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<title>Apache 1.3 API notes</title>

<summary>

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

<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>

</note>

<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>

</summary>

<section id="basics"><title>Basic concepts</title>

<p>We begin with an overview of the basic concepts behind the API, and how

they are manifested in the code.</p>

<section id="HMR"><title>Handlers, Modules, and Requests</title>

<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 <directive module="mod_env"

>SetEnv</directive>, 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>

</section>

<section id="moduletour"><title>A brief tour of a module</title>

<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 <directive module="mod_alias"

>ScriptAlias</directive> 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 <directive

module="mod_alias">ScriptAlias</directive>, it also has handlers for the

name translation phase (to recognize <directive module="mod_alias"

>ScriptAlias</directive>ed URIs), the type-checking phase (any

<directive module="mod_alias">ScriptAlias</directive>ed request is typed

as a CGI script).</p>

<p>The module needs to maintain some per (virtual) server information,

namely, the <directive module="mod_alias">ScriptAlias</directive>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 <directive

module="mod_alias">ScriptAlias</directive>es declared).</p>

<p>Finally, this module contains code to handle the <directive

module="mod_alias">ScriptAlias</directive> 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 <directive

module="mod_alias">ScriptAlias</directive>). With no further ado, the

module itself:</p>

<example>

/* 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 />

<indent>

{ "application/x-httpd-cgi", cgi_handler },<br />

{ NULL }<br />

</indent>

};<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 />

<indent>

{ "ScriptAlias", script_alias, NULL, RSRC_CONF, TAKE2,<br />

<indent>"a fakename and a realname"},<br /></indent>

{ NULL }<br />

</indent>

};<br />

<br />

module cgi_module = {

<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>

</example>

</section>

</section>

<section id="handlers"><title>How handlers work</title>

<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>

<section id="req_tour"><title>A brief tour of the request_rec</title>

<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>

<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>

<example>

r-&gt;content_type = "text/html";

</example>

</note>

<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 <directive type="section" module="core"

>Directory</directive> 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>

<example>

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;

<pre>char *args; /* QUERY_ARGS, if any */

struct stat finfo; /* Set by server core;

* st_mode set to zero if no such file */</pre>

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;* <directive module="core">ErrorDocument</directive> 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 />

<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>

/* 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 />

<pre>void *per_dir_config; /* Options set in config files, <em>etc.</em> */

void *request_config; /* Notes on *this* request */</pre>

<br />

};

</example>

</section>

<section id="req_orig"><title>Where request_rec structures come from</title>

<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>

</section>

<section id="req_return"><title>Handling requests, declining, and returning

error codes</title>

<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>

</section>

<section id="resp_handlers"><title>Special considerations for response

handlers</title>

<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>

<example>

int default_handler (request_rec *r)<br />

{<br />

<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 />

<indent>

log_reason("file permissions deny server access", r-&gt;filename, r);<br />

return FORBIDDEN;<br />

</indent>

}<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 />

</indent>

}

</example>

<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>

</section>

<section id="auth_handlers"><title>Special considerations for authentication

handlers</title>

<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>

</section>

<section id="log_handlers"><title>Special considerations for logging

handlers</title>

<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>

</section>

</section>

<section id="pools"><title>Resource allocation and resource pools</title>

<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>

<section><title>Allocation of memory in pools</title>

<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>

<example>

int my_handler(request_rec *r)<br />

{<br />

<indent>

struct my_structure *foo;<br />

...<br />

<br />

foo = (foo *)ap_palloc (r-&gt;pool, sizeof(my_structure));<br />

</indent>

}

</example>

<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>

</section>

<section><title>Allocating initialized memory</title>

<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>

<example>

ap_pstrcat (r-&gt;pool, "foo", "/", "bar", NULL);

</example>

<p>returns a pointer to 8 bytes worth of memory, initialized to

<code>"foo/bar"</code>.</p>

</section>

<section id="pools-used"><title>Commonly-used pools in the Apache Web

server</title>

<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 <module>mod_include</module>

/ <module>mod_cgi</module>. 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 <module>mod_include</module> 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>

</section>

<section id="pool-files"><title>Tracking open files, etc.</title>

<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>

<example>

...<br />

FILE *f = ap_pfopen (r-&gt;pool, r-&gt;filename, "r");<br />

<br />

if (f == NULL) { ... } else { ... }<br />

</example>

<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>

</section>

<section><title>Other sorts of resources -- cleanup functions</title>

<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>

</section>

<section><title>Fine control -- creating and dealing with sub-pools, with

a note on sub-requests</title>

<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>

</section>

</section>

<section id="config"><title>Configuration, commands and the like</title>

<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

<directive module="mod_mime">AddType</directive> and <directive

module="core">DefaultType</directive> 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

