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-
- <h3>Apache HTTP Server Version 2.0</h3>
- </div>
-
-
-
- <blockquote>
- <strong>Warning:</strong> 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.
- </blockquote>
-
- <h1 align="center">Apache API notes</h1>
- 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>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>
-
- <h2><a id="basics" name="basics">Basic concepts.</a></h2>
- We begin with an overview of the basic concepts behind the API,
- and how they are manifested in the code.
-
- <h3><a id="HMR" name="HMR">Handlers, Modules, and
- Requests</a></h3>
- 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:
-
- <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>SetEnv</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>
- 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:
-
- <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>
- 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>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 id="moduletour" name="moduletour">A brief tour of a
- module</a></h3>
- 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>ScriptAlias</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>Let's begin with handlers. In order to handle the CGI
- scripts, the module declares a response handler for them.
- Because of <code>ScriptAlias</code>, it also has handlers for
- the name translation phase (to recognize
- <code>ScriptAlias</code>ed URIs), the type-checking phase (any
- <code>ScriptAlias</code>ed request is typed as a CGI
- script).</p>
-
- <p>The module needs to maintain some per (virtual) server
- information, namely, the <code>ScriptAlias</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>ScriptAlias</code>es declared).</p>
-
- <p>Finally, this module contains code to handle the
- <code>ScriptAlias</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>ScriptAlias</code>). With no
- further ado, the module itself:</p>
-<pre>
-/* Declarations of handlers. */
-
-int translate_scriptalias (request_rec *);
-int type_scriptalias (request_rec *);
-int cgi_handler (request_rec *);
-
-/* Subsidiary dispatch table for response-phase handlers, by MIME type */
-
-handler_rec cgi_handlers[] = {
-{ "application/x-httpd-cgi", cgi_handler },
-{ NULL }
-};
-
-/* Declarations of routines to manipulate the module's configuration
- * info. Note that these are returned, and passed in, as void *'s;
- * the server core keeps track of them, but it doesn't, and can't,
- * know their internal structure.
- */
-
-void *make_cgi_server_config (pool *);
-void *merge_cgi_server_config (pool *, void *, void *);
-
-/* Declarations of routines to handle config-file commands */
-
-extern char *script_alias(cmd_parms *, void *per_dir_config, char *fake,
- char *real);
-
-command_rec cgi_cmds[] = {
-{ "ScriptAlias", script_alias, NULL, RSRC_CONF, TAKE2,
- "a fakename and a realname"},
-{ NULL }
-};
-
-module cgi_module = {
- STANDARD_MODULE_STUFF,
- NULL, /* initializer */
- NULL, /* dir config creator */
- NULL, /* dir merger --- default is to override */
- 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>
-
- <h2><a id="handlers" name="handlers">How handlers work</a></h2>
- 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.
-
- <h3><a id="req_tour" name="req_tour">A brief tour of the
- <code>request_rec</code></a></h3>
- 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>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>
-
- <blockquote>
- Note that the <samp>Content-type</samp> header value
- <em>cannot</em> be set by module content-handlers using the
- <samp>ap_table_*()</samp> routines. Rather, it is set by
- pointing the <samp>content_type</samp> field in the
- <samp>request_rec</samp> structure to an appropriate string.
- <em>E.g.</em>,
-<pre>
- r->content_type = "text/html";
-</pre>
- </blockquote>
- 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><Directory></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>Here is an abridged declaration, giving the fields most
- commonly used:</p>
-<pre>
-struct request_rec {
-
- pool *pool;
- conn_rec *connection;
- server_rec *server;
-
- /* What object is being requested */
-
- char *uri;
- char *filename;
- char *path_info;
- char *args; /* QUERY_ARGS, if any */
- struct stat finfo; /* Set by server core;
- * st_mode set to zero if no such file */
-
- char *content_type;
- char *content_encoding;
-
- /* MIME header environments, in and out. Also, an array containing
- * environment variables to be passed to subprocesses, so people can
- * write modules to add to that environment.
- *
- * The difference between headers_out and err_headers_out is that
- * the latter are printed even on error, and persist across internal
- * redirects (so the headers printed for ErrorDocument handlers will
- * have them).
- */
-
- table *headers_in;
- table *headers_out;
- table *err_headers_out;
- table *subprocess_env;
-
- /* Info about the request itself... */
-
- 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> */
-
- /* Info for logging */
-
- char *the_request;
- int bytes_sent;
-
- /* A flag which modules can set, to indicate that the data being
- * returned is volatile, and clients should be told not to cache it.
- */
-
- int no_cache;
-
- /* Various other config info which may change with .htaccess files
- * These are config vectors, with one void* pointer for each module
- * (the thing pointed to being the module's business).
