list of <code>request_rec</code> structures which are threaded
+ through the <code>r->prev</code> and <code>r->next</code> pointers.
+ The <code>request_rec</code> which is passed to the logging handlers in
+ such cases is the one which was originally built for the initial request
+ from the client; note that the <code>bytes_sent</code> field will only be
+ correct in the last request in the chain (the one for which a response was
+ actually sent).</p>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="pools" id="pools">Resource allocation and resource pools</a></h2>
+ <p>One of the problems of writing and designing a server-pool server is
+ that of preventing leakage, that is, allocating resources (memory, open
+ files, <em>etc.</em>), without subsequently releasing them. The resource
+ pool machinery is designed to make it easy to prevent this from happening,
+ by allowing resource to be allocated in such a way that they are
+ <em>automatically</em> released when the server is done with them.</p>
+
+ <p>The way this works is as follows: the memory which is allocated, file
+ opened, <em>etc.</em>, to deal with a particular request are tied to a
+ <em>resource pool</em> which is allocated for the request. The pool is a
+ data structure which itself tracks the resources in question.</p>
+
+ <p>When the request has been processed, the pool is <em>cleared</em>. At
+ that point, all the memory associated with it is released for reuse, all
+ files associated with it are closed, and any other clean-up functions which
+ are associated with the pool are run. When this is over, we can be confident
+ that all the resource tied to the pool have been released, and that none of
+ them have leaked.</p>
+
+ <p>Server restarts, and allocation of memory and resources for per-server
+ configuration, are handled in a similar way. There is a <em>configuration
+ pool</em>, which keeps track of resources which were allocated while reading
+ the server configuration files, and handling the commands therein (for
+ instance, the memory that was allocated for per-server module configuration,
+ log files and other files that were opened, and so forth). When the server
+ restarts, and has to reread the configuration files, the configuration pool
+ is cleared, and so the memory and file descriptors which were taken up by
+ reading them the last time are made available for reuse.</p>
+
+ <p>It should be noted that use of the pool machinery isn't generally
+ obligatory, except for situations like logging handlers, where you really
+ need to register cleanups to make sure that the log file gets closed when
+ the server restarts (this is most easily done by using the function <code><a href="#pool-files">ap_pfopen</a></code>, which also arranges for the
+ underlying file descriptor to be closed before any child processes, such as
+ for CGI scripts, are <code>exec</code>ed), or in case you are using the
+ timeout machinery (which isn't yet even documented here). However, there are
+ two benefits to using it: resources allocated to a pool never leak (even if
+ you allocate a scratch string, and just forget about it); also, for memory
+ allocation, <code>ap_palloc</code> is generally faster than
+ <code>malloc</code>.</p>
+
+ <p>We begin here by describing how memory is allocated to pools, and then
+ discuss how other resources are tracked by the resource pool machinery.</p>
+
+ <h3>Allocation of memory in pools</h3>
+ <p>Memory is allocated to pools by calling the function
+ <code>ap_palloc</code>, which takes two arguments, one being a pointer to
+ a resource pool structure, and the other being the amount of memory to
+ allocate (in <code>char</code>s). Within handlers for handling requests,
+ the most common way of getting a resource pool structure is by looking at
