Question

does an entry in Instruments “leaked block” during application running imply memory leak?

That is, if one is half way through using the iPhone application, where you might have some variables that have been retained but it hasn’t got to the part of the application where it gets released, then do these show up as leaked blocks or not?

If the answer is that variables which have not been finished with do show up here as leaked blocks, then this would be quite confusing then if you are stopping/pausing instruments, in which case how would one run instruments in a manner whereby any leaked blocks you see are valid memory leaks? (e.g. need to kill application to end everything first and then look at instruments?)

Answer 1

A leak in Instruments indicates that Instruments cannot find a pointer to the allocated memory starting at any of a group of “root” pointers. Specifically, from the Memory Usage Performance Guidelines:

The Leaks instrument records all allocation events that occur in your application and then periodically searches the application’s writable memory, registers, and stack for references to any active memory blocks. If it does not find a reference to a block in one of these places, it deems the block a “leak” and displays the relevant information in the Detail pane.

So retaining something is not a leak as long as you have a pointer to it in one of your ivars, local variables or static variables. But there are ways that the Leaks instrument can get confused, and sometimes Apple has a leak in their frameworks, and sometimes Instruments has a bug.

The thing you’re looking for with Leaks are steady and significant increases in the amount of leaked memory either over time or when doing specific actions. Tiny, one-time leaks are generally not worth chasing.