extending-php

Extending PHP
Wez Furlong
wez@thebrainroom.com
PHP{Con West, Santa Clara, October 2003

Extending PHP
Sooner or later, you will find that PHP doesn't do everything that you need
it to do.

• No bindings for a particular library
• Might want to integrate PHP more tightly into your existing
systems (or vice versa)
• Maybe PHP script is too slow for performance critical parts of
your application

Rather than abandon (the otherwise perfect!) PHP and search for an
alternative language, it could easily be worthwhile to extend PHP to make
it do what you want.

What we will cover
• Generating an extension skeleton (ext_skel)
• config.m4 and the PHP build system
• Extension Anatomy 101
• PHP_FUNCTION
• Resources
• Parameter passing

We have 3 hours of interactive tutorial time; feel free to ask questions at
any point. Shout if I don't seem to have noticed you!

To give us focus, I've picked on an asynchronous DNS resolving library
and we'll create a PHP extension for it today, right now.

libares
Starting with PHP 4.3.0, it is now possible to write some pretty smart
multiplexing networking applications stream_select(). The only problem is
that the standard DNS functions gethostbyname() block the whole script
until the domain name resolves.

Wouldn't it be nice to have an alternative method of resolving DNS that
won't cause your existing sockets to time out?

We'll be using libares (written by Greg Hudson) as the basis of a PHP
extension that provides such an alternative method. libares is released
under an MIT license.

ares C API
Before we can write the extension, we have to have some knowledge of
the code that we want to expose to our PHP scripts.

Focusing on replacing gethostbyname(), ares provides the following
interesting functions:

interesting ares API
typedef void (*ares_host_callback)(void *arg, int status,
struct hostent *hostent);

int ares_init(ares_channel *channelptr);

void ares_destroy(ares_channel channel);

void ares_gethostbyname(ares_channel channel, const char *name,
int family, ares_host_callback callback, void *arg);

int ares_fds(ares_channel channel, fd_set *read_fds, fd_set *write_fds);

void ares_process(ares_channel channel, fd_set *read_fds,
fd_set *write_fds);

Typical usage for these functions (taken from the ahost example code in
ares distribution) looks something like this:

example of ares API in action
#include <ares.h>

/* gets called when the name has been resolved */
static void callback(void *arg, int status, struct hostent *host)
{
char *mem, **p;
struct in_addr addr;

if (status != ARES_SUCCESS) {
fprintf(stderr, "%s: %sn", (char*)arg,
ares_strerror(status, &mem));
ares_free_errmem(mem);
}

for (p = host->h_addr_list; *p; p++) {
memcpy(&addr, *p, sizeof(addr));
printf("%s: %sn", host->h_name, inet_ntoa(addr));
}
}

int main(int argc, char **argv)
{
ares_channel channel;
char *errmem;
int status, nfds;
fd_set read_fds, write_fds;
struct timeval *tvp, tv;

/* create/initialize a channel for DNS communications */

status = ares_init(&channel);
fprintf(stderr, "ares_init: %sn",
ares_strerror(status, &errmem));
ares_free_errmem(errmem);
return 1;
}

/* for each command line arg */
for (argv++; *argv; argv++) {
/* look up the domain name */
ares_gethostbyname(channel, *argv, AF_INET, callback, *argv);
}

/* wait for the queries to complete */
while (1) {
FD_ZERO(&read_fds);
FD_ZERO(&write_fds);
nfds = ares_fds(channel, &read_fds, &write_fds);
if (nfds == 0)
break;
tvp = ares_timeout(channel, NULL, &tv);
select(nfds, &read_fds, &write_fds, NULL, tvp);
/* calls the callback as appropriate */
ares_process(channel, &read_fds, &write_fds);
}

/* all done */
ares_destroy(channel);
return 0;
}

extension functions
Based on the C api, a nice convenient PHP alternative might look like this:

PHP version of ares resolving
<?php

function gothost($hostname, $status, $hostent)
{
if ($status != ARES_SUCCESS) {
echo "Failed to resolve $hostname: "
. ares_strerror($status) . "n";
return;
}
foreach ($hostent['addr_list'] as $ip) {
echo "$hostent[name] -> $ipn";
}
}