+ the <code>pool</code> slot of the relevant <code>request_rec</code>; hence
+ the repeated appearance of the following idiom in module code:</p>
+
+ <div class="example"><p><code>
+ int my_handler(request_rec *r)<br />
+ {<br />
+ <span class="indent">
+ struct my_structure *foo;<br />
+ ...<br />
+ <br />
+ foo = (foo *)ap_palloc (r->pool, sizeof(my_structure));<br />
+ </span>
+ }
+ </code></p></div>
+
+ <p>Note that <em>there is no <code>ap_pfree</code></em> --
+ <code>ap_palloc</code>ed memory is freed only when the associated resource
+ pool is cleared. This means that <code>ap_palloc</code> does not have to
+ do as much accounting as <code>malloc()</code>; all it does in the typical
+ case is to round up the size, bump a pointer, and do a range check.</p>
+
+ <p>(It also raises the possibility that heavy use of
+ <code>ap_palloc</code> could cause a server process to grow excessively
+ large. There are two ways to deal with this, which are dealt with below;
+ briefly, you can use <code>malloc</code>, and try to be sure that all of
+ the memory gets explicitly <code>free</code>d, or you can allocate a
+ sub-pool of the main pool, allocate your memory in the sub-pool, and clear
+ it out periodically. The latter technique is discussed in the section
+ on sub-pools below, and is used in the directory-indexing code, in order
+ to avoid excessive storage allocation when listing directories with
+ thousands of files).</p>
+
+
+ <h3>Allocating initialized memory</h3>
+ <p>There are functions which allocate initialized memory, and are
+ frequently useful. The function <code>ap_pcalloc</code> has the same
+ interface as <code>ap_palloc</code>, but clears out the memory it
+ allocates before it returns it. The function <code>ap_pstrdup</code>
+ takes a resource pool and a <code>char *</code> as arguments, and
+ allocates memory for a copy of the string the pointer points to, returning
+ a pointer to the copy. Finally <code>ap_pstrcat</code> is a varargs-style
+ function, which takes a pointer to a resource pool, and at least two
+ <code>char *</code> arguments, the last of which must be
+ <code>NULL</code>. It allocates enough memory to fit copies of each of
+ the strings, as a unit; for instance:</p>
+
+ <div class="example"><p><code>
+ ap_pstrcat (r->pool, "foo", "/", "bar", NULL);
+ </code></p></div>
+
+ <p>returns a pointer to 8 bytes worth of memory, initialized to
+ <code>"foo/bar"</code>.</p>
+
+
+ <h3><a name="pools-used" id="pools-used">Commonly-used pools in the Apache Web
+ server</a></h3>
+ <p>A pool is really defined by its lifetime more than anything else.
+ There are some static pools in http_main which are passed to various
+ non-http_main functions as arguments at opportune times. Here they
+ are:</p>
+
+ <dl>
+ <dt><code>permanent_pool</code></dt>
+ <dd>never passed to anything else, this is the ancestor of all pools</dd>
+
+ <dt><code>pconf</code></dt>
+ <dd>
+ <ul>
+ <li>subpool of permanent_pool</li>
+
+ <li>created at the beginning of a config "cycle"; exists
+ until the server is terminated or restarts; passed to all
+ config-time routines, either via cmd->pool, or as the
+ "pool *p" argument on those which don't take pools</li>
+
+ <li>passed to the module init() functions</li>
+ </ul>
+ </dd>
+
+ <dt><code>ptemp</code></dt>
+ <dd>
+ <ul>
+ <li>sorry I lie, this pool isn't called this currently in
+ 1.3, I renamed it this in my pthreads development. I'm
+ referring to the use of ptrans in the parent... contrast
+ this with the later definition of ptrans in the
+ child.</li>
+
+ <li>subpool of permanent_pool</li>
+
+ <li>created at the beginning of a config "cycle"; exists
+ until the end of config parsing; passed to config-time
+ routines <em>via</em> cmd->temp_pool. Somewhat of a
+ "bastard child" because it isn't available everywhere.
