posix: implement mmap()

[ycsin]

There are currently 20 missing POSIX functions with trivial implementations, mmap() is one of them.

mmap - map pages of memory

The mmap() function shall establish a mapping between an address space of a process and a memory object.
...

See https://pubs.opengroup.org/onlinepubs/9699919799/functions/mmap.html

See RFC #51211 for more info.

[KaiserS0ze]

Hi @ycsin I am a first-time contributor. Can I take a crack at this?

[ycsin]

Hi @ycsin I am a first-time contributor. Can I take a crack at this?

Hi, please feel free to submit PR for anything

[cfriedt]

mmap() can actually get very complicated. It certainly isn't trivial.

We can definitely start out simple though.

Where should we get the file descriptor? How do we differentiate between real and special files? Some of these things might be tricky and there could be dependencies on other POSIX API calls (e.g. stat?)

1. memory (identity / physical)
2. memory (virtual)
3. Device memory (physical)
4. Regular File from file system, read-only (physical)
   ..

[KaiserS0ze]

Thanks. Yes, I think there are a lot of edge cases. I will start off with a simple implementation.