/* for each argument, resolve it */
function lookup_hosts()
{
$args = func_get_args();
$resolver = ares_init();
foreach ($args as $arg) {
ares_gethostbyname($resolver, $arg, 'gothost', $arg);
}

// wait up to 2 seconds for hosts to be resolved
while (ares_process_with_timeout($resolver, 2) > 0) {
// timed out (2 seconds are up)
// could do other stuff here while waiting
echo ".";
}

ares_destroy($resolver);
}
?>

We have arrived at a list of 5 functions that we need to implement in our
PHP extension:

• ares_init()
• ares_gethostbyname()
• ares_process_with_timeout()
• ares_destroy()
• ares_strerror()

Of these, ares_process_with_timeout() is special, as it wraps up a
number of bits and pieces that are difficult to bring into user space. Later,
we will look at plugging ares into stream_select(), but lets get this basic
step out of the way first.

ext_skel
Make sure you have these things ready:

• Latest PHP 4 source release
• ares headers and library
• Working build environment(!)

First thing to do is to generate an extension skeleton which will contain all
the stuff that PHP needs to be able to see your code.

generating a skeleton
% cd php4.3.x/ext
% ./ext_skel --extname=ares

ext_skel will produce output like this:

ext_skel output
Creating directory ares
Creating basic files: config.m4 .cvsignore ares.c php_ares.h CREDITS EXPERIMENTAL
tests/001.phpt ares.php [done].

To use your new extension, you will have to execute the following steps:

1. $ cd ..
2. $ vi ext/ares/config.m4
3. $ ./buildconf
4. $ ./configure --[with|enable]-ares
5. $ make
6. $ ./php -f ext/ares/ares.php
7. $ vi ext/ares/ares.c
8. $ make

Repeat steps 3-6 until you are satisfied with ext/ares/config.m4 and
step 6 confirms that your module is compiled into PHP. Then, start writing
code and repeat the last two steps as often as necessary.

As it suggests, looking in the config.m4 file is the first thing we need to do -
we want configure to be able to find libares, so it is an essential step.

config.m4: the PHP build system
PHP has an excellent flexible build system. Each extension defines a
config.m4 containing shell script (and m4 macros) to help locate libraries
and headers that are required.

ext_skel has conveniently generated a template for us to use; all we need
to do is uncomment the parts that apply to our extension.

ares config.m4
dnl $Id$
dnl config.m4 for extension ares

PHP_ARG_WITH(ares, for ares support,
[ --with-ares Include ares support])

if test "$PHP_ARES" != "no"; then
# --with-ares -> check with-path
SEARCH_PATH="/usr/local /usr"
SEARCH_FOR="/include/ares.h"
if test -r $PHP_ARES/; then # path given as parameter
ARES_DIR=$PHP_ARES
else # search default path list
AC_MSG_CHECKING([for ares files in default path])
for i in $SEARCH_PATH ; do
if test -r $i/$SEARCH_FOR; then
ARES_DIR=$i
AC_MSG_RESULT(found in $i)
fi
done
fi

if test -z "$ARES_DIR"; then
AC_MSG_RESULT([not found])
AC_MSG_ERROR([Please reinstall the ares distribution])
fi

# --with-ares -> add include path
PHP_ADD_INCLUDE($ARES_DIR/include)

# --with-ares -> check for lib and symbol presence
LIBNAME=ares
LIBSYMBOL=ares_init

PHP_CHECK_LIBRARY($LIBNAME,$LIBSYMBOL,
[
PHP_ADD_LIBRARY_WITH_PATH($LIBNAME, $ARES_DIR/lib, ARES_SHARED_LIBADD)
AC_DEFINE(HAVE_ARESLIB,1,[ ])
],[
AC_MSG_ERROR([wrong ares lib version or lib not found])
],[
-L$ARES_DIR/lib -lm
])

PHP_SUBST(ARES_SHARED_LIBADD)

