Add shm_open/shm_unlink (POSIX shared memory)

Fixes #705.

Cargo.toml

@@ -149,6 +149,9 @@ thread = ["linux-raw-sys/prctl"]
 # Enable `rustix::process::*`.
 process = ["linux-raw-sys/prctl"]
 
+# Enable `rustix::shm::*`.
+shm = ["fs"]
+
 # Enable `rustix::time::*`.
 time = []
 
@@ -198,6 +201,7 @@ all-apis = [
     "pty",
     "rand",
     "runtime",
+    "shm",
     "system",
     "stdio",
     "termios",

src/backend/libc/mod.rs

@@ -184,6 +184,14 @@ pub(crate) mod pid;
 #[cfg(any(feature = "process", feature = "thread"))]
 #[cfg(linux_kernel)]
 pub(crate) mod prctl;
+#[cfg(not(any(
+    windows,
+    target_os = "android",
+    target_os = "espidf",
+    target_os = "wasi"
+)))]
+#[cfg(feature = "shm")]
+pub(crate) mod shm;
 #[cfg(any(feature = "fs", feature = "thread", feature = "process"))]
 #[cfg(not(any(windows, target_os = "wasi")))]
 pub(crate) mod ugid;

src/backend/libc/shm/mod.rs

@@ -0,0 +1,2 @@
+pub(crate) mod syscalls;
+pub(crate) mod types;

src/backend/libc/shm/syscalls.rs

@@ -0,0 +1,25 @@
+use crate::ffi::CStr;
+
+use crate::backend::c;
+use crate::backend::conv::{c_str, ret, ret_owned_fd};
+use crate::fd::OwnedFd;
+use crate::fs::Mode;
+use crate::io;
+use crate::shm::ShmOFlags;
+
+pub(crate) fn shm_open(name: &CStr, oflags: ShmOFlags, mode: Mode) -> io::Result<OwnedFd> {
+    // On this platforms, `mode_t` is `u16` and can't be passed directly to a
+    // variadic function.
+    #[cfg(apple)]
+    let mode: c::c_uint = mode.bits().into();
+
+    // Otherwise, cast to `mode_t` as that's what `open` is documented to take.
+    #[cfg(not(apple))]
+    let mode: c::mode_t = mode.bits() as _;
+
+    unsafe { ret_owned_fd(c::shm_open(c_str(name), bitflags_bits!(oflags), mode)) }
+}
+
+pub(crate) fn shm_unlink(name: &CStr) -> io::Result<()> {
+    unsafe { ret(c::shm_unlink(c_str(name))) }
+}

src/backend/libc/shm/types.rs

@@ -0,0 +1,30 @@
+use crate::backend::c;
+use bitflags::bitflags;
+
+bitflags! {
+    /// `O_*` constants for use with [`shm_open`].
+    ///
+    /// [`shm_open`]: crate:shm::shm_open
+    #[repr(transparent)]
+    #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+    pub struct ShmOFlags: u32 {
+        /// `O_CREAT`
+        #[doc(alias = "CREAT")]
+        const CREATE = bitcast!(c::O_CREAT);
+
+        /// `O_EXCL`
+        const EXCL = bitcast!(c::O_EXCL);
+
+        /// `O_RDONLY`
+        const RDONLY = bitcast!(c::O_RDONLY);
+
+        /// `O_RDWR`
+        const RDWR = bitcast!(c::O_RDWR);
+
+        /// `O_TRUNC`
+        const TRUNC = bitcast!(c::O_TRUNC);
+
+        /// <https://docs.rs/bitflags/latest/bitflags/#externally-defined-flags>
+        const _ = !0;
+    }
+}

src/backend/linux_raw/mod.rs

@@ -67,6 +67,8 @@ pub(crate) mod pty;
 pub(crate) mod rand;
 #[cfg(feature = "runtime")]
 pub(crate) mod runtime;
+#[cfg(feature = "shm")]
+pub(crate) mod shm;
 #[cfg(feature = "system")]
 pub(crate) mod system;
 #[cfg(feature = "termios")]

src/backend/linux_raw/shm/mod.rs

@@ -0,0 +1,2 @@
+pub(crate) mod syscalls;
+pub(crate) mod types;