<directive module="mod_alias">Alias</directive>es and <directive

module="mod_alias">Redirect</directive>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 <directive

type="section" module="core">Directory</directive> 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>

<section id="per-dir"><title>Per-directory configuration structures</title>

<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 <directive module="mod_mime"

>AddType</directive> and <directive module="mod_mime"

>AddEncoding</directive> 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>

<example>

<pre>typedef struct {

table *forced_types; /* Additional AddTyped stuff */

table *encoding_types; /* Added with AddEncoding... */

} mime_dir_config;</pre>

</example>

<p>When the server is reading a configuration file, or <directive

type="section" module="core">Directory</directive> 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>

<example>

void *create_mime_dir_config (pool *p, char *dummy)<br />

{<br />

<indent>

mime_dir_config *new =<br />

<indent>

(mime_dir_config *) ap_palloc (p, sizeof(mime_dir_config));<br />

</indent>

<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 />

</indent>

}

</example>

<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 <directive module="mod_mime">AddType</directive>

or <directive module="mod_mime">AddEncoding</directive> 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>

<example>

void *merge_mime_dir_configs (pool *p, void *parent_dirv, void *subdirv)<br />

{<br />

<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 />

<indent>

(mime_dir_config *)ap_palloc (p, sizeof(mime_dir_config));<br />

</indent>

<br />

new-&gt;forced_types = ap_overlay_tables (p, subdir-&gt;forced_types,<br />

<indent>

parent_dir-&gt;forced_types);<br />

</indent>

new-&gt;encoding_types = ap_overlay_tables (p, subdir-&gt;encoding_types,<br />

<indent>

parent_dir-&gt;encoding_types);<br />

</indent>

<br />

return new;<br />

</indent>

}

</example>

<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>

</section>

<section id="commands"><title>Command handling</title>

<p>Now that we have these structures, we need to be able to figure out how

to fill them. That involves processing the actual <directive

module="mod_mime">AddType</directive> and <directive module="mod_mime"

>AddEncoding</directive> 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 <directive module="mod_mime">AddType</directive>

command handler, which looks like this (the <directive module="mod_mime"

>AddEncoding</directive> command looks basically the same, and won't be

shown here):</p>

<example>

char *add_type(cmd_parms *cmd, mime_dir_config *m, char *ct, char *ext)<br />

{<br />

<indent>

if (*ext == '.') ++ext;<br />

ap_table_set (m-&gt;forced_types, ext, ct);<br />

return NULL;<br />

</indent>

}

</example>

<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>

<example>

command_rec mime_cmds[] = {<br />

<indent>

{ "AddType", add_type, NULL, OR_FILEINFO, TAKE2,<br />

<indent>"a mime type followed by a file extension" },<br /></indent>

{ "AddEncoding", add_encoding, NULL, OR_FILEINFO, TAKE2,<br />

<indent>

"an encoding (<em>e.g.</em>, gzip), followed by a file extension" },<br />

</indent>

{ NULL }<br />

</indent>

};

</example>

<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>

<example>

int find_ct(request_rec *r)<br />

{<br />

<indent>

int i;<br />

char *fn = ap_pstrdup (r-&gt;pool, r-&gt;filename);<br />

mime_dir_config *conf = (mime_dir_config *)<br />

<indent>

ap_get_module_config(r-&gt;per_dir_config, &amp;mime_module);<br />

</indent>

char *type;<br />

<br />

if (S_ISDIR(r-&gt;finfo.st_mode)) {<br />

<indent>

r-&gt;content_type = DIR_MAGIC_TYPE;<br />

return OK;<br />

</indent>

}<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 />

<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 />

</indent>

}<br />

<br />

if ((type = ap_table_get (conf-&gt;forced_types, &amp;fn[i])))<br />

{<br />

<indent>

r-&gt;content_type = type;<br />

</indent>

}<br />

<br />

return OK;

</indent>

}

</example>

</section>

<section id="servconf"><title>Side notes -- per-server configuration,

virtual servers, <em>etc</em>.</title>

<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>

<example>

char *add_redirect(cmd_parms *cmd, void *dummy, char *f, char *url)<br />

{<br />

<indent>

server_rec *s = cmd-&gt;server;<br />

alias_server_conf *conf = (alias_server_conf *)<br />

<indent>

ap_get_module_config(s-&gt;module_config,&amp;alias_module);<br />

</indent>

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 />

</indent>

}

</example>

</section>

</section>

</manualpage>