PHP_NEW_EXTENSION(ares, ares.c, $ext_shared)
fi

Extension Anatomy 101
php_ares.h as generated by ext_skel
/*
+----------------------------------------------------------------------+
| PHP Version 4 |
+----------------------------------------------------------------------+
| Copyright (c) 1997-2003 The PHP Group |
+----------------------------------------------------------------------+
| This source file is subject to version 2.02 of the PHP license, |
| that is bundled with this package in the file LICENSE, and is |
| available at through the world-wide-web at |
| http://www.php.net/license/2_02.txt. |
| If you did not receive a copy of the PHP license and are unable to |
| obtain it through the world-wide-web, please send a note to |
| license@php.net so we can mail you a copy immediately. |
+----------------------------------------------------------------------+
| Author: |
+----------------------------------------------------------------------+

$Id: header,v 1.10.8.1 2003/07/14 15:59:18 sniper Exp $
*/

#ifndef PHP_ARES_H
#define PHP_ARES_H

extern zend_module_entry ares_module_entry;
#define phpext_ares_ptr &ares_module_entry

#ifdef PHP_WIN32
#define PHP_ARES_API __declspec(dllexport)
#else
#define PHP_ARES_API
#endif

#ifdef ZTS
#include "TSRM.h"
#endif

PHP_MINIT_FUNCTION(ares);
PHP_MSHUTDOWN_FUNCTION(ares);
PHP_RINIT_FUNCTION(ares);
PHP_RSHUTDOWN_FUNCTION(ares);
PHP_MINFO_FUNCTION(ares);

PHP_FUNCTION(confirm_ares_compiled); /* For testing, remove later. */

/*
Declare any global variables you may need between the BEGIN
and END macros here:

ZEND_BEGIN_MODULE_GLOBALS(ares)
long global_value;
char *global_string;
ZEND_END_MODULE_GLOBALS(ares)
*/

/* In every utility function you add that needs to use variables
in php_ares_globals, call TSRM_FETCH(); after declaring other
variables used by that function, or better yet, pass in TSRMLS_CC
after the last function argument and declare your utility function
with TSRMLS_DC after the last declared argument. Always refer to
the globals in your function as ARES_G(variable). You are
encouraged to rename these macros something shorter, see
examples in any other php module directory.
*/

#ifdef ZTS
#define ARES_G(v) TSRMG(ares_globals_id, zend_ares_globals *, v)
#else
#define ARES_G(v) (ares_globals.v)
#endif

#endif /* PHP_ARES_H */

/*
* Local variables:
* tab-width: 4
* c-basic-offset: 4
* indent-tabs-mode: t
* End:
*/

ares.c as generated by ext_skel
/*
+----------------------------------------------------------------------+
| PHP Version 4 |
+----------------------------------------------------------------------+
| Copyright (c) 1997-2003 The PHP Group |
+----------------------------------------------------------------------+
| This source file is subject to version 2.02 of the PHP license, |
| that is bundled with this package in the file LICENSE, and is |
| available at through the world-wide-web at |
| http://www.php.net/license/2_02.txt. |
| If you did not receive a copy of the PHP license and are unable to |
| obtain it through the world-wide-web, please send a note to |
| license@php.net so we can mail you a copy immediately. |
+----------------------------------------------------------------------+
| Author: |
+----------------------------------------------------------------------+

$Id: header,v 1.10.8.1 2003/07/14 15:59:18 sniper Exp $
*/

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include "php.h"
#include "php_ini.h"
#include "ext/standard/info.h"
#include "php_ares.h"

/* If you declare any globals in php_ares.h uncomment this:
ZEND_DECLARE_MODULE_GLOBALS(ares)
*/

/* True global resources - no need for thread safety here */
static int le_ares;

/* {{{ ares_functions[]
*
* Every user visible function must have an entry in ares_functions[].
*/
function_entry ares_functions[] = {
PHP_FE(confirm_ares_compiled, NULL) /* For testing, remove later. */
{NULL, NULL, NULL} /* Must be the last line in ares_functions[] */
};
/* }}} */

