std.os.linux: resurrect shm

[RossComputerGuy]

This reintroduces Linux's shared memory system calls which was in #7214. It also adds the externs necessary for C.

[nektro]

• the types used are very wrong
• the constants should be in a namespace
• the functions should not use @bitcast

(was going to enumerate the way the types were different but it ended up as 9 comments and thought that would be over-bearing)

[RossComputerGuy]

@nektro I've resolved the last two points you made. Not sure what to do about the types though.

[nektro]

std.c should use c_int/c_uint
shmdt should accept *const anyopaque
shmctl's buf accepts a struct ptr, not a byte one
shmid should be unsigned
returning usize then accepting u32 feels sus

[RossComputerGuy]

Ok, I've updated shmdt and shmid. Made a few of the std.c changes. Not sure what you're meaning about the shmctl buffer.
Edit: found the header for shm stuff, gonna look at that for the C stuff and port the struct over.

[andrewrk]

@RossComputerGuy can you share the steps you used to come up with this patch?

[RossComputerGuy]

@andrewrk I used the original implementation and then updated it by looking at the header files which define the methods and stuff. I then ran std tests on my system to see if the shm test passes.

[andrewrk]

A core zig team member will need to check this work. We've unfortunately seen a lot of mistakes in the ABI bits from contributors.

[Aransentin]

Note that if you want to do SHM for X11 it actually supports the modern POSIX shared memory too, it's just that the 1.2 version of MIT-SHM that introduced it is entirely undocumented. You'd have to take a peek at the libXCB source code instead or the like.

[RossComputerGuy]

Note that if you want to do SHM for X11

Trying to do Wayland shm.

[Aransentin]

Note that if you want to do SHM for X11

Trying to do Wayland shm.

Seems to be POSIX shared memory on Wayland as well, though? https://wayland-book.com/surfaces/shared-memory.html

[ifreund]

Trying to do Wayland shm.

You don't want these syscalls for Wayland shared memory for software rendering buffers (wl_shm). You want memfd_create() on operating systems that support it and something shm_open() equivalent on operating systems that don't (see e.g. https://github.com/emersion/hello-wayland/blob/5f3a35def81116f0a74fcaf5a421d66c6700482d/shm.c)

[RossComputerGuy]

Oh it might been that I needed this for something else but I don't remember.