Memory Layout of Linux (malloc() used in C, but does not start with the expected address)

Question

I used this chunk of code:

    int
main(int argc, char *argv[])
{
    int *p;                   // memory for pointer is on "stack"
    p = malloc(sizeof(int));  // malloc'd memory is on "heap"
    assert(p != NULL);
    printf("(pid:%d) addr of p:        %llx\n", (int) getpid(), 
       (unsigned long long) &p);
    printf("(pid:%d) addr stored in p: %llx\n", (int) getpid(), 
       (unsigned long long) p);
    return 0;
}

However, I get:

addr of p:        7ffc0c53e3e0
addr stored in p: 558ae195c260

Now, first of all, since the program only does this, I do not understand why malloc() does not start with the address 00200000 ? Second, can I say that the 7ffc0c53e3e0 address is in heap , and the address 558ae195c260 is in stack ? Third, if my guess with 00200000 is wrong, is there any logic with the addresses that I get, or is it completely random?

When I think about it, the address is not even 32 bits, it is 48 bits. Even if it is to be more than 32 bits (I have 8 GB memory, so I believe it must be more than 32 anyways), why is it not expressed in 64 bits, since the processor is 64 bits.

Thank you for your help.

Answer 1

No, p is in stack (or global) and it points to a bunch of memory in the heap.

Regarding malloc() , if you are working on an OS, it depends on the kernel, and how it is managing the memory.

Finally, obviously some of your data is wrong. A 32bit bus data cannot manage 8GB of RAM (no more than 2^32=4GB). However, it makes sense in a 64bits bus, because a 64bits variable has enough space to contain an 8GB address.

Answer 2

Linux implements ASLR , so as far as I'm aware you always get random addresses. 558ae195c260 is actually allocated on the heap via malloc() , whereas 7ffc0c53e3e0 is allocated on the stack when you declare int *p; . 48 bits is still enough for 256 TB of RAM, but beyond that some architectures don't allow all address lines to be a full 64 bits (like AMD64).

Hope that helps. If anything I've said is wrong or misleading please correct me in the comments.