/* {{{ ares_module_entry
*/
zend_module_entry ares_module_entry = {
#if ZEND_MODULE_API_NO >= 20010901
STANDARD_MODULE_HEADER,
#endif
"ares",

ares_functions,
PHP_MINIT(ares),
PHP_MSHUTDOWN(ares),
PHP_RINIT(ares), /* Replace with NULL if there's nothing to do at request
start */
PHP_RSHUTDOWN(ares), /* Replace with NULL if there's nothing to do at request end
*/
PHP_MINFO(ares),
#if ZEND_MODULE_API_NO >= 20010901
"0.1", /* Replace with version number for your extension */
#endif
STANDARD_MODULE_PROPERTIES
};
/* }}} */

#ifdef COMPILE_DL_ARES
ZEND_GET_MODULE(ares)
#endif

/* {{{ PHP_INI
*/
/* Remove comments and fill if you need to have entries in php.ini
PHP_INI_BEGIN()
STD_PHP_INI_ENTRY("ares.global_value", "42", PHP_INI_ALL, OnUpdateInt,
global_value, zend_ares_globals, ares_globals)
STD_PHP_INI_ENTRY("ares.global_string", "foobar", PHP_INI_ALL, OnUpdateString,
global_string, zend_ares_globals, ares_globals)
PHP_INI_END()
*/
/* }}} */

/* {{{ php_ares_init_globals
*/
/* Uncomment this function if you have INI entries
static void php_ares_init_globals(zend_ares_globals *ares_globals)
{
ares_globals->global_value = 0;
ares_globals->global_string = NULL;
}
*/
/* }}} */

/* {{{ PHP_MINIT_FUNCTION
*/
PHP_MINIT_FUNCTION(ares)
{
/* If you have INI entries, uncomment these lines
ZEND_INIT_MODULE_GLOBALS(ares, php_ares_init_globals, NULL);
REGISTER_INI_ENTRIES();
*/
return SUCCESS;
}
/* }}} */

/* {{{ PHP_MSHUTDOWN_FUNCTION
*/
PHP_MSHUTDOWN_FUNCTION(ares)
{
/* uncomment this line if you have INI entries
UNREGISTER_INI_ENTRIES();
*/
return SUCCESS;
}
/* }}} */

/* Remove if there's nothing to do at request start */
/* {{{ PHP_RINIT_FUNCTION
*/
PHP_RINIT_FUNCTION(ares)
{
return SUCCESS;
}
/* }}} */

/* Remove if there's nothing to do at request end */

/* {{{ PHP_RSHUTDOWN_FUNCTION
*/
PHP_RSHUTDOWN_FUNCTION(ares)
{
return SUCCESS;
}
/* }}} */

/* {{{ PHP_MINFO_FUNCTION
*/
PHP_MINFO_FUNCTION(ares)
{
php_info_print_table_start();
php_info_print_table_header(2, "ares support", "enabled");
php_info_print_table_end();

/* Remove comments if you have entries in php.ini
DISPLAY_INI_ENTRIES();
*/
}
/* }}} */

/* Remove the following function when you have succesfully modified config.m4
so that your module can be compiled into PHP, it exists only for testing
purposes. */

/* Every user-visible function in PHP should document itself in the source */
/* {{{ proto string confirm_ares_compiled(string arg)
Return a string to confirm that the module is compiled in */
PHP_FUNCTION(confirm_ares_compiled)
{
char *arg = NULL;
int arg_len, len;
char string[256];

if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s", &arg, &arg_len) ==
FAILURE) {
return;
}

len = sprintf(string, "Congratulations! You have successfully modified
ext/%.78s/config.m4. Module %.78s is now compiled into PHP.", "ares", arg);
RETURN_STRINGL(string, len, 1);
}
/* }}} */
/* The previous line is meant for vim and emacs, so it can correctly fold and
unfold functions in source code. See the corresponding marks just before
function definition, where the functions purpose is also documented. Please
follow this convention for the convenience of others editing your code.
*/

/*
* Local variables:
* tab-width: 4
* c-basic-offset: 4
* End:
* vim600: noet sw=4 ts=4 fdm=marker
* vim<600: noet sw=4 ts=4
*/

ares_init - resources
The first function that we are going to implement is ares_init(). In the C
library, ares_init initializes a structure and then all other functions operate
on a pointer to that structure. Since we don't have structures and pointers
in PHP, we need to declare a resource type.

