+use crate::io;
+use crate::num::NonZeroUsize;
+
+/// Returns the number of hardware threads available to the program.
+///
+/// This value should be considered only a hint.
+///
+/// # Platform-specific behavior
+///
+/// If interpreted as the number of actual hardware threads, it may undercount on
+/// Windows systems with more than 64 hardware threads. If interpreted as the
+/// available concurrency for that process, it may overcount on Windows systems
+/// when limited by a process wide affinity mask or job object limitations, and
+/// it may overcount on Linux systems when limited by a process wide affinity
+/// mask or affected by cgroups limits.
+///
+/// # Errors
+///
+/// This function will return an error in the following situations, but is not
+/// limited to just these cases:
+///
+/// - If the number of hardware threads is not known for the target platform.
+/// - The process lacks permissions to view the number of hardware threads
+/// available.
+///
+/// # Examples
+///
+/// ```
+/// # #![allow(dead_code)]
+/// #![feature(available_concurrency)]
+/// use std::thread;
+///
+/// let count = thread::available_concurrency().map(|n| n.get()).unwrap_or(1);
+/// ```
+#[unstable(feature = "available_concurrency", issue = "74479")]
+pub fn available_concurrency() -> io::Result<NonZeroUsize> {
+ available_concurrency_internal()
+}
+
+cfg_if::cfg_if! {
+ if #[cfg(windows)] {
+ #[allow(nonstandard_style)]
+ fn available_concurrency_internal() -> io::Result<NonZeroUsize> {
+ #[repr(C)]
+ struct SYSTEM_INFO {
+ wProcessorArchitecture: u16,
+ wReserved: u16,
+ dwPageSize: u32,
+ lpMinimumApplicationAddress: *mut u8,
+ lpMaximumApplicationAddress: *mut u8,
+ dwActiveProcessorMask: *mut u8,
+ dwNumberOfProcessors: u32,
+ dwProcessorType: u32,
+ dwAllocationGranularity: u32,
+ wProcessorLevel: u16,
+ wProcessorRevision: u16,
+ }
+ extern "system" {
+ fn GetSystemInfo(info: *mut SYSTEM_INFO) -> i32;
+ }
+ let res = unsafe {
+ let mut sysinfo = crate::mem::zeroed();
+ GetSystemInfo(&mut sysinfo);
+ sysinfo.dwNumberOfProcessors as usize
+ };
+ match res {
+ 0 => Err(io::Error::new(io::ErrorKind::NotFound, "The number of hardware threads is not known for the target platform")),
+ cpus => Ok(unsafe { NonZeroUsize::new_unchecked(cpus) }),
+ }
+ }
+ } else if #[cfg(any(
+ target_os = "android",
+ target_os = "cloudabi",
+ target_os = "emscripten",
+ target_os = "fuchsia",
+ target_os = "ios",
+ target_os = "linux",
+ target_os = "macos",
+ target_os = "solaris",
+ target_os = "illumos",
+ ))] {
+ fn available_concurrency_internal() -> io::Result<NonZeroUsize> {
+ match unsafe { libc::sysconf(libc::_SC_NPROCESSORS_ONLN) } {
+ -1 => Err(io::Error::last_os_error()),
+ 0 => Err(io::Error::new(io::ErrorKind::NotFound, "The number of hardware threads is not known for the target platform")),
+ cpus => Ok(unsafe { NonZeroUsize::new_unchecked(cpus as usize) }),
+ }
+ }
+ } else if #[cfg(any(target_os = "freebsd", target_os = "dragonfly", target_os = "netbsd"))] {
+ fn available_concurrency_internal() -> io::Result<NonZeroUsize> {
+ use crate::ptr;
+
+ let mut cpus: libc::c_uint = 0;
+ let mut cpus_size = crate::mem::size_of_val(&cpus);
+
+ unsafe {
+ cpus = libc::sysconf(libc::_SC_NPROCESSORS_ONLN) as libc::c_uint;
+ }
+
+ // Fallback approach in case of errors or no hardware threads.
+ if cpus < 1 {
+ let mut mib = [libc::CTL_HW, libc::HW_NCPU, 0, 0];
+ let res = unsafe {
+ libc::sysctl(
+ mib.as_mut_ptr(),
+ 2,
+ &mut cpus as *mut _ as *mut _,
+ &mut cpus_size as *mut _ as *mut _,
+ ptr::null_mut(),
+ 0,
+ )
+ };
+
+ // Handle errors if any.
+ if res == -1 {
+ return Err(io::Error::last_os_error());
+ } else if cpus == 0 {
+ return Err(io::Error::new(io::ErrorKind::NotFound, "The number of hardware threads is not known for the target platform"));
+ }
+ }
+ Ok(unsafe { NonZeroUsize::new_unchecked(cpus as usize) })
+ }
+ } else if #[cfg(target_os = "openbsd")] {
+ fn available_concurrency_internal() -> io::Result<NonZeroUsize> {
+ use crate::ptr;
+
+ let mut cpus: libc::c_uint = 0;
+ let mut cpus_size = crate::mem::size_of_val(&cpus);
+ let mut mib = [libc::CTL_HW, libc::HW_NCPU, 0, 0];
+
+ let res = unsafe {
+ libc::sysctl(
+ mib.as_mut_ptr(),
+ 2,
+ &mut cpus as *mut _ as *mut _,
+ &mut cpus_size as *mut _ as *mut _,
+ ptr::null_mut(),
+ 0,
+ )
+ };
+
+ // Handle errors if any.
+ if res == -1 {
+ return Err(io::Error::last_os_error());
+ } else if cpus == 0 {
+ return Err(io::Error::new(io::ErrorKind::NotFound, "The number of hardware threads is not known for the target platform"));
+ }
+
+ Ok(unsafe { NonZeroUsize::new_unchecked(cpus as usize) })
+ }
+ } else {
+ // FIXME: implement on vxWorks, Redox, HermitCore, Haiku, l4re
+ fn available_concurrency_internal() -> io::Result<NonZeroUsize> {
+ Err(io::Error::new(io::ErrorKind::NotFound, "The number of hardware threads is not known for the target platform"))
+ }
+ }
+}