https://github.com/wheybags/vmhax

https://github.com/wheybags/vmhax

Last synced: 22 days ago
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• Host: GitHub
• URL: https://github.com/wheybags/vmhax
• Owner: wheybags
• Created: 2021-10-19T20:35:57.000Z (over 3 years ago)
• Default Branch: master
• Last Pushed: 2023-07-15T19:16:00.000Z (over 1 year ago)
• Last Synced: 2024-11-12T04:23:29.161Z (3 months ago)
• Language: C++
• Size: 27.3 KB
• Stars: 5
• Watchers: 4
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: readme.md

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README

        
# vmhax

Experiments with virtual memory.

## pinned.h

This is an allocator / vector class that retains the same base pointer when you reallocate.

```c

pinned_alloc_info allocation;

int ret = pinned_alloc(1024, PINNED_MAXSIZE_SMALL, &allocation);

assert(ret);

do_stuff_with_buffer(allocation.data, allocation.size);

void* pointer_inside_buffer = ((char*)allocation.data) + 20);

ret = pinned_realloc(&allocation, 8192);

assert(ret);

do_more_stuff_with_bigger_buffer(allocation.data, allocation.size);

do_more_stuff_with_old_pointer(pointer_inside_buffer); // valid, because pinned_realloc does not reallocate pointers

pinned_free(&allocation);

```

When using the header from c++, it also defines a `pinned_vec` template class, that works like `std::vector`.

```c++

pinned_vec v;

for (int i = 0; i < 10; i++)

  v.push_back(i);

int* p = &v[5];

for (int i = 0; i < 10; i++)

  v.push_back(i); // iterators / pointers *not* invalidated

  

do_something_with(p); // not an error

```

It works by reserving a large chunk of virtual memory, and only actually mapping a few pages at the start of the reserved chunk. When you "realloc", it just maps some more pages on the end of that chunk. This can work because typical 64-bit computers (yeah, this will not work on a 32-bit machine :p) have way more virtual address space than physical memory, so we can afford to waste large chunks of it.