Resources are implemented in PHP using a global (per request) linked list
of resource identifiers. Each individual resource has associated with it:

• A resource "type" id
• A pointer value

In order to return a resource to the script, we will need to register a
resource type when our extension is loaded. We need to do this so that the
Zend Engine knows how to release the resource when the script
terminates - we register the resource with a destructor function.

registering a resource
/* True global resources - no need for thread safety here */
static int le_ares_channel;

static void ares_channel_dtor(zend_rsrc_list_entry *rsrc TSRMLS_DC)
{
/* TODO: rsrc->ptr need to be released correctly */
}

*/
{
le_ares_channel = zend_register_list_destructors_ex(ares_channel_dtor,
NULL, "ares channel", module_number);

return SUCCESS;
}
*/

Now we need to write the PHP function itself.

implementing ares_init()
/* {{{ proto resource ares_init()
Creates a DNS resolving communications channel */
PHP_FUNCTION(ares_init)
{
int status;

status = ares_init(&channel);


char *errmem;

ares_strerror(status, &errmem);
php_error_docref(NULL TSRMLS_CC, E_WARNING, "failed to init ares channel: %s",
errmem);

RETURN_NULL();
}

ZEND_REGISTER_RESOURCE(return_value, channel, le_ares_channel);
}
/* }}} */

• ares_channel is typedef'd as a pointer type
• If the channel couldn't be created, a regular php E_WARNING is
raised, and NULL is returned.

For the sake of completeness, lets implement ares_destroy() now too. It
demonstrates how to accept a resource type as a parameter.

ares_destroy()
/* {{{ proto void ares_destroy(resource $channel)
Destroys a DNS resolving channel */
PHP_FUNCTION(ares_destroy)
{
zval *r;

if (FAILURE == zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC,
"r", &r)) {
return;
}

ZEND_FETCH_RESOURCE(channel, ares_channel, &r, -1, "ares channel",
le_ares_channel);

zend_list_delete(Z_LVAL_P(r));
}
/* }}} */

• ZEND_FETCH_RESOURCE will automatically emit an error and return
from the function if the $channel parameter is not a valid channel
resource (eg: stream or image)

Almost done - now that we know how we are creating the channel, we
should fill in that dtor so it really does some work.

static void ares_channel_dtor(zend_rsrc_list_entry *rsrc TSRMLS_DC)
{
ares_channel channel = (ares_channel)rsrc->ptr;
ares_destroy(channel);
}

ares_gethostbyname()
Now we can write the real core of the extension - the resolver. We
decided that the API for the function would work like this:

function prototype for ares_gethostbyname
<?php
ares_gethostbyname($channel, $hostname, $callback, $callbackarg);
?>

So we need to write a PHP_FUNCTION that accepts a resource, a string,
and two generic zval values as parameters.

ares_gethostbyname() in C
/* {{{ proto void ares_gethostbyname(resource $channel, string $hostname, mixed
$callback, mixed $arg)
Initiate resolution of $hostname; will call $callback with $arg when complete */
PHP_FUNCTION(ares_gethostbyname)
{
zval *zchannel, *zcallback, *zarg;
char *hostname;
long hostname_len;

if (FAILURE == zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "rszz",
&zchannel, &hostname, &hostname_len, &zcallback, &zarg)) {
return;
}
ZEND_FETCH_RESOURCE(channel, ares_channel, &zchannel, -1, "ares channel",
le_ares_channel);

/* more code to go here ... */

}
/* }}} */

Now, we've hit a little bit of a hurdle - we need to call back into the engine
when the resolution completes. There is a little bit of magic required, since
we can't call a PHP_FUNCTION directly from C (well, we can, but we can't
call it directly from this callback). We need to define a structure to hold the
function name (or object + method name) of the callback, as well as the
argument. We'll pass those to ares and have a little stub C function to act
as the real callback:

structure and callback for gethostbyname
struct php_ares_callback_struct {
zval *callback;
zval *arg;
};

static void php_ares_hostcallback(void *arg, int status, struct hostent *host)

{
struct php_ares_callback_struct *cb = (struct php_ares_callback_struct *)arg;

