Why does this php function leak memory

Question

I have a long running operation in PHP and it always crashed with an out of memory.

So i started logging mem usage with:

        $result = memory_get_usage() / 1024;
        echo $result;

By commenting parts of the code I found the "guilty" one, responsible for eating up all my ram.

This is the code:

        static private function from_camel_case($str)
        {
            $str[0] = strtolower($str[0]);
            $func = create_function('$c', 'return "_" . strtolower($c[1]);');
            $result = preg_replace_callback('/([A-Z])/', $func, $str);
            return $result;
        }

It basically converts text in camelcase to lowercase with underscores.

Why is this leaking?

I am running PHP 5.3.5 in MAMP on my Mac OS X Lion

You can use preg_replace to prefix upper cased letter with _ and then apply strtolower to the whole string. It will prevent of using callbacks and creating functions — zerkms
– zerkms, Commented Apr 28, 2012 at 11:57
It is not leaking memory, you just don't know where the memory is being used. See the answers. — salathe
– salathe, Commented Apr 28, 2012 at 12:00

Simon Forsberg · Accepted Answer · 2012-04-28 12:06:56Z

4

Because you're creating a new function every time that function runs.

Since you're using 5.3, you can replace create_function with an anonymous function and see if that helps:

    static private function from_camel_case($str)
    {
        $str[0] = strtolower($str[0]);
        $result = preg_replace_callback('/([A-Z])/', function($matches) {
          return '_' . strtolower($matches[1]);
        }, $str);
        return $result;
    }

Or extract the callback into a normal function:

    static private function from_camel_case($str)
    {
        $str[0] = strtolower($str[0]);
        $result = preg_replace_callback('/([A-Z])/', array(__CLASS__, 'replace_case'), $str);
        return $result;
    }

    static private function replace_case($matches) {
      return '_' . strtolower($matches[1]);
    }

edited Apr 28, 2012 at 12:06

Simon Forsberg

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answered Apr 28, 2012 at 12:02

user1233508

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1 Comment

Simon Forsberg Over a year ago

You forgot to remove the old create_function call.

kuba · Accepted Answer · 2012-04-28 11:57:22Z

3

You are creating a function using create_function every time you call it. Functions in PHP are always global, which means it will exist till the end of the script. That's why every time you call it, it will allocate some memory and never deallocate it.

You should create the function only once, or I's sure you can rewrite the whole thing using callbacks to get rid of the memory leak.

answered Apr 28, 2012 at 11:57

kuba

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1 Comment

Joris Over a year ago

Indeed! That is the problem. Thanks! Still have to wait 8 minutes before I can accept this answer though :(

Simon Forsberg · Accepted Answer · 2012-04-28 11:56:50Z

3

If you're calling that code over and over again the problem is this line:

$func = create_function('$c', 'return "_" . strtolower($c[1]);');

That code creates a new anonymous function every time you call it. According to http://www.php.net/create_function the code is put into the global namespace, so the function is not removed when the method is completed.

answered Apr 28, 2012 at 11:56

Simon Forsberg

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