PHP7.4 new extension method FFI detailed explanation

With PHP7.4 comes an extension that I think is very useful: PHP FFI (Foreign Function interface) , quoting a description in the PHP FFI RFC:

For PHP, FFI opens a way to write PHP extensions and bindings to C libraries in pure PHP.

Yes, FFI provides high-level languages ​​to directly call each other, while for PHP In other words, FFI allows us to easily call various libraries written in C language.

In fact, there are a large number of PHP extensions that are packages of some existing C libraries. Some commonly used mysqli, curl, gettext, etc. are also included in PECL. Tons of similar extensions.

In the traditional way, when we need to use the capabilities of some existing C language libraries, we need to write wrappers in C language and package them into extensions. This process requires everyone to learn How to write PHP extensions? Of course, there are some convenient ways now, some kind of Zephir. But there are still some learning costs, and with FFI, we can directly call functions in libraries written in C language in PHP scripts.

In the decades of history of C language, excellent libraries have been accumulated, and FFI directly allows us to conveniently enjoy this huge resource.

Getting back to the subject, today I will use an example to introduce how we use PHP to call libcurl to crawl the content of a web page. Why use libcurl? Doesn’t PHP already have a curl extension? Well, first of all, I am familiar with libcurl's API. Secondly, it is precisely because of it that it is easy to compare. Isn't the traditional expansion method AS and FFI methods directly easier to use?

First of all, let’s take the current article you are reading as an example. I now need to write a piece of code to capture its content. If we use the traditional PHP curl extension, we will probably write like this :

<?php
 
$url = "https://www.laruence.com/2020/03/11/5475.html";
$ch = curl_init();
 
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
 
curl_exec($ch);
 
curl_close($ch);

(Because my website is https, there will be one more operation to set SSL_VERIFYPEER) What if I use FFI?

First enable ext/ffi of PHP7.4. It should be noted that PHP-FFI requires libffi-3 or above.

Then, we need to tell PHP FFI what the prototype of the function we want to call looks like. We can use FFI::cdef, and its prototype is:

FFI::cdef([string $cdef = "" [, string $lib = null]]): FFI

In the string $cdef, we can write a C language functional declaration, FFI will parse it, and understand that we want to use this in the string $lib What is the signature of the function called in the library? In this example, we use three libcurl functions. We can find their declarations in the libcurl documentation. Some information about curl_easy_init .

Specifically for this example, we write a curl.php, which contains all the things to be declared. The code is as follows:

$libcurl = FFI::cdef(<<<CTYPE
void *curl_easy_init();
int curl_easy_setopt(void *curl, int option, ...);
int curl_easy_perform(void *curl);
void curl_easy_cleanup(void *handle);
CTYPE
 , "libcurl.so"
 );

Here is a place where it is written in the document The return value is CURL *, but in fact since our example does not dereference it, it is just passed, then use void * instead to avoid trouble.

However, another troublesome thing is that PHP is predefined:

Copy after login

Okay, even if the definition part is completed, now we complete the actual logic part, and the entire code will be ：

<?php
require "curl.php";
 
$url = "https://www.laruence.com/2020/03/11/5475.html";
 
$ch = $libcurl->curl_easy_init();
$libcurl->curl_easy_setopt($ch, CURLOPT_URL, $url);
$libcurl->curl_easy_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
 
$libcurl->curl_easy_perform($ch);
 
$libcurl->curl_easy_cleanup($ch);

How about using curl expansion instead, is it just as concise?

Next, we make it a little more complicated, until, if we don’t want the result to be output directly, but returned as a string, for PHP’s curl extension, we only need to call curl_setopSet CURLOPT_RETURNTRANSFER to 1, but in libcurl there is no ability to directly return a string, or an alternative function to WRITEFUNCTION is provided. When there is data returned When libcurl calls this function, in fact, the PHP curl extension does the same.

Currently we cannot directly pass a PHP function as an additional function to libcurl through FFI, then we have two ways to do it:

1. Use WRITEDATA , the default libcurl will call fwrite as a variable function, and we can give libcurl an fd through WRITEDATA, so that it does not write stdout, but writes Enter this fd

2. We write a simple C function ourselves, pass the FFI date in, and pass it to libcurl.

Let’s use the first method first. First we need to use fopen. This time we declare the prototype by defining a C header file (file.h) :

void *fopen(char *filename, char *mode);
void fclose(void * fp);

Like file.h, we also put all libcurl function declarations in curl.h

#define FFI_LIB "libcurl.so"
 
void *curl_easy_init();
int curl_easy_setopt(void *curl, int option, ...);
int curl_easy_perform(void *curl);
void curl_easy_cleanup(CURL *handle);

Then we can use FFI::load to load the .h file:

static function load(string $filename): FFI;

But how to tell FFI to load the corresponding library? As above, we tell FFI that these functions come from libcurl.so by defining a FFI_LIB macro. When we use FFI::load to load this h file, PHP FFI will automatically load libcurl.so

So why does fopen not need to specify the loading library? That is because FFI will also look for symbols in the variable symbol table, and fopen is a standard library function that has existed for a long time.