/* TODO: map hostent to php and call the callback */
}

Let's jump back to the gethostbyname function and fill in the callback
structure:

ares_gethostbyname() continued
/* {{{ proto void ares_gethostbyname(resource $channel, string $hostname, mixed
$callback, mixed $arg)
Initiate resolution of $hostname; will call $callback with $arg when complete */
PHP_FUNCTION(ares_gethostbyname)
{
zval *zchannel, *zcallback, *zarg;
char *hostname, *callback_name;
long hostname_len;
struct php_ares_callback_struct *cb;

if (FAILURE == zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "rszz",
&zchannel, &hostname, &hostname_len, &zcallback, &zarg)) {
return;
}
le_ares_channel);

/* verify that the callback will work */
if (!zend_is_callable(zcallback, 0, &callback_name)) {
php_error_docref1(NULL TSRMLS_CC, callback_name, E_WARNING,
"3rd argument is not a valid callback");
efree(callback_name);
return;
}

/* copy the values into the structure */
cb = (struct php_ares_callback_struct*)emalloc(sizeof(*cb));

MAKE_STD_ZVAL(cb->callback);
*cb->callback = *zcallback;
zval_copy_ctor(*cb->callback);

MAKE_STD_ZVAL(cb->arg);
*cb->arg = *zarg;
zval_copy_ctor(*cb->arg);

ares_get_hostbyname(channel, hostname, AF_INET,
php_ares_hostcallback, cb);
}
/* }}} */

We can come back to the callback now - we have another little problem -
we can't pass a struct hostent directly to a script, so we need to copy the
data from it into a zval that can be passed. Looking back at our API
sample, we chose to use an array with keys named similarly to the C
structure:

the sample PHP version of the callback
<?php

function gothost($hostname, $status, $hostent)
{

if ($status != ARES_SUCCESS) {
echo "Failed to resolve $hostname: "
. ares_strerror($status) . "n";
return;
}
foreach ($hostent['addr_list'] as $ip) {
echo "$hostent[name] -> $ipn";
}
}
?>

So the task is now to create an array and set it up as follows:

layout of the hostent parameter
<?php
$hostent = array();
$hostent['name'] = $hostname;
$hostent['addr_list'] = array();
$hostent['addr_list'][] = $ip1;
?>

The C translation of that code looks like this:

building a hostent array in C
{
struct in_addr addr;
char **p;
zval *hearray, *addr_list;

MAKE_STD_ZVAL(hearray);
array_init(hearray);

add_assoc_string(hearray, "name", host->h_name, 1);

MAKE_STD_ZVAL(addr_list);
array_init(addr_list);

for (p = host->h_addr_list; *p; p++) {
memcpy(&addr, *p, sizeof(addr));
add_next_index_string(addr_list, inet_ntoa(addr), 1);
}

add_assoc_zval(hearray, "addr_list", addr_list);

}

Now we get to the most important part - calling the callback.

using call_user_function to call the callback
{
zval retval;
zval *arguments[3];

/* insert code for mapping the hostent to an array (from above) here */

arguments[0] = cb->arg;

MAKE_STD_ZVAL(arguments[1]);
ZVAL_LONG(arguments[1], status);

arguments[1] = hearray;

if (call_user_function(EG(function_table), NULL,
cb->callback, &retval, 3, arguments TSRMLS_CC) == SUCCESS) {
/* release any returned zval - it's not important to us */
zval_dtor(&retval);
}

FREE_ZVAL(arguments[1]);

/* and clean up the structure */
zval_dtor(cb->callback);
zval_dtor(cb->arg);
zval_dtor(hearray);
efree(cb);
}

ares_process_with_timeout
This function provides the easy way to check on the status of the DNS
resolution, and call the callbacks if it has completed. For convenience we
can specify a timeout so we can "sleep" on it while waiting.