src/backend/linux_raw/shm/syscalls.rs

@@ -0,0 +1,49 @@
+use crate::ffi::CStr;
+
+use crate::backend::fs::{
+    syscalls::{open, unlink},
+    types::{Mode, OFlags},
+};
+use crate::fd::OwnedFd;
+use crate::io;
+use crate::shm::ShmOFlags;
+
+const NAME_MAX: usize = 255;
+const SHM_DIR: &[u8] = b"/dev/shm/";
+
+fn get_shm_name(name: &CStr) -> io::Result<([u8; NAME_MAX + SHM_DIR.len() + 1], usize)> {
+    let name = name.to_bytes();
+
+    if name.len() > NAME_MAX {
+        return Err(io::Errno::NAMETOOLONG);
+    }
+
+    let num_slashes = name.into_iter().take_while(|x| **x == b'/').count();
+    let after_slashes = &name[num_slashes..];
+    if after_slashes.is_empty()
+        || after_slashes == b"."
+        || after_slashes == b".."
+        || after_slashes.contains(&b'/')
+    {
+        return Err(io::Errno::INVAL);
+    }
+
+    let mut path = [0; NAME_MAX + SHM_DIR.len() + 1];
+    path[..SHM_DIR.len()].copy_from_slice(SHM_DIR);
+    path[SHM_DIR.len()..SHM_DIR.len() + name.len()].copy_from_slice(name);
+    Ok((path, SHM_DIR.len() + name.len() + 1))
+}
+
+pub(crate) fn shm_open(name: &CStr, oflags: ShmOFlags, mode: Mode) -> io::Result<OwnedFd> {
+    let (path, len) = get_shm_name(name)?;
+    open(
+        CStr::from_bytes_with_nul(&path[..len]).unwrap(),
+        OFlags::from_bits(oflags.bits()).unwrap() | OFlags::CLOEXEC,
+        mode,
+    )
+}
+
+pub(crate) fn shm_unlink(name: &CStr) -> io::Result<()> {
+    let (path, len) = get_shm_name(name)?;
+    unlink(CStr::from_bytes_with_nul(&path[..len]).unwrap())
+}

src/backend/linux_raw/shm/types.rs

@@ -0,0 +1,30 @@
+use crate::backend::c;
+use bitflags::bitflags;
+
+bitflags! {
+    /// `O_*` constants for use with [`shm_open`].
+    ///
+    /// [`shm_open`]: crate:shm::shm_open
+    #[repr(transparent)]
+    #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+    pub struct ShmOFlags: c::c_uint {
+        /// `O_CREAT`
+        #[doc(alias = "CREAT")]
+        const CREATE = linux_raw_sys::general::O_CREAT;
+
+        /// `O_EXCL`
+        const EXCL = linux_raw_sys::general::O_EXCL;
+
+        /// `O_RDONLY`
+        const RDONLY = linux_raw_sys::general::O_RDONLY;
+
+        /// `O_RDWR`
+        const RDWR = linux_raw_sys::general::O_RDWR;
+
+        /// `O_TRUNC`
+        const TRUNC = linux_raw_sys::general::O_TRUNC;
+
+        /// <https://docs.rs/bitflags/latest/bitflags/#externally-defined-flags>
+        const _ = !0;
+    }
+}

src/lib.rs

@@ -246,6 +246,14 @@ pub mod pty;
 #[cfg(feature = "rand")]
 #[cfg_attr(doc_cfg, doc(cfg(feature = "rand")))]
 pub mod rand;
+#[cfg(not(any(
+    windows,
+    target_os = "android",
+    target_os = "espidf",
+    target_os = "wasi"
+)))]
+#[cfg(feature = "shm")]
+pub mod shm;
 #[cfg(not(windows))]
 #[cfg(feature = "stdio")]
 #[cfg_attr(doc_cfg, doc(cfg(feature = "stdio")))]

src/shm.rs

@@ -0,0 +1,39 @@
+//! POSIX shared memory
+
+use crate::fd::OwnedFd;
+use crate::{backend, io, path};
+
+pub use crate::backend::fs::types::Mode;
+pub use crate::backend::shm::types::ShmOFlags;
+
+/// `shm_open(name, oflags, mode)`—Opens a shared memory object.
+///
+/// For portability, `name` should begin with a slash, contain no other slashes,
+/// and be no longer than an implementation-defined limit (255 on Linux).
+///
+/// Exactly one of [ShmOFlags::RDONLY] and [ShmOFlags::RDWR] should be passed. The file
+/// descriptor will be opened with `FD_CLOEXEC` set.
+///
+/// # References
+///  - [POSIX]
+///  - [Linux]
+///
+/// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/shm_open.html
+/// [Linux]: https://man7.org/linux/man-pages/man3/shm_open.3.html
+#[inline]
+pub fn shm_open<P: path::Arg>(name: P, flags: ShmOFlags, mode: Mode) -> io::Result<OwnedFd> {
+    name.into_with_c_str(|name| backend::shm::syscalls::shm_open(name, flags, mode))
+}
+
+/// `shm_unlink(name)`—Unlinks a shared memory object.
+///
+/// # References
+///  - [POSIX]
+///  - [Linux]
+///
+/// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/shm_unlink.html
+/// [Linux]: https://man7.org/linux/man-pages/man3/shm_unlink.3.html
+#[inline]
+pub fn shm_unlink<P: path::Arg>(name: P) -> io::Result<()> {
+    name.into_with_c_str(|name| backend::shm::syscalls::shm_unlink(name))
+}