Use POSIX shared memory in X11 backend #165
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The X11 backend used System-V shared memory to communicate shared buffers to the server, but rustix does not support SysV SHM so we had to use libc in this backend. However, there is an alternate X11 API which uses POSIX shared memory, which rustix does support. This PR switches the X11 backend to POSIX shared memory and purges the previous usages of libc. The goal is to remove libc totally. Therefore this PR also removes the default libc dependency from rustix. Signed-off-by: John Nunley <[email protected]>
| @@ -758,6 +767,45 @@ impl Drop for X11Impl { | |||
| } | |||
| } | |||
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| /// Create a shared memory identifier. | |||
| fn create_shm_id() -> io::Result<fd::OwnedFd> { | |||
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We should move the create_memfile logic in the wayland backend to somewhere it can be used in both backends, instead of duplicating things.
Using memfd_create is better than shm_open where supported.
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I'm not sure if memfiles are supported for X11 MIT-SHM
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I can't really find any documentation for ShmAttachFd beyond:
Create a shared memory segment. The file descriptor will be mapped at offset
zero, and the size will be obtained using fstat(). A zero size will result in a
Value error.
The file descriptor the server should mmap().
So any fd it can fstat to get the size and mmap should be fine.
https://lists.freedesktop.org/archives/xorg-devel/2013-October/038490.html
I was hoping for a patch to shmproto.txt, but there isn't one, le sigh.
We should really fix the MIT-SHM docs, Jon's reverse-engineered spec pdf
is full of enough handwave to be embarassing for us.
Lol. I guess the fact https://gitlab.freedesktop.org/xorg/proto/xcbproto/-/blob/master/src/shm.xml exists means the blocker there is addressed, but no one has added documentation for version 1.2 of the protocol yet.
Personally I don't see that much value to completely avoiding the |
I think there's progress on this front. |
The X11 backend used System-V shared memory to communicate shared buffers to the server, but rustix does not support SysV SHM so we had to use libc in this backend. However, there is an alternate X11 API which uses POSIX shared memory, which rustix does support. This PR switches the X11 backend to POSIX shared memory and purges the previous usages of libc.
The goal is to remove libc totally. Therefore this PR also removes the default libc dependency from rustix.