C++ How to stop over-allocating in heap?

Question

I have the following code:

#include <iostream>

int main()
{
    try {
        while (true)
            int* ptr = new int[10000000L];
    } catch (const std::bad_alloc& e) {
        std::cout << e.what();
        return EXIT_FAILURE;

    }
    return EXIT_SUCCESS;
}

The main problem is that the std::bad_alloc exception is never caught, the program continues to run endlessly without any errors. Of course, all of my 8GB RAM is filled to 95%, then it starts using the utilizing swap memory. As a result, the error about the lack of memory does not pop up.

I'm using a common Windows 10 OS.

How to fix that program to disable using utilizing swap memory?

In x64 its not easy to get a `std::bad_alloc` in practice. You may have 8GB of ram but I assume you also have at least 8GB of swap. — drescherjm, Mar 02 '23 at 15:44
Over-commit is common. As long as you don't actually use the memory you have allocated, the OS will simply not map any memory pages to your process. — Some programmer dude, Mar 02 '23 at 15:44
"then it starts automatically forever reallocated" - what do you mean by that? how do you know it is happening? — user253751, Mar 02 '23 at 15:46
A very good optimizing compiler will remove that entire code that allocates memory, since you don't really do anything with it. — PaulMcKenzie, Mar 02 '23 at 15:47
You _might_ be asking how to stop a program from utilizing swap memory, but I'm honestly not sure what you are trying to do with this program. — Drew Dormann, Mar 02 '23 at 15:52
Realize virtual memory is not the same size as physical RAM. — franji1, Mar 02 '23 at 16:00
@PaulSanders allocation functions are considered to never have side effects when called as a result of new-expression as of C++14. — Revolver_Ocelot, Mar 02 '23 at 16:39

score 4 · Answer 1 · answered Mar 02 '23 at 16:08

First, modern computers use virtual memory. In a virtual memory system, the pointer value of an address in memory space has an offset into a page and a page index in it. (Typically pages are 4k in size, so 12 bits of page offset).

The physical location of the page in physical memory may not exist, or it might exist at a completely different offset. There is a table mapping the page number to where the data actually is. This table can include a physical page offset, or a trap that causes code to be executed in the OS if an attempt is made to access that page.

So while you have only 8 GB of physical memory, this does not limit the amount of memory a process can address.

This is also how different processes can't touch each other's memory; they exist in different address spaces. The same pointer value in one process does not refer to the same page, because they have distinct page tables. (In windows, the processes share a kernel address space; but outside of kernel mode you can't access those pages anyhow, nor do safe OS APIs let you inject user-mode values into any kernel-mode pointer)

Second, you are only asking for address space to be reserved. A 64 bit computer can have up to 2^64 memory addresses (each 8 bits), or over 10^19. You are allocating 10^7 at a time. This won't run out of 64 bit address space for 10^12 iterations (1000 billion). If you can do 10^7 allocations per second, this should take about a day.

Now, not all 64 bit systems have the full 64 bit memory space. So it could happen earlier.

Many OS's permit you to request memory space, and if never touched don't actually assign you any physical memory or even any space on disk.

Third, in C++ if you allocate memory and lose track of all pointers to it, this is equivalent to never allocating the memory. The C++ standard permits compilers to detect noop allocations and not do them.

In short, if you want to know how much memory you have, find an OS API that tells you.

If you want to actually reserve physical memory, that generally isn't possible from user-mode code. You'll have to have special privileges, like be a driver or be part of the windows OS itself. And drivers have to be signed by the manufacturer of the hardware.

There are ways to request that your memory not be swapped to disk, but these are only going to be requests. Look up Virtual Memory Functions in the windows API. VirtualLock, for example. Using this is a really bad idea in 999/1000 cases.

C++ How to stop over-allocating in heap?

1 Answers1