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I'm studying up on OS memory management, and I wish to verify that I got the basic mechanism of allocation \ virtual memory \ paging straight.

Let's say a process calls malloc(), what happens behind the scenes? my answer: The runtime library finds an appropriately sized block of memory in its virtual memory address space. (This is where allocation algorithms such as first-fit, best-fit that deal with fragmentation come into play)

Now let's say the process accesses that memory, how is that done? my answer: The memory address, as seen by the process, is in fact virtual. The OS checks if that address is currently mapped to a physical memory address and if so performs the access. If it isn't mapped - a page fault is raised.

Am I getting this straight? i.e. the compiler\runtime library are in charge of allocating virtual memory blocks, and the OS is in charge of a mapping between processes' virtual address and physical addresses (and the paging algorithm that entails)?

Thanks!

bloodcell
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2 Answers2

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About right. The memory needs to exist in the virtual memory of the process for a page fault to actually allocate a physical page though. You can't just start poking around anywhere and expect the kernel to put physical memory where you happen to access.

There is much more to it than this. Read up on mmap(), anonymous and not, shared and private. And brk() too. malloc() builds on brk() and mmap().

Thomas
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You've almost got it. The one thing you missed is how the process asks the system for more virtual memory in the first place. As Thomas pointed out, you can't just write where you want. There's no reason an OS couldn't be designed to allow that, but it's much more efficient if it has some idea where you're going to be writing and the space where you do it is contiguous.

On Unixy systems, userland processes have a region called the data segment, which is what it sounds like: it's where the data goes. When a process needs memory for data, it calls brk(), which asks the system to extend the data segment to a specified pointer value. (For example, if your existing data segment was empty and you wanted to extend it to 2M, you'd call brk(0x200000).)

Note that while very common, brk() is not a standard; in fact it was yanked out of POSIX.1 a decade ago because C specifies malloc() and there's no reason to mandate the interface for data segment allocation.

Blrfl
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