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Derick Rethans:
Understanding Valgrind errors (1)
February 04, 2014 @ 11:03:02

Using the Valgrind tool can give you a good window into the inner workings of your PHP script (at a very low level) but the results it gives can be a bit confusing to interpret. In his latest post Derick Rethans uses the memcheck tool that comes as a part of Valgrind to monitor memory usage and interprets some of the results.

Memcheck is really valuable for C and C++ developers and something you should learn, especially when you write PHP extensions. Memcheck's error messages can sometimes be difficult to understand, so with this (infrequent series), I hope to shed some light on it. Let's have a look at the following Valgrind error output, which I encountered while debugging issue PHP-963 of the MongoDB driver for PHP.

He walks though the output and how to spot the actual error - a reference to a null pointer in the C code. He tracks it back through the code and finds the spot where it references the wrong memory segment in the Mongo PHP extension code.

In short, an error where Address 0x38 is not stack'd, malloc'd or (recently) free'd has a low address in the message often means a NULL-pointer dereference.
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valgrind error memcheck mongo extension

Link: http://derickrethans.nl/valgrind-null.html

Derick Rethans:
Mongo is dead, long live MongoClient
November 28, 2012 @ 09:13:51

In this recent post to his site Derick Rethans mentions the shiny new "MongoClient" class that the latest release of the Mongo PHP drivers provides.

This afternoon we published version 1.3.0 of the MongoDB PHP driver. Besides a number of bug fixes since RC2 and RC3, this new release also includes a new MongoClient class. This new MongoClient class serves as a replacement for the Mongo class. The old Mongo class is now deprecated and will be removed in a future release, although we are keeping it in place for now because of backwards compatibility reasons. We have already removed it mostly from the documentation, and are working to update all our other material as well.

The main change that comes with the MongoClient class is that it now has acknowledged writes on by default (a "safe mode"). This option determines wether or not the client waits for a confirmation from the server when a write has happened. He includes a bit of code showing how to: turn it off, keep it on or using replica set acknowledged writes. You can also set it on a per-query basis.

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mongo class driver release mongoclient class confirm write



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