In C++, on Linux, after I open a file with open() and then memory-map it with mmap(), I am given a void* that I can typecast to e.g. char* and read as desired.
But, suppose I want to use std::vector to access that data. How could I accomplish that?
At first I thought I could make a custom allocator for my std::vector. But I see two problems with that:
allocate() method will only receive a size argument. That is insufficient, as it also needs to know the already-mapped address, rather than allocating a new one (as allocators usually do.)deallocate() method -- to do this correctly -- would need to both unmap the buffer (easy, once #1 above is solved,) and close() the file descriptor (hard, since it has no way of knowing that value.)I thought about maybe storing this metadata (existing mapped address, and file descriptor) in a known place to be referenced by my custom allocator, but that would not be thread-safe without complicating things with TLS.
Is the above possible some way? Or ill-advised (if so, please explain)? Is there perhaps some other standard-library class that would serve my purpose?
My reason for wanting to do this, is to avoid a raw/POD pointer in a class that interprets the binary data in that memory-mapped region.
Is there perhaps some other standard-library class that would serve my purpose?
Yes: std::span is a wrapper that provides container like access without ownership.