+ Used for temporary scratch space which may be needed by
+ some config routines but which is deleted at the end of
+ config.</li>
+ </ul>
+ </dd>
+
+ <dt><code>pchild</code></dt>
+ <dd>
+ <ul>
+ <li>subpool of permanent_pool</li>
+
+ <li>created when a child is spawned (or a thread is
+ created); lives until that child (thread) is
+ destroyed</li>
+
+ <li>passed to the module child_init functions</li>
+
+ <li>destruction happens right after the child_exit
+ functions are called... (which may explain why I think
+ child_exit is redundant and unneeded)</li>
+ </ul>
+ </dd>
+
+ <dt><code>ptrans</code></dt>
+ <dd>
+ <ul>
+ <li>should be a subpool of pchild, but currently is a
+ subpool of permanent_pool, see above</li>
+
+ <li>cleared by the child before going into the accept()
+ loop to receive a connection</li>
+
+ <li>used as connection->pool</li>
+ </ul>
+ </dd>
+
+ <dt><code>r->pool</code></dt>
+ <dd>
+ <ul>
+ <li>for the main request this is a subpool of
+ connection->pool; for subrequests it is a subpool of
+ the parent request's pool.</li>
+
+ <li>exists until the end of the request (<em>i.e.</em>,
+ ap_destroy_sub_req, or in child_main after
+ process_request has finished)</li>
+
+ <li>note that r itself is allocated from r->pool;
+ <em>i.e.</em>, r->pool is first created and then r is
+ the first thing palloc()d from it</li>
+ </ul>
+ </dd>
+ </dl>
+
+ <p>For almost everything folks do, <code>r->pool</code> is the pool to
+ use. But you can see how other lifetimes, such as pchild, are useful to
+ some modules... such as modules that need to open a database connection
+ once per child, and wish to clean it up when the child dies.</p>
+
+ <p>You can also see how some bugs have manifested themself, such as
+ setting <code>connection->user</code> to a value from
+ <code>r->pool</code> -- in this case connection exists for the
+ lifetime of <code>ptrans</code>, which is longer than
+ <code>r->pool</code> (especially if <code>r->pool</code> is a
+ subrequest!). So the correct thing to do is to allocate from
+ <code>connection->pool</code>.</p>
+
+ <p>And there was another interesting bug in <code class="module"><a href="../mod/mod_include.html">mod_include</a></code>
+ / <code class="module"><a href="../mod/mod_cgi.html">mod_cgi</a></code>. You'll see in those that they do this test
+ to decide if they should use <code>r->pool</code> or
+ <code>r->main->pool</code>. In this case the resource that they are
+ registering for cleanup is a child process. If it were registered in
+ <code>r->pool</code>, then the code would <code>wait()</code> for the
+ child when the subrequest finishes. With <code class="module"><a href="../mod/mod_include.html">mod_include</a></code> this
+ could be any old <code>#include</code>, and the delay can be up to 3
+ seconds... and happened quite frequently. Instead the subprocess is
+ registered in <code>r->main->pool</code> which causes it to be
+ cleaned up when the entire request is done -- <em>i.e.</em>, after the
+ output has been sent to the client and logging has happened.</p>
+
+
+ <h3><a name="pool-files" id="pool-files">Tracking open files, etc.</a></h3>
+ <p>As indicated above, resource pools are also used to track other sorts
+ of resources besides memory. The most common are open files. The routine
+ which is typically used for this is <code>ap_pfopen</code>, which takes a
+ resource pool and two strings as arguments; the strings are the same as
+ the typical arguments to <code>fopen</code>, <em>e.g.</em>,</p>
+
+ <div class="example"><p><code>
+ ...<br />
+ FILE *f = ap_pfopen (r->pool, r->filename, "r");<br />
+ <br />
+ if (f == NULL) { ... } else { ... }<br />
+ </code></p></div>
+
+ <p>There is also a <code>ap_popenf</code> routine, which parallels the
+ lower-level <code>open</code> system call. Both of these routines arrange
+ for the file to be closed when the resource pool in question is
+ cleared.</p>
+
+ <p>Unlike the case for memory, there <em>are</em> functions to close files
+ allocated with <code>ap_pfopen</code>, and <code>ap_popenf</code>, namely
+ <code>ap_pfclose</code> and <code>ap_pclosef</code>. (This is because, on
+ many systems, the number of files which a single process can have open is
+ quite limited). It is important to use these functions to close files
+ allocated with <code>ap_pfopen</code> and <code>ap_popenf</code>, since to
+ do otherwise could cause fatal errors on systems such as Linux, which
+ react badly if the same <code>FILE*</code> is closed more than once.</p>
+
+ <p>(Using the <code>close</code> functions is not mandatory, since the
+ file will eventually be closed regardless, but you should consider it in
+ cases where your module is opening, or could open, a lot of files).</p>
+
+
+ <h3>Other sorts of resources -- cleanup functions</h3>
+ <p>More text goes here. Describe the cleanup primitives in terms of
+ which the file stuff is implemented; also, <code>spawn_process</code>.</p>
+
+ <p>Pool cleanups live until <code>clear_pool()</code> is called:
+ <code>clear_pool(a)</code> recursively calls <code>destroy_pool()</code>
+ on all subpools of <code>a</code>; then calls all the cleanups for
+ <code>a</code>; then releases all the memory for <code>a</code>.