- */
-
- void *per_dir_config; /* Options set in config files, <em>etc.</em> */
- void *request_config; /* Notes on *this* request */
-
-};
-
-</pre>
-
- <h3><a id="req_orig" name="req_orig">Where request_rec
- structures come from</a></h3>
- 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:
-
- <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>
- 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>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 id="req_return" name="req_return">Handling requests,
- declining, and returning error codes</a></h3>
- 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
-
- <ul>
- <li>OK --- the request was handled successfully. This may or
- may not terminate the phase.</li>
-
- <li>DECLINED --- 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>
- 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>.
-
- <h3><a id="resp_handlers" name="resp_handlers">Special
- considerations for response handlers</a></h3>
- 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>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>
-<pre>
-int default_handler (request_rec *r)
-{
- int errstatus;
- FILE *f;
-
- if (r->method_number != M_GET) return DECLINED;
- if (r->finfo.st_mode == 0) return NOT_FOUND;
-
- if ((errstatus = ap_set_content_length (r, r->finfo.st_size))
- || (errstatus = ap_set_last_modified (r, r->finfo.st_mtime)))
- return errstatus;
-
- f = fopen (r->filename, "r");
-
- if (f == NULL) {
- log_reason("file permissions deny server access",
- r->filename, r);
- return FORBIDDEN;
- }
-
- register_timeout ("send", r);
- ap_send_http_header (r);
-
- if (!r->header_only) send_fd (f, r);
- ap_pfclose (r->pool, f);
- return OK;
-}
-</pre>
- 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>(Invoking <code>ap_internal_redirect</code> from handlers
- which are <em>not</em> response handlers will lead to serious
- confusion).</p>
-
- <h3><a id="auth_handlers" name="auth_handlers">Special
- considerations for authentication handlers</a></h3>
- Stuff that should be discussed here in detail:
-
- <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 id="log_handlers" name="log_handlers">Special
- considerations for logging handlers</a></h3>
- 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
- bytes_sent field will only be correct in the last request in
- the chain (the one for which a response was actually sent).
-
- <h2><a id="pools" name="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>
-<pre>
-int my_handler(request_rec *r)
-{
- struct my_structure *foo;
- ...
-
- foo = (foo *)ap_palloc (r->pool, sizeof(my_structure));
-}
-</pre>
-
- <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>
-<pre>
- ap_pstrcat (r->pool, "foo", "/", "bar", NULL);
-</pre>
-
- <p>returns a pointer to 8 bytes worth of memory, initialized to
- <code>"foo/bar"</code>.</p>
-
- <h3><a id="pools-used" name="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 compact="compact">
- <dt>permanent_pool</dt>
-
- <dd>
- <ul>
- <li>never passed to anything else, this is the ancestor
- of all pools</li>
- </ul>
- </dd>
-
- <dt>pconf</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>ptemp</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>pchild</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>ptrans</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>r->pool</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, r->pool 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 connection->user to a value from r->pool
- -- in this case connection exists for the lifetime of ptrans,
- which is longer than r->pool (especially if r->pool is a
- subrequest!). So the correct thing to do is to allocate from
- connection->pool.</p>
-
- <p>And there was another interesting bug in
- mod_include/mod_cgi. You'll see in those that they do this test
- to decide if they should use r->pool or r->main->pool.
- In this case the resource that they are registering for cleanup
- is a child process. If it were registered in r->pool, then
- the code would wait() for the child when the subrequest
- finishes. With mod_include this could be any old #include, and
- the delay can be up to 3 seconds... and happened quite
- frequently. Instead the subprocess is registered in
- r->main->pool 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 id="pool-files" name="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>
-<pre>
- ...
- FILE *f = ap_pfopen (r->pool, r->filename, "r");
-
- if (f == NULL) { ... } else { ... }
-</pre>
-
- <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>
-
- <blockquote>
- More text goes here. Describe the the cleanup primitives in
- terms of which the file stuff is implemented; also,
- <code>spawn_process</code>.