Okay, now the entire code will be:

<?php
const CURLOPT_URL = 10002;
const CURLOPT_SSL_VERIFYPEER = 64;
const CURLOPT_WRITEDATA = 10001;
 
$libc = FFI::load("file.h");
$libcurl = FFI::load("curl.h");
 
$url = "https://www.laruence.com/2020/03/11/5475.html";
$tmpfile = "/tmp/tmpfile.out";
 
$ch = $libcurl->curl_easy_init();
$fp = $libc->fopen($tmpfile, "a");
 
$libcurl->curl_easy_setopt($ch, CURLOPT_URL, $url);
$libcurl->curl_easy_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
$libcurl->curl_easy_setopt($ch, CURLOPT_WRITEDATA, $fp);
$libcurl->curl_easy_perform($ch);
 
$libcurl->curl_easy_cleanup($ch);
 
$libc->fclose($fp);
 
$ret = file_get_contents($tmpfile);
@unlink($tmpfile);

但这种方式呢就是需要一个临时的中转文件，还是不够优雅，现在我们用第二种方式，要用第二种方式，我们需要自己用C写一个替代函数传递给libcurl：

#include <stdlib.h>
#include <string.h>
#include "write.h"
 
size_t own_writefunc(void *ptr, size_t size, size_t nmember, void *data) {
        own_write_data *d = (own_write_data*)data;
        size_t total = size * nmember;
 
        if (d->buf == NULL) {
                d->buf = malloc(total);
                if (d->buf == NULL) {
                        return 0;
                }
                d->size = total;
                memcpy(d->buf, ptr, total);
        } else {
                d->buf = realloc(d->buf, d->size + total);
                if (d->buf == NULL) {
                        return 0;
                }
                memcpy(d->buf + d->size, ptr, total);
                d->size += total;
        }
 
        return total;
}
 
void * init() {
        return &own_writefunc;
}

注意此处的初始函数，因为在PHP FFI中，就目前的版本（2020-03-11）我们没有办法直接获得一个函数指针，所以我们定义了这个函数，返回own_writefunc的地址。

最后我们定义上面用到的头文件write.h：

#define FFI_LIB "write.so"
 
typedef struct _writedata {
        void *buf;
        size_t size;
} own_write_data;
 
void *init();

注意到我们在头文件中也定义了FFI_LIB，这样这个头文件就可以同时被write.c和接下来我们的PHP FFI共同使用了。

然后我们编译write函数为一个动态库：

gcc -O2 -fPIC -shared  -g  write.c -o write.so

好了，现在整个的代码会变成：

<?php
const CURLOPT_URL = 10002;
const CURLOPT_SSL_VERIFYPEER = 64;
const CURLOPT_WRITEDATA = 10001;
const CURLOPT_WRITEFUNCTION = 20011;
 
$libcurl = FFI::load("curl.h");
$write  = FFI::load("write.h");
 
$url = "https://www.laruence.com/2020/03/11/5475.html";
 
$data = $write->new("own_write_data");
 
$ch = $libcurl->curl_easy_init();
 
$libcurl->curl_easy_setopt($ch, CURLOPT_URL, $url);
$libcurl->curl_easy_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
$libcurl->curl_easy_setopt($ch, CURLOPT_WRITEDATA, FFI::addr($data));
$libcurl->curl_easy_setopt($ch, CURLOPT_WRITEFUNCTION, $write->init());
$libcurl->curl_easy_perform($ch);
 
$libcurl->curl_easy_cleanup($ch);
 
ret = FFI::string($data->buf, $data->size);

此处，我们使用FFI :: new（$ write-> new）来分配了一个结构_write_data的内存：

function FFI::new(mixed $type [, bool $own = true [, bool $persistent = false]]): FFI\CData

$own表示这个内存管理是否采用PHP的内存管理，有时的情况下，我们申请的内存会经过PHP的生命周期管理，不需要主动释放，但是有的时候你也可能希望自己管理，那么可以设置$own为flase，那么在适当的时候，你需要调用FFI :: free去主动释放。

然后我们把$data作为WRITEDATA传递给libcurl，这里我们使用了FFI :: addr来获取$data的实际内存地址：

static function addr(FFI\CData $cdata): FFI\CData;

然后我们把own_write_func作为WRITEFUNCTION传递给了libcurl，这样再有返回的时候，libcurl就会调用我们的own_write_func来处理返回，同时会把write_data作为自定义参数传递给我们的替代函数。

