Add rustix::process::exit to terminate processes

Author: danielschemmel

Diff for: src/backend/libc/process/syscalls.rs

@@ -742,3 +742,14 @@ pub(crate) fn getgroups(buf: &mut [Gid]) -> io::Result<usize> {
 
     unsafe { ret_usize(c::getgroups(len, buf.as_mut_ptr().cast()) as isize) }
 }
+
+#[inline]
+pub(crate) fn _exit(status: i32) -> ! {
+    unsafe {
+        libc::_exit(status);
+    }
+    #[allow(unreachable_code)]
+    {
+        unreachable!("_exit failed to exit the process")
+    }
+}

Diff for: src/backend/linux_raw/arch/mod.rs

@@ -306,7 +306,7 @@ macro_rules! syscall_readonly {
 }
 
 /// Like `syscall`, but indicates that the syscall does not return.
-#[cfg(feature = "runtime")]
+#[cfg(any(feature = "runtime", feature = "process"))]
 macro_rules! syscall_noreturn {
     ($nr:ident, $a0:expr) => {
         $crate::backend::arch::choose::syscall1_noreturn(

Diff for: src/backend/linux_raw/process/syscalls.rs

@@ -620,3 +620,14 @@ pub(crate) fn getgroups(buf: &mut [Gid]) -> io::Result<usize> {
         ))
     }
 }
+
+#[inline]
+pub(crate) fn _exit(status: i32) -> ! {
+    unsafe {
+        syscall_noreturn!(__NR_exit_group, c_int(status));
+    };
+    #[allow(unreachable_code)]
+    {
+        unreachable!("_exit failed to exit the process")
+    }
+}

Diff for: src/process/exit.rs

@@ -1,8 +1,6 @@
 use crate::backend;
 
-/// `EXIT_SUCCESS` for use with [`exit`].
-///
-/// [`exit`]: std::process::exit
+/// `EXIT_SUCCESS` for use with [`exit`] or [`std::process::exit`].
 ///
 /// # References
 ///  - [POSIX]
@@ -12,9 +10,7 @@ use crate::backend;
 /// [Linux]: https://man7.org/linux/man-pages/man3/exit.3.html
 pub const EXIT_SUCCESS: i32 = backend::c::EXIT_SUCCESS;
 
-/// `EXIT_FAILURE` for use with [`exit`].
-///
-/// [`exit`]: std::process::exit
+/// `EXIT_FAILURE` for use with [`exit`] or [`std::process::exit`].
 ///
 /// # References
 ///  - [POSIX]
@@ -34,3 +30,30 @@ pub const EXIT_FAILURE: i32 = backend::c::EXIT_FAILURE;
 /// [`Signal::Abort`]: crate::process::Signal::Abort
 #[cfg(not(any(target_os = "espidf", target_os = "wasi")))]
 pub const EXIT_SIGNALED_SIGABRT: i32 = backend::c::EXIT_SIGNALED_SIGABRT;
+
+/// Immediately exits the process. Exiting via this function does not unwind the
+/// stack and does not call any further user code. This behavior is similar to
+/// the POSIX/C `_Exit` and `_exit` functions.
+///
+/// Notably, this function does:
+///  - *Not* flush any buffers, such as Rust or C standard output or files.
+///  - *Not* call any destructors, neither in the form of stack unwinding, nor
+///    any global destructors.
+///  - *Not* call functions registered with [`atexit`] or [`at_quick_exit`]
+///
+/// In general, most code should call [`std::process::exit`] instead, if it is
+/// available.
+///
+/// # References
+///  - [POSIX]
+///  - [Linux]
+///
+/// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stdlib.h.html
+/// [Linux]: https://www.man7.org/linux/man-pages/man2/exit.2.html
+/// [`atexit`]: https://www.man7.org/linux/man-pages/man3/atexit.3.html
+/// [`at_quick_exit`]: https://en.cppreference.com/w/c/program/at_quick_exit
+#[doc(alias = "_exit")]
+#[inline]
+pub fn exit(status: i32) -> ! {
+    backend::process::syscalls::_exit(status);
+}