+ <code>destroy_pool(a)</code> calls <code>clear_pool(a)</code> and then
+ releases the pool structure itself. <em>i.e.</em>,
+ <code>clear_pool(a)</code> doesn't delete <code>a</code>, it just frees
+ up all the resources and you can start using it again immediately.</p>
+
+
+ <h3>Fine control -- creating and dealing with sub-pools, with
+ a note on sub-requests</h3>
+ <p>On rare occasions, too-free use of <code>ap_palloc()</code> and the
+ associated primitives may result in undesirably profligate resource
+ allocation. You can deal with such a case by creating a <em>sub-pool</em>,
+ allocating within the sub-pool rather than the main pool, and clearing or
+ destroying the sub-pool, which releases the resources which were
+ associated with it. (This really <em>is</em> a rare situation; the only
+ case in which it comes up in the standard module set is in case of listing
+ directories, and then only with <em>very</em> large directories.
+ Unnecessary use of the primitives discussed here can hair up your code
+ quite a bit, with very little gain).</p>
+
+ <p>The primitive for creating a sub-pool is <code>ap_make_sub_pool</code>,
+ which takes another pool (the parent pool) as an argument. When the main
+ pool is cleared, the sub-pool will be destroyed. The sub-pool may also be
+ cleared or destroyed at any time, by calling the functions
+ <code>ap_clear_pool</code> and <code>ap_destroy_pool</code>, respectively.
+ (The difference is that <code>ap_clear_pool</code> frees resources
+ associated with the pool, while <code>ap_destroy_pool</code> also
+ deallocates the pool itself. In the former case, you can allocate new
+ resources within the pool, and clear it again, and so forth; in the
+ latter case, it is simply gone).</p>
+
+ <p>One final note -- sub-requests have their own resource pools, which are
+ sub-pools of the resource pool for the main request. The polite way to
+ reclaim the resources associated with a sub request which you have
+ allocated (using the <code>ap_sub_req_...</code> functions) is
+ <code>ap_destroy_sub_req</code>, which frees the resource pool. Before
+ calling this function, be sure to copy anything that you care about which
+ might be allocated in the sub-request's resource pool into someplace a
+ little less volatile (for instance, the filename in its
+ <code>request_rec</code> structure).</p>
+
+ <p>(Again, under most circumstances, you shouldn't feel obliged to call
+ this function; only 2K of memory or so are allocated for a typical sub
+ request, and it will be freed anyway when the main request pool is
+ cleared. It is only when you are allocating many, many sub-requests for a
+ single main request that you should seriously consider the
+ <code>ap_destroy_...</code> functions).</p>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="config" id="config">Configuration, commands and the like</a></h2>
+ <p>One of the design goals for this server was to maintain external
+ compatibility with the NCSA 1.3 server --- that is, to read the same
+ configuration files, to process all the directives therein correctly, and
+ in general to be a drop-in replacement for NCSA. On the other hand, another
+ design goal was to move as much of the server's functionality into modules
+ which have as little as possible to do with the monolithic server core. The
+ only way to reconcile these goals is to move the handling of most commands
+ from the central server into the modules.</p>
+
+ <p>However, just giving the modules command tables is not enough to divorce
+ them completely from the server core. The server has to remember the
+ commands in order to act on them later. That involves maintaining data which
