Derick Rethans: Understanding Valgrind errors (1)

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.