PHP Extensions - RETURN_STRING

Question

In a PHP extension, what is the difference between this:

PHP_METHOD(TestExtension, test)
{
  MyClass *myclass;
  MyClass_Object *obj = (MyClass_Object*)zend_object_store_get_object(getThis() TSRMLS_CC);
  myclass = obj->myclass;
  if (myclass != NULL) 
  {
    string retval = myclass->test();
    RETURN_STRING(retval.c_str(), 1);
  }
  RETURN_NULL();
}

and this:

PHP_METHOD(TestExtension, test)
{
  MyClass *myclass;
  MyClass_Object *obj = (MyClass_Object*)zend_object_store_get_object(getThis() TSRMLS_CC);
  myclass = obj->myclass;
  if (myclass != NULL) 
  {
    RETURN_STRING(myclass->test().c_str(), 1);
  }
  RETURN_NULL();
}

?

Both seem to work, but when I run valgrind with:

valgrind --tool=memcheck --num-callers=30 --log-file=./php.log /usr/bin/php test.php

where test.php is:

<?php
  $obj = new TestExtension("testing");
  echo $obj->test() . "\n";
?>

then the latter gives me a whole bunch of errors, all of which are:

Address 0xe4c3e98 is 24 bytes inside a block of size 66 free'd

The valgrind summary is as follows:

126 ==23067== HEAP SUMMARY:
127 ==23067==     in use at exit: 9,031 bytes in 15 blocks
128 ==23067==   total heap usage: 25,131 allocs, 25,116 frees, 4,435,757 bytes allocated
129 ==23067==
130 ==23067== LEAK SUMMARY:
131 ==23067==    definitely lost: 0 bytes in 0 blocks
132 ==23067==    indirectly lost: 0 bytes in 0 blocks
133 ==23067==      possibly lost: 0 bytes in 0 blocks
134 ==23067==    still reachable: 9,031 bytes in 15 blocks
135 ==23067==         suppressed: 0 bytes in 0 blocks
136 ==23067== Rerun with --leak-check=full to see details of leaked memory
137 ==23067==
138 ==23067== For counts of detected and suppressed errors, rerun with: -v
139 ==23067== ERROR SUMMARY: 48 errors from 5 contexts (suppressed: 0 from 0)

missing a comma after myClass->test() in RETURN_STRING() ?

damndaewoo
– damndaewoo

2015-01-21 11:03:34 +00:00
Commented Jan 21, 2015 at 11:03 — damndaewoo
– damndaewoo, Commented Jan 21, 2015 at 11:03
There is no missing comma.

Vanessa Deagan
– Vanessa Deagan

2015-01-21 11:16:29 +00:00
Commented Jan 21, 2015 at 11:16 — Vanessa Deagan
– Vanessa Deagan, Commented Jan 21, 2015 at 11:16

Lightness Races in Orbit · Accepted Answer · 2015-01-21 16:26:53Z

2

Although it's not stated in the documentation for the RETURN_STRING macro (which seems like an omission: it should definitely state lifetime requirements) it would appear that the macro expands to more than one line.

Say, for example:

RETURN_STRING(myclass->test().c_str(), 1);

becomes:

const char* arg = myclass->test().c_str();
someCode();
someMoreCode();
finalCode(arg);

arg is not valid on the last line because (assuming it returns by value) the temporary result of myclass->test() only exists for the duration of the first line. The result of myclass->test().c_str() is thus, also, only valid for the duration of the first line. arg becomes a dangling pointer immediately thereafter.

Your workaround is a correct way to fix this. I would recommend, when using any of these macros, ensuring that you pass it a pointer to data that is assuredly still in existence for at least as long as the macro runs, no matter how many statements/expressions that may involve.

Here's what I'd do:

if (myclass != NULL) {
   const string& retval = myclass->test();
   RETURN_STRING(retval.c_str(), 1);
}

Now, no matter what myclass->test() is, it'll survive all of the macro's expanded statements, and you've not had to copy it into a new std::string object.

edited Jan 21, 2015 at 16:26

answered Jan 21, 2015 at 11:28

Lightness Races in Orbit

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3 Comments

Vanessa Deagan Over a year ago

Thank you! I should have looked at the macro. I'm kinda still fumbling my way around PHP extensions. Just out of interest, what documentation do you use?

Lightness Races in Orbit Over a year ago

@VanessaDeagan: I don't write PHP extensions. So, none. :)

Christopher Thomas Over a year ago

I write some php extensions, I would suggest github and stackoverflow to search for example code, there is little organised documentation, some of it is in the phpinternals book and some other parts are in the php hacker manual, but yeah, you're going to find documentation hard to find :(

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