+ is private to the modules, and which can be either per-server, or
+ per-directory. Most things are per-directory, including in particular access
+ control and authorization information, but also information on how to
+ determine file types from suffixes, which can be modified by
+ <code class="directive"><a href="../mod/mod_mime.html#addtype">AddType</a></code> and <code class="directive"><a href="../mod/core.html#defaulttype">DefaultType</a></code> directives, and so forth. In general,
+ the governing philosophy is that anything which <em>can</em> be made
+ configurable by directory should be; per-server information is generally
+ used in the standard set of modules for information like
+ <code class="directive"><a href="../mod/mod_alias.html#alias">Alias</a></code>es and <code class="directive"><a href="../mod/mod_alias.html#redirect">Redirect</a></code>s which come into play before the
+ request is tied to a particular place in the underlying file system.</p>
+
+ <p>Another requirement for emulating the NCSA server is being able to handle
+ the per-directory configuration files, generally called
+ <code>.htaccess</code> files, though even in the NCSA server they can
+ contain directives which have nothing at all to do with access control.
+ Accordingly, after URI -> filename translation, but before performing any
+ other phase, the server walks down the directory hierarchy of the underlying
+ filesystem, following the translated pathname, to read any
+ <code>.htaccess</code> files which might be present. The information which
+ is read in then has to be <em>merged</em> with the applicable information
+ from the server's own config files (either from the <code class="directive"><a href="../mod/core.html#directory"><Directory></a></code> sections in
+ <code>access.conf</code>, or from defaults in <code>srm.conf</code>, which
+ actually behaves for most purposes almost exactly like <code><Directory
+ /></code>).</p>
+
+ <p>Finally, after having served a request which involved reading
+ <code>.htaccess</code> files, we need to discard the storage allocated for
+ handling them. That is solved the same way it is solved wherever else
+ similar problems come up, by tying those structures to the per-transaction
+ resource pool.</p>
+
+ <h3><a name="per-dir" id="per-dir">Per-directory configuration structures</a></h3>
+ <p>Let's look out how all of this plays out in <code>mod_mime.c</code>,
+ which defines the file typing handler which emulates the NCSA server's
+ behavior of determining file types from suffixes. What we'll be looking
+ at, here, is the code which implements the <code class="directive"><a href="../mod/mod_mime.html#addtype">AddType</a></code> and <code class="directive"><a href="../mod/mod_mime.html#addencoding">AddEncoding</a></code> commands. These commands can appear in
+ <code>.htaccess</code> files, so they must be handled in the module's
+ private per-directory data, which in fact, consists of two separate
+ tables for MIME types and encoding information, and is declared as
+ follows:</p>
+
+ <div class="example"><pre>typedef struct {
+ table *forced_types; /* Additional AddTyped stuff */
+ table *encoding_types; /* Added with AddEncoding... */
+} mime_dir_config;</pre></div>
+
+ <p>When the server is reading a configuration file, or <code class="directive"><a href="../mod/core.html#directory"><Directory></a></code> section, which includes
+ one of the MIME module's commands, it needs to create a
+ <code>mime_dir_config</code> structure, so those commands have something
+ to act on. It does this by invoking the function it finds in the module's
+ `create per-dir config slot', with two arguments: the name of the
