Question

In modern-day operating systems, memory is available as an abstracted resource. A process is exposed to a virtual address space (which is independent from address space of all other processes) and a whole mechanism exists for mapping any virtual address to some actual physical address.
My doubt is:

• If each process has its own address space, then it should be free to access any address in the same. So apart from permission restricted sections like that of .data, .bss, .text etc, one should be free to change value at any address. But this usually gives segmentation fault, why?

• For acquiring the dynamic memory, we need to do a malloc. If the whole virtual space is made available to a process, then why can’t it directly access it?

• Different runs of a program results in different addresses for variables (both on stack and heap). Why is it so, when the environments for each run is same? Does it not affect the amount of addressable memory available for usage? (Does it have something to do with address space randomization?)

• Some links on memory allocation (e.g. in heap).

The data available at different places is very confusing, as they talk about old and modern times, often not distinguishing between them. It would be helpful if someone could clarify the doubts while keeping modern systems in mind, say Linux.

Thanks.

Answer 1

Technically, the operating system is able to allocate any memory page on access, but there are important reasons why it shouldn’t or can’t:

different memory regions serve different purposes.

• code. It can be read and executed, but shouldn’t be written to.
• literals (strings, const arrays). This memory is read-only and should be.
• the heap. It can be read and written, but not executed.
• the thread stack. There is no reason for two threads to access each other’s stack, so the OS might as well forbid that. Moreover, the tread stack can be de-allocated when the tread ends.
• memory-mapped files. Any changes to this region should affect a specific file. If the file is open for reading, the same memory page may be shared between processes because it’s read-only.
• the kernel space. Normally the application should not (or can not) access that region – only kernel code can. It’s basically a scratch space for the kernel and it’s shared between processes. The network buffer may reside there, so that it’s always available for writes, no matter when the packet arrives.
• …

The OS might assume that all unrecognised memory access is an attempt to allocate more heap space, but:

• if an application touches the kernel memory from user code, it must be killed. On 32-bit Windows, all memory above 1<<31 (top bit set) or above 3<<30 (top two bits set) is kernel memory. You should not assume any unallocated memory region is in the user space.
• if an application thinks about using a memory region but doesn’t tell the OS, the OS may allocate something else to that memory (OS: sure, your file is at 0x12341234; App: but I wanted to store my data there). You could tell the OS by touching the end of your array (which is unreliable anyways), but it’s easier to just call an OS function. It’s just a good idea that the function call is “give me 10MB of heap”, not “give me 10MB of heap starting at 0x12345678“
• If the application allocates memory by using it then it typically does not de-allocate at all. This can be problematic as the OS still has to hold the unused pages (but the Java Virtual Machine does not de-allocate either, so hey).

Different runs of a program results in different addresses for variables

This is called memory layout randomisation and is used, alongside of proper permissions (stack space is not executable), to make buffer overflow attacks much more difficult. You can still kill the app, but not execute arbitrary code.

Some links on memory allocation (e.g. in heap).

Do you mean, what algorithm the allocator uses? The easiest algorithm is to always allocate at the soonest available position and link from each memory block to the next and store the flag if it’s a free block or used block. More advanced algorithms always allocate blocks at the size of a power of two or a multiple of some fixed size to prevent memory fragmentation (lots of small free blocks) or link the blocks in a different structures to find a free block of sufficient size faster.

An even simpler approach is to never de-allocate and just point to the first (and only) free block and holds its size. If the remaining space is too small, throw it away and ask the OS for a new one.

There’s nothing magical about memory allocators. All they do is to:

• ask the OS for a large region and
• partition it to smaller chunks
• without
  • wasting too much space or
  • taking too long.

Anyways, the Wikipedia article about memory allocation is http://en.wikipedia.org/wiki/Memory_management .

One interesting algorithm is called “(binary) buddy blocks”. It holds several pools of a power-of-two size and splits them recursively into smaller regions. Each region is then either fully allocated, fully free or split in two regions (buddies) that are not both fully free. If it’s split, then one byte suffices to hold the size of the largest free block within this block.