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<?xml version="1.0" encoding="UTF-8"?>
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<meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />

<title>Apache 1.3 API notes - Apache HTTP Server Version 2.4</title>
<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
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<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"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><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><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</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 -&gt; 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 />
        &nbsp;* 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 />
        &nbsp;* module's configuration info.  Note that these are<br />
        &nbsp;* returned, and passed in, as void *'s; the server<br />
        &nbsp;* core keeps track of them, but it doesn't, and can't,<br />
        &nbsp;* know their internal structure.<br />
        &nbsp;*/<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-&gt;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">&lt;Directory&gt;</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 />
        &nbsp;* an array containing environment variables to<br />
        &nbsp;* be passed to subprocesses, so people can write<br />
        &nbsp;* modules to add to that environment.<br />
        &nbsp;*<br />
        &nbsp;* The difference between headers_out and <br />
        &nbsp;* err_headers_out is that the latter are printed <br />
        &nbsp;* even on error, and persist across internal<br />
        &nbsp;* redirects (so the headers printed for <br />
        &nbsp;* <code class="directive"><a href="../mod/core.html#errordocument">ErrorDocument</a></code> handlers will have
         them).<br />
        &nbsp;*/<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 />
        &nbsp;* the data being returned is volatile, and clients<br />
        &nbsp;* should be told not to cache it.<br />
        &nbsp;*/<br />
        <br />
        int no_cache;<br />
        <br />
        /* Various other config info which may change<br />
        &nbsp;* with .htaccess files<br />
        &nbsp;* These are config vectors, with one void*<br />
        &nbsp;* pointer for each module (the thing pointed<br />
        &nbsp;* to being the module's business).<br />
        &nbsp;*/<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-&gt;method_number != M_GET) return DECLINED;<br />
          if (r-&gt;finfo.st_mode == 0) return NOT_FOUND;<br />
          <br />
          if ((errstatus = ap_set_content_length (r, r-&gt;finfo.st_size))<br />
          &nbsp;&nbsp;&nbsp;&nbsp;||
             (errstatus = ap_set_last_modified (r, r-&gt;finfo.st_mtime)))<br />
          return errstatus;<br />
          <br />
          f = fopen (r-&gt;filename, "r");<br />
          <br />
          if (f == NULL) {<br />
          <span class="indent">
            log_reason("file permissions deny server access", r-&gt;filename, r);<br />
            return FORBIDDEN;<br />
          </span>
          }<br />
          <br />
          register_timeout ("send", r);<br />
          ap_send_http_header (r);<br />
          <br />
          if (!r-&gt;header_only) send_fd (f, r);<br />
          ap_pfclose (r-&gt;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-&gt;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-&gt;prev</code> and <code>r-&gt;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-&gt;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-&gt;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-&gt;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-&gt;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-&gt;pool</li>
        </ul>
      </dd>

<dt><code>r-&gt;pool</code></dt>
      <dd>
        <ul>
          <li>for the main request this is a subpool of
          connection-&gt;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-&gt;pool;
          <em>i.e.</em>, r-&gt;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-&gt;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-&gt;user</code> to a value from
      <code>r-&gt;pool</code> -- in this case connection exists for the
      lifetime of <code>ptrans</code>, which is longer than
      <code>r-&gt;pool</code> (especially if <code>r-&gt;pool</code> is a
      subrequest!). So the correct thing to do is to allocate from
      <code>connection-&gt;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-&gt;pool</code> or
      <code>r-&gt;main-&gt;pool</code>. In this case the resource that they are
      registering for cleanup is a child process. If it were registered in
      <code>r-&gt;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-&gt;main-&gt;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-&gt;pool, r-&gt;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#forcetype">ForceType</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 -&gt; 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">&lt;Directory&gt;</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>&lt;Directory
    /&gt;</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">&lt;Directory&gt;</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-&gt;forced_types = ap_make_table (p, 4);<br />
          new-&gt;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-&gt;forced_types = ap_overlay_tables (p, subdir-&gt;forced_types,<br />
          <span class="indent">
            parent_dir-&gt;forced_types);<br />
          </span>
          new-&gt;encoding_types = ap_overlay_tables (p, subdir-&gt;encoding_types,<br />
          <span class="indent">
            parent_dir-&gt;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-&gt;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-&gt;pool, r-&gt;filename);<br />
          mime_dir_config *conf = (mime_dir_config *)<br />
          <span class="indent">
            ap_get_module_config(r-&gt;per_dir_config, &amp;mime_module);<br />
          </span>
          char *type;<br />
          <br />
          if (S_ISDIR(r-&gt;finfo.st_mode)) {<br />
          <span class="indent">
            r-&gt;content_type = DIR_MAGIC_TYPE;<br />
            return OK;<br />
          </span>
          }<br />
          <br />
          if((i=ap_rind(fn,'.')) &lt; 0) return DECLINED;<br />
          ++i;<br />
          <br />
          if ((type = ap_table_get (conf-&gt;encoding_types, &amp;fn[i])))<br />
          {<br />
          <span class="indent">
            r-&gt;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,'.')) &lt; 0) return OK;<br />
            ++i;<br />
          </span>
          }<br />
          <br />
          if ((type = ap_table_get (conf-&gt;forced_types, &amp;fn[i])))<br />
          {<br />
          <span class="indent">
            r-&gt;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-&gt;server;<br />
          alias_server_conf *conf = (alias_server_conf *)<br />
          <span class="indent">
            ap_get_module_config(s-&gt;module_config,&amp;alias_module);<br />
          </span>
          alias_entry *new = ap_push_array (conf-&gt;redirects);<br />
          <br />
          if (!ap_is_url (url)) return "Redirect to non-URL";<br />
          <br />
          new-&gt;fake = f; new-&gt;real = url;<br />
          return NULL;<br />
        </span>
        }
      </code></p></div>
    
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