+ directory to which this configuration information applies (or
+ <code>NULL</code> for <code>srm.conf</code>), and a pointer to a
+ resource pool in which the allocation should happen.</p>
+
+ <p>(If we are reading a <code>.htaccess</code> file, that resource pool
+ is the per-request resource pool for the request; otherwise it is a
+ resource pool which is used for configuration data, and cleared on
+ restarts. Either way, it is important for the structure being created to
+ vanish when the pool is cleared, by registering a cleanup on the pool if
+ necessary).</p>
+
+ <p>For the MIME module, the per-dir config creation function just
+ <code>ap_palloc</code>s the structure above, and a creates a couple of
+ tables to fill it. That looks like this:</p>
+
+ <div class="example"><p><code>
+ void *create_mime_dir_config (pool *p, char *dummy)<br />
+ {<br />
+ <span class="indent">
+ mime_dir_config *new =<br />
+ <span class="indent">
+ (mime_dir_config *) ap_palloc (p, sizeof(mime_dir_config));<br />
+ </span>
+ <br />
+ new->forced_types = ap_make_table (p, 4);<br />
+ new->encoding_types = ap_make_table (p, 4);<br />
+ <br />
+ return new;<br />
+ </span>
+ }
+ </code></p></div>
+
+ <p>Now, suppose we've just read in a <code>.htaccess</code> file. We
+ already have the per-directory configuration structure for the next
+ directory up in the hierarchy. If the <code>.htaccess</code> file we just
+ read in didn't have any <code class="directive"><a href="../mod/mod_mime.html#addtype">AddType</a></code>
+ or <code class="directive"><a href="../mod/mod_mime.html#addencoding">AddEncoding</a></code> commands, its
+ per-directory config structure for the MIME module is still valid, and we
+ can just use it. Otherwise, we need to merge the two structures
+ somehow.</p>
+
+ <p>To do that, the server invokes the module's per-directory config merge
+ function, if one is present. That function takes three arguments: the two
+ structures being merged, and a resource pool in which to allocate the
+ result. For the MIME module, all that needs to be done is overlay the
+ tables from the new per-directory config structure with those from the
+ parent:</p>
+
+ <div class="example"><p><code>
+ void *merge_mime_dir_configs (pool *p, void *parent_dirv, void *subdirv)<br />
+ {<br />
+ <span class="indent">
+ mime_dir_config *parent_dir = (mime_dir_config *)parent_dirv;<br />
+ mime_dir_config *subdir = (mime_dir_config *)subdirv;<br />
+ mime_dir_config *new =<br />
+ <span class="indent">
+ (mime_dir_config *)ap_palloc (p, sizeof(mime_dir_config));<br />
+ </span>
+ <br />
+ new->forced_types = ap_overlay_tables (p, subdir->forced_types,<br />
+ <span class="indent">
+ parent_dir->forced_types);<br />
+ </span>
+ new->encoding_types = ap_overlay_tables (p, subdir->encoding_types,<br />
+ <span class="indent">
+ parent_dir->encoding_types);<br />
+ </span>
+ <br />
+ return new;<br />
+ </span>
+ }
+ </code></p></div>
+
+ <p>As a note -- if there is no per-directory merge function present, the
+ server will just use the subdirectory's configuration info, and ignore
+ the parent's. For some modules, that works just fine (<em>e.g.</em>, for
+ the includes module, whose per-directory configuration information
+ consists solely of the state of the <code>XBITHACK</code>), and for those
+ modules, you can just not declare one, and leave the corresponding
+ structure slot in the module itself <code>NULL</code>.</p>
+
+
+ <h3><a name="commands" id="commands">Command handling</a></h3>
+ <p>Now that we have these structures, we need to be able to figure out how