Lets take a look at the fragment of code from the ahost example again:

fragment from ahost
int main(int argc, char **argv)
{
int status, nfds;
struct timeval *tvp, tv;

/* ... */

/* wait for the queries to complete */
while (1) {
FD_ZERO(&read_fds);
if (nfds == 0)
break;
tvp = ares_timeout(channel, NULL, &tv);
select(nfds, &read_fds, &write_fds, NULL, tvp);
}

/* ... */
}

We'll do things a little differently - we will be specifying our own timeout.

implementing the function
/* {{{ proto long ares_process_with_timeout(resource $channel, long $secs)
check for completion of DNS resolution and call callbacks. Returns number of
outstanding requests. */
PHP_FUNCTION(ares_process_with_timeout)
{
zval *zchannel;
long timeout_secs;
struct timeval tv;
int status, nfds;

if (FAILURE == zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "rl",
&zchannel, &timeout_secs)) {
return;
}

le_ares_channel);

FD_ZERO(&read_fds);

if (nfds) {
tv.tv_secs = timeout_secs;
tv.tv_usec = 0;
if (select(nfds, &read_fds, &write_fds, NULL, &tv) > 0) {
}
RETURN_LONG(nfds);
} else {
RETURN_LONG(0);
}
}
/* }}} */

Nice things for error handling
If the DNS resolution fails for some reason, we might want to notify the
user of the error message, or we might want to act upon the error code
and take some alternative course of action. To do these things we need to
have a function to retrieve the error message text, and it would also be
nice to have some symbolic constants to descirbe the error codes.

implementing ares_strerror
/* {{{ proto string ares_strerror(long $statuscode)
returns a string description for an error code */
PHP_FUNCTION(ares_strerror)
{
long statuscode;
char *errmem = NULL;

if (FAILURE == zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "l",
&statuscode)) {
return;
}

ares_strerror(status, &errmem);
RETVAL_STRING(return_value, errmem, 1);
}
/* }}} */

The other nice thing to have is symbolic constants. We can do the
equivalent of define() when our extension is loaded. We do this in our
module init (MINIT) function, just after the point where we register our
resource type.

error codes defined by libares
#define ARES_SUCCESS 0

/* Server error codes (ARES_ENODATA indicates no relevant answer) */
#define ARES_ENODATA 1
#define ARES_EFORMERR 2
#define ARES_ESERVFAIL 3
#define ARES_ENOTFOUND 4
#define ARES_ENOTIMP 5
#define ARES_EREFUSED 6

/* Locally generated error codes */
#define ARES_EBADQUERY 7
#define ARES_EBADNAME 8
#define ARES_EBADFAMILY 9
#define ARES_EBADRESP 10
#define ARES_ECONNREFUSED 11
#define ARES_ETIMEOUT 12
#define ARES_EOF 13
#define ARES_EFILE 14
#define ARES_ENOMEM 15
#define ARES_EDESTRUCTION 16

To map these into user space:

*/
{
le_ares_channel = zend_register_list_destructors_ex(ares_channel_dtor,
NULL, "ares channel", module_number);

REGISTER_LONG_CONSTANT("ARES_SUCCESS", ARES_SUCCESS, CONST_CS|CONST_PERSISTENT);
REGISTER_LONG_CONSTANT("ARES_ENODATA", ARES_ENODATA, CONST_CS|CONST_PERSISTENT);
REGISTER_LONG_CONSTANT("ARES_EFORMERR", ARES_EFORMERR, CONST_CS|CONST_PERSISTENT);
/* ... */

return SUCCESS;
}
*/

Summary
We can now make asynchronous DNS resolution requests, and even run
them concurrently. Along the way we have learned about the major parts
of writing a PHP extension:

• config.m4
• general structure
• PHP_FUNCTION
• zend_parse_parameters
• making callbacks
• registering resources
• registering constants
• returning values to the script

how to use with stream_select()
<?php
$s1 = fsockopen($host1, $port);
$s2 = fsockopen($host2, $port);
$chan = ares_init();

ares_gethostbyname($chan, 'sample.com', 'sample.com');

/* wait for things to come back */
while (true) {
/* don't wait for anything - return immediately */
ares_process_with_timeout($chan, 0);
/* wait 5 seconds for data on the sockets */
$r = array($s1, $s2);
$n = stream_select($r, $w = null, $e = null, 5);
if ($n) {
/* do something with it */
}
}
?>

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