Memory usage of C++ / Qt application

Question

I'm using OS X 10.5.6. I have a C++ application with a GUI made with Qt. When I start my application it uses 30 MB of memory (reported by OS X Activity Monitor RSIZE).

I use this application to read in text files to memory, parse the data and finally visualize it. If I open (read to memory, parse, visualize) a 9 MB text file Activity Monitor reports that my application grows from the initial 30 MB of memory used to 103 MB.

Now if the file is closed and the parsed and visualized data is deleted, the size of the application stays at 103 MB. This sounds like a memory leak to me. But if I open the file again, reading it to memory, parsing it and visualizing it the application stays at 103 MB. No matter how many times I open the file (or another file of the same size) my applications memory use stays more or less unchanged. Does this mean that it's not a memory leak? If it was a leak the memory usage should keep growing each time the file is opened should it not? The only time it grows is if I open a larger file than the previous one.

Is this normal? Is this platform or library dependent? Is this some sort of caching done by the OS or libraries?

Answer 1

This seems relatively normal, but all OS are slightly different.

In the usual application life cycle the application requests memory from the OS and is given memory in huge chunks that it manages (via the C/C++ standard libraries). As the application acquires/releases memory this is all done internally within the application without recourse to the OS until the application has non left then a call is made to the OS for another huge chunk.

Memory is not usually returned to the OS until the application quits (though most OS do provide the mechanisms to do this if required and some C/C++ standard libraries will use this facility). Instead of returning memory to the OS the application uses everything it has been given and does its own memory management.

Though note: just because an application has memory does not mean that this is currently taking up RAM on a chip. Memory that is sporadically used or has not been used in a while will be temporarily saved onto secondary/tertiary storage.

Activity Monitor: Is not a very useful tool for checking memory usage, as you have discovered it only displays the total actually allocated to the application. It does not display any information about how the application has internal allocated this memory (most of which could be deallocated). Check the folder where XCode lives, there are a broad set of tools for examining how an application works provided with the development environment.

NB: I have avoided using terms like page etc as these are nothing to-do with C/C++/Objective C and are all OS/hardware specific.