+ to fill them. That involves processing the actual <code class="directive"><a href="../mod/mod_mime.html#addtype">AddType</a></code> and <code class="directive"><a href="../mod/mod_mime.html#addencoding">AddEncoding</a></code> commands. To find commands, the server looks in
+ the module's command table. That table contains information on how many
+ arguments the commands take, and in what formats, where it is permitted,
+ and so forth. That information is sufficient to allow the server to invoke
+ most command-handling functions with pre-parsed arguments. Without further
+ ado, let's look at the <code class="directive"><a href="../mod/mod_mime.html#addtype">AddType</a></code>
+ command handler, which looks like this (the <code class="directive"><a href="../mod/mod_mime.html#addencoding">AddEncoding</a></code> command looks basically the same, and won't be
+ shown here):</p>
+
+ <div class="example"><p><code>
+ char *add_type(cmd_parms *cmd, mime_dir_config *m, char *ct, char *ext)<br />
+ {<br />
+ <span class="indent">
+ if (*ext == '.') ++ext;<br />
+ ap_table_set (m->forced_types, ext, ct);<br />
+ return NULL;<br />
+ </span>
+ }
+ </code></p></div>
+
+ <p>This command handler is unusually simple. As you can see, it takes
+ four arguments, two of which are pre-parsed arguments, the third being the
+ per-directory configuration structure for the module in question, and the
+ fourth being a pointer to a <code>cmd_parms</code> structure. That
+ structure contains a bunch of arguments which are frequently of use to
+ some, but not all, commands, including a resource pool (from which memory
+ can be allocated, and to which cleanups should be tied), and the (virtual)
+ server being configured, from which the module's per-server configuration
+ data can be obtained if required.</p>
+
+ <p>Another way in which this particular command handler is unusually
+ simple is that there are no error conditions which it can encounter. If
+ there were, it could return an error message instead of <code>NULL</code>;
+ this causes an error to be printed out on the server's
+ <code>stderr</code>, followed by a quick exit, if it is in the main config
+ files; for a <code>.htaccess</code> file, the syntax error is logged in
+ the server error log (along with an indication of where it came from), and
+ the request is bounced with a server error response (HTTP error status,
+ code 500).</p>
+
+ <p>The MIME module's command table has entries for these commands, which
+ look like this:</p>
+
+ <div class="example"><p><code>
+ command_rec mime_cmds[] = {<br />
+ <span class="indent">
+ { "AddType", add_type, NULL, OR_FILEINFO, TAKE2,<br />
+ <span class="indent">"a mime type followed by a file extension" },<br /></span>
+ { "AddEncoding", add_encoding, NULL, OR_FILEINFO, TAKE2,<br />
+ <span class="indent">
+ "an encoding (<em>e.g.</em>, gzip), followed by a file extension" },<br />
+ </span>
+ { NULL }<br />
+ </span>
+ };
+ </code></p></div>
+
+ <p>The entries in these tables are:</p>
+ <ul>
+ <li>The name of the command</li>
+ <li>The function which handles it</li>
+ <li>a <code>(void *)</code> pointer, which is passed in the
+ <code>cmd_parms</code> structure to the command handler ---
+ this is useful in case many similar commands are handled by
+ the same function.</li>
+
+ <li>A bit mask indicating where the command may appear. There
+ are mask bits corresponding to each
+ <code>AllowOverride</code> option, and an additional mask
+ bit, <code>RSRC_CONF</code>, indicating that the command may
+ appear in the server's own config files, but <em>not</em> in
+ any <code>.htaccess</code> file.</li>
+
+ <li>A flag indicating how many arguments the command handler
+ wants pre-parsed, and how they should be passed in.