- </blockquote>
-
- <p>Pool cleanups live until clear_pool() is called:
- clear_pool(a) recursively calls destroy_pool() on all subpools
- of a; then calls all the cleanups for a; then releases all the
- memory for a. destroy_pool(a) calls clear_pool(a) and then
- releases the pool structure itself. <em>i.e.</em>,
- clear_pool(a) doesn't delete a, 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>
- 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>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>
-
- <h2><a id="config" name="config">Configuration, commands and
- the like</a></h2>
- 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>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>AddType</code> and <code>DefaultType</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>Alias</code>es and <code>Redirect</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><Directory></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 id="per-dir" name="per-dir">Per-directory configuration
- structures</a></h3>
- 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>AddType</code> and
- <code>AddEncoding</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 <code>table</code>s for MIME types and encoding
- information, and is declared as follows:
-<pre>
-typedef struct {
- table *forced_types; /* Additional AddTyped stuff */
- table *encoding_types; /* Added with AddEncoding... */
-} mime_dir_config;
-</pre>
- When the server is reading a configuration file, or
- <code><Directory></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>(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 <code>table</code>s to fill it. That looks like
- this:</p>
-<pre>
-void *create_mime_dir_config (pool *p, char *dummy)
-{
- mime_dir_config *new =
- (mime_dir_config *) ap_palloc (p, sizeof(mime_dir_config));
-
- new->forced_types = ap_make_table (p, 4);
- new->encoding_types = ap_make_table (p, 4);
-
- return new;
-}
-</pre>
- 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>AddType</code> or <code>AddEncoding</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>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>
-<pre>
-void *merge_mime_dir_configs (pool *p, void *parent_dirv, void *subdirv)
-{
- mime_dir_config *parent_dir = (mime_dir_config *)parent_dirv;
- mime_dir_config *subdir = (mime_dir_config *)subdirv;
- mime_dir_config *new =
- (mime_dir_config *)ap_palloc (p, sizeof(mime_dir_config));
-
- new->forced_types = ap_overlay_tables (p, subdir->forced_types,
- parent_dir->forced_types);
- new->encoding_types = ap_overlay_tables (p, subdir->encoding_types,
- parent_dir->encoding_types);
-
- return new;
-}
-</pre>
- 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>.
-
- <h3><a id="commands" name="commands">Command handling</a></h3>
- Now that we have these structures, we need to be able to figure
- out how to fill them. That involves processing the actual
- <code>AddType</code> and <code>AddEncoding</code> commands. To
- find commands, the server looks in the module's <code>command
- table</code>. 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>AddType</code> command handler, which looks like this
- (the <code>AddEncoding</code> command looks basically the same,
- and won't be shown here):
-<pre>
-char *add_type(cmd_parms *cmd, mime_dir_config *m, char *ct, char *ext)
-{
- if (*ext == '.') ++ext;
- ap_table_set (m->forced_types, ext, ct);
- return NULL;
-}
-</pre>
- 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>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>
-<pre>
-command_rec mime_cmds[] = {
-{ "AddType", add_type, NULL, OR_FILEINFO, TAKE2,
- "a mime type followed by a file extension" },
-{ "AddEncoding", add_encoding, NULL, OR_FILEINFO, TAKE2,
- "an encoding (<em>e.g.</em>, gzip), followed by a file extension" },
-{ NULL }
-};
-</pre>
- The entries in these tables are:
-
- <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>
- 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.
-
-<pre>
-int find_ct(request_rec *r)
-{
- int i;
- char *fn = ap_pstrdup (r->pool, r->filename);
- mime_dir_config *conf = (mime_dir_config *)
- ap_get_module_config(r->per_dir_config, &mime_module);
- char *type;
-
- if (S_ISDIR(r->finfo.st_mode)) {
- r->content_type = DIR_MAGIC_TYPE;
- return OK;
- }
-
- if((i=ap_rind(fn,'.')) < 0) return DECLINED;
- ++i;
-
- if ((type = ap_table_get (conf->encoding_types, &fn[i])))
- {
- r->content_encoding = type;
-
- /* go back to previous extension to try to use it as a type */
-
- fn[i-1] = '\0';
- if((i=ap_rind(fn,'.')) < 0) return OK;
- ++i;
- }
-
- if ((type = ap_table_get (conf->forced_types, &fn[i])))
- {
- r->content_type = type;
- }
-
- return OK;
-}
-
-</pre>
-
- <h3><a id="servconf" name="servconf">Side notes --- per-server
- configuration, virtual servers, <em>etc</em>.</a></h3>
- 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>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>
-<pre>
-char *add_redirect(cmd_parms *cmd, void *dummy, char *f, char *url)
-{
- server_rec *s = cmd->server;
- alias_server_conf *conf = (alias_server_conf *)
- ap_get_module_config(s->module_config,&alias_module);
- alias_entry *new = ap_push_array (conf->redirects);
-
- if (!ap_is_url (url)) return "Redirect to non-URL";
-
- new->fake = f; new->real = url;
- return NULL;
-}
-</pre>
- <hr />
-
- <h3 align="center">Apache HTTP Server Version 2.0</h3>
- <a href="./"><img src="../images/index.gif" alt="Index" /></a>
- <a href="../"><img src="../images/home.gif" alt="Home" /></a>
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