最后我们使用了FFI :: string来把一段内存转换成PHP的string：

static function FFI::string(FFI\CData $src [, int $size]): string

好了，跑一下吧？

然而毕竟直接在PHP中每次请求都加载so的话，会是一个很大的性能问题，所以我们也可以采用preload的方式，这种模式下，我们通过opcache.preload来在PHP启动的时候就加载好：

ffi.enable=1
opcache.preload=ffi_preload.inc

ffi_preload.inc：

<?php
FFI::load("curl.h");
FFI::load("write.h");

但我们引用加载的FFI呢？因此我们需要修改一下这俩个.h头文件，加入FFI_SCOPE，比如curl.h：

#define FFI_LIB "libcurl.so"
#define FFI_SCOPE "libcurl"
 
void *curl_easy_init();
int curl_easy_setopt(void *curl, int option, ...);
int curl_easy_perform(void *curl);
void curl_easy_cleanup(void *handle);

对应的我们给write.h也加入FFI_SCOPE为“ write”，然后我们的脚本现在看起来应该是这样的：

<?php
const CURLOPT_URL = 10002;
const CURLOPT_SSL_VERIFYPEER = 64;
const CURLOPT_WRITEDATA = 10001;
const CURLOPT_WRITEFUNCTION = 20011;
 
$libcurl = FFI::scope("libcurl");
$write  = FFI::scope("write");
 
$url = "https://www.laruence.com/2020/03/11/5475.html";
 
$data = $write->new("own_write_data");
 
$ch = $libcurl->curl_easy_init();
 
$libcurl->curl_easy_setopt($ch, CURLOPT_URL, $url);
$libcurl->curl_easy_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
$libcurl->curl_easy_setopt($ch, CURLOPT_WRITEDATA, FFI::addr($data));
$libcurl->curl_easy_setopt($ch, CURLOPT_WRITEFUNCTION, $write->init());
$libcurl->curl_easy_perform($ch);
 
$libcurl->curl_easy_cleanup($ch);
 
ret = FFI::string($data->buf, $data->size);

也就是，我们现在使用FFI :: scope来代替FFI :: load，引用对应的函数。

static function scope(string $name): FFI;

然后还有另外一个问题，FFI虽然给了我们很大的规模，但是毕竟直接调用C库函数，还是非常具有风险性的，我们应该只允许用户调用我们确认过的函数，于是，ffi.enable = preload就该上场了，当我们设置ffi.enable = preload的话，那就只有在opcache.preload的脚本中的函数才能调用FFI，而用户写的函数是没有办法直接调用的。

我们稍微修改下ffi_preload.inc变成ffi_safe_preload.inc

<?php
class CURLOPT {
     const URL = 10002;
     const SSL_VERIFYHOST = 81;
     const SSL_VERIFYPEER = 64;
     const WRITEDATA = 10001;
     const WRITEFUNCTION = 20011;
}
 
FFI::load("curl.h");
FFI::load("write.h");
 
function get_libcurl() : FFI {
     return FFI::scope("libcurl");
}
 
function get_write_data($write) : FFI\CData {
     return $write->new("own_write_data");
}
 
function get_write() : FFI {
     return FFI::scope("write");
}
 
function get_data_addr($data) : FFI\CData {
     return FFI::addr($data);
}
 
function paser_libcurl_ret($data) :string{
     return FFI::string($data->buf, $data->size);
}

也就是，我们把所有会调用FFI API的函数都定义在preload脚本中，然后我们的示例会变成（ffi_safe.php）：

<?php
$libcurl = get_libcurl();
$write  =  get_write();
$data = get_write_data($write);
 
$url = "https://www.laruence.com/2020/03/11/5475.html";
 
 
$ch = $libcurl->curl_easy_init();
 
$libcurl->curl_easy_setopt($ch, CURLOPT::URL, $url);
$libcurl->curl_easy_setopt($ch, CURLOPT::SSL_VERIFYPEER, 0);
$libcurl->curl_easy_setopt($ch, CURLOPT::WRITEDATA, get_data_addr($data));
$libcurl->curl_easy_setopt($ch, CURLOPT::WRITEFUNCTION, $write->init());
$libcurl->curl_easy_perform($ch);
 
$libcurl->curl_easy_cleanup($ch);
 
$ret = paser_libcurl_ret($data);

这样一来通过ffi.enable = preload，我们就可以限制，所有的FFI API只能被我们可控制的preload脚本调用，用户不能直接调用。从而我们可以在这些函数内部做好适当的安全保证工作，从而保证一定的安全性。

好了，经历了这个例子，大家应该对FFI有一个比较深入的理解了，详细的PHP API说明，大家可以参考：PHP-FFI Manual，有兴趣的话，就去找一个C库，试试吧？

本文的例子，你可以在我的github上下载到:FFI example

最后还是多说一句，例子只是为了演示功能，所以省掉了很多错误分支的判断捕获，大家自己写的时候还是要加入。毕竟使用FFI的话，会让你会有1000种方式让PHP segfault crash，所以be careful

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