+ <code>TAKE2</code> indicates two pre-parsed arguments. Other
+ options are <code>TAKE1</code>, which indicates one
+ pre-parsed argument, <code>FLAG</code>, which indicates that
+ the argument should be <code>On</code> or <code>Off</code>,
+ and is passed in as a boolean flag, <code>RAW_ARGS</code>,
+ which causes the server to give the command the raw, unparsed
+ arguments (everything but the command name itself). There is
+ also <code>ITERATE</code>, which means that the handler looks
+ the same as <code>TAKE1</code>, but that if multiple
+ arguments are present, it should be called multiple times,
+ and finally <code>ITERATE2</code>, which indicates that the
+ command handler looks like a <code>TAKE2</code>, but if more
+ arguments are present, then it should be called multiple
+ times, holding the first argument constant.</li>
+
+ <li>Finally, we have a string which describes the arguments
+ that should be present. If the arguments in the actual config
+ file are not as required, this string will be used to help
+ give a more specific error message. (You can safely leave
+ this <code>NULL</code>).</li>
+ </ul>
+
+ <p>Finally, having set this all up, we have to use it. This is ultimately
+ done in the module's handlers, specifically for its file-typing handler,
+ which looks more or less like this; note that the per-directory
+ configuration structure is extracted from the <code>request_rec</code>'s
+ per-directory configuration vector by using the
+ <code>ap_get_module_config</code> function.</p>
+
+ <div class="example"><p><code>
+ int find_ct(request_rec *r)<br />
+ {<br />
+ <span class="indent">
+ int i;<br />
+ char *fn = ap_pstrdup (r->pool, r->filename);<br />
+ mime_dir_config *conf = (mime_dir_config *)<br />
+ <span class="indent">
+ ap_get_module_config(r->per_dir_config, &mime_module);<br />
+ </span>
+ char *type;<br />
+ <br />
+ if (S_ISDIR(r->finfo.st_mode)) {<br />
+ <span class="indent">
+ r->content_type = DIR_MAGIC_TYPE;<br />
+ return OK;<br />
+ </span>
+ }<br />
+ <br />
+ if((i=ap_rind(fn,'.')) < 0) return DECLINED;<br />
+ ++i;<br />
+ <br />
+ if ((type = ap_table_get (conf->encoding_types, &fn[i])))<br />
+ {<br />
+ <span class="indent">
+ r->content_encoding = type;<br />
+ <br />
+ /* go back to previous extension to try to use it as a type */<br />
+ fn[i-1] = '\0';<br />
+ if((i=ap_rind(fn,'.')) < 0) return OK;<br />
+ ++i;<br />
+ </span>
+ }<br />
+ <br />
+ if ((type = ap_table_get (conf->forced_types, &fn[i])))<br />
+ {<br />
+ <span class="indent">
+ r->content_type = type;<br />
+ </span>
+ }<br />
+ <br />
+ return OK;
+ </span>
+ }
+ </code></p></div>
+
+
+ <h3><a name="servconf" id="servconf">Side notes -- per-server configuration,
+ virtual servers, <em>etc</em>.</a></h3>
+ <p>The basic ideas behind per-server module configuration are basically
+ the same as those for per-directory configuration; there is a creation
+ function and a merge function, the latter being invoked where a virtual
+ server has partially overridden the base server configuration, and a
+ combined structure must be computed. (As with per-directory configuration,
+ the default if no merge function is specified, and a module is configured
+ in some virtual server, is that the base configuration is simply
+ ignored).</p>
+
+ <p>The only substantial difference is that when a command needs to
+ configure the per-server private module data, it needs to go to the
+ <code>cmd_parms</code> data to get at it. Here's an example, from the
+ alias module, which also indicates how a syntax error can be returned
+ (note that the per-directory configuration argument to the command
+ handler is declared as a dummy, since the module doesn't actually have
+ per-directory config data):</p>
+
+ <div class="example"><p><code>
+ char *add_redirect(cmd_parms *cmd, void *dummy, char *f, char *url)<br />
+ {<br />
+ <span class="indent">
+ server_rec *s = cmd->server;<br />
+ alias_server_conf *conf = (alias_server_conf *)<br />
+ <span class="indent">
+ ap_get_module_config(s->module_config,&alias_module);<br />
+ </span>
+ alias_entry *new = ap_push_array (conf->redirects);<br />
+ <br />
+ if (!ap_is_url (url)) return "Redirect to non-URL";<br />
+ <br />
+ new->fake = f; new->real = url;<br />
+ return NULL;<br />
+ </span>
+ }
+ </code></p></div>
+
+</div></div>
+<div class="bottomlang">
+<p><span>Available Languages: </span><a href="../en/developer/API.html" title="English"> en </a></p>
+</div><div id="footer">
+<p class="apache">Copyright 2009 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
+<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div>
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