What does Malloc do?

**Albinoswordfish** · 03-09-2010

Now I've read this thread: What does malloc() do?

I already know from the high-level perspective what malloc does. However I want to know how an OS (Unix or Windows) interprets a malloc call.

I'm asking this because I was looking at my process list while I was stepping through a C program and noticed that the amount of memory a process takes up doesn't change after a malloc call. The process memory only changes when I attempt to read or write to an address which I allocated with malloc.

So from this I'm guessing the OS doesn't actually allocate virtual page entries for a process until it actually uses it (load/store ops). If this is the case then what does malloc() actually do at the low-level with the OS?

**quzah** · 03-09-2010

Let's make a simple program:

Code:

while not done
    present menu
        add item
        remove item
        show item
        exit

There. That's our program. How could your program know how many times I choose 'add item' ahead of time? It can't. So it can't create a fixed number of pages, because there's no way to know how many times I feel like choosing 'add'.

Quzah.

**Salem** · 03-09-2010

The pages are allocated to the process as a result of the malloc call.

When you touch those pages, the virtual page gets mapped to some real memory.

**Albinoswordfish** · 03-09-2010

Oh ok now I see, thanks for the reply

**rags_to_riches** · 03-10-2010

This is a good read on this subject

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