armv7a/usr/lib/rustlib/src/rust/library/std/src/sys/unix/weak.rs - manifest_repos/toolchain - Git at Google

 //! Support for "weak linkage" to symbols on Unix
 //!
 //! Some I/O operations we do in libstd require newer versions of OSes but we
 //! need to maintain binary compatibility with older releases for now. In order
 //! to use the new functionality when available we use this module for
 //! detection.
 //!
 //! One option to use here is weak linkage, but that is unfortunately only
 //! really workable with ELF. Otherwise, use dlsym to get the symbol value at
 //! runtime. This is also done for compatibility with older versions of glibc,
 //! and to avoid creating dependencies on GLIBC_PRIVATE symbols. It assumes that
 //! we've been dynamically linked to the library the symbol comes from, but that
 //! is currently always the case for things like libpthread/libc.
 //!
 //! A long time ago this used weak linkage for the __pthread_get_minstack
 //! symbol, but that caused Debian to detect an unnecessarily strict versioned
 //! dependency on libc6 (#23628) because it is GLIBC_PRIVATE. We now use `dlsym`
 //! for a runtime lookup of that symbol to avoid the ELF versioned dependency.

 // There are a variety of `#[cfg]`s controlling which targets are involved in
 // each instance of `weak!` and `syscall!`. Rather than trying to unify all of
 // that, we'll just allow that some unix targets don't use this module at all.
 #![allow(dead_code, unused_macros)]

 use crate::ffi::CStr;
 use crate::marker::PhantomData;
 use crate::mem;
 use crate::ptr;
 use crate::sync::atomic::{self, AtomicPtr, Ordering};

 // We can use true weak linkage on ELF targets.
 #[cfg(all(not(any(target_os = "macos", target_os = "ios")), not(bootstrap)))]
 pub(crate) macro weak {
     (fn $name:ident($($t:ty),*) -> $ret:ty) => (
         let ref $name: ExternWeak<unsafe extern "C" fn($($t),*) -> $ret> = {
             extern "C" {
                 #[linkage = "extern_weak"]
                 static $name: Option<unsafe extern "C" fn($($t),*) -> $ret>;
             }
             #[allow(unused_unsafe)]
             ExternWeak::new(unsafe { $name })
         };
     )
 }

 #[cfg(all(not(any(target_os = "macos", target_os = "ios")), bootstrap))]
 pub(crate) macro weak {
     (fn $name:ident($($t:ty),*) -> $ret:ty) => (
         let ref $name: ExternWeak<unsafe extern "C" fn($($t),*) -> $ret> = {
             extern "C" {
                 #[linkage = "extern_weak"]
                 static $name: *const libc::c_void;
             }
             #[allow(unused_unsafe)]
             ExternWeak::new(unsafe { $name })
         };
     )
 }

 // On non-ELF targets, use the dlsym approximation of weak linkage.
 #[cfg(any(target_os = "macos", target_os = "ios"))]
 pub(crate) use self::dlsym as weak;

 #[cfg(not(bootstrap))]
 pub(crate) struct ExternWeak<F: Copy> {
     weak_ptr: Option<F>,
 }

 #[cfg(not(bootstrap))]
 impl<F: Copy> ExternWeak<F> {
     #[inline]
     pub(crate) fn new(weak_ptr: Option<F>) -> Self {
         ExternWeak { weak_ptr }
     }

     #[inline]
     pub(crate) fn get(&self) -> Option<F> {
         self.weak_ptr
     }
 }

 #[cfg(bootstrap)]
 pub(crate) struct ExternWeak<F> {
     weak_ptr: *const libc::c_void,
     _marker: PhantomData<F>,
 }

 #[cfg(bootstrap)]
 impl<F> ExternWeak<F> {
     #[inline]
     pub(crate) fn new(weak_ptr: *const libc::c_void) -> Self {
         ExternWeak { weak_ptr, _marker: PhantomData }
     }
 }

 #[cfg(bootstrap)]
 impl<F> ExternWeak<F> {
     #[inline]
     pub(crate) fn get(&self) -> Option<F> {
         unsafe {
             if self.weak_ptr.is_null() {
                 None
             } else {
                 Some(mem::transmute_copy::<*const libc::c_void, F>(&self.weak_ptr))
             }
         }
     }
 }

 pub(crate) macro dlsym {
     (fn $name:ident($($t:ty),*) -> $ret:ty) => (
          dlsym!(fn $name($($t),*) -> $ret, stringify!($name));
     ),
     (fn $name:ident($($t:ty),*) -> $ret:ty, $sym:expr) => (
         static DLSYM: DlsymWeak<unsafe extern "C" fn($($t),*) -> $ret> =
             DlsymWeak::new(concat!($sym, '\0'));
         let $name = &DLSYM;
     )
 }
 pub(crate) struct DlsymWeak<F> {
     name: &'static str,
     func: AtomicPtr<libc::c_void>,
     _marker: PhantomData<F>,
 }

 impl<F> DlsymWeak<F> {
     pub(crate) const fn new(name: &'static str) -> Self {
         DlsymWeak { name, func: AtomicPtr::new(ptr::invalid_mut(1)), _marker: PhantomData }
     }

     #[inline]
     pub(crate) fn get(&self) -> Option<F> {
         unsafe {
             // Relaxed is fine here because we fence before reading through the
             // pointer (see the comment below).
             match self.func.load(Ordering::Relaxed) {
                 func if func.addr() == 1 => self.initialize(),
                 func if func.is_null() => None,
                 func => {
                     let func = mem::transmute_copy::<*mut libc::c_void, F>(&func);
                     // The caller is presumably going to read through this value
                     // (by calling the function we've dlsymed). This means we'd
                     // need to have loaded it with at least C11's consume
                     // ordering in order to be guaranteed that the data we read
                     // from the pointer isn't from before the pointer was
                     // stored. Rust has no equivalent to memory_order_consume,
                     // so we use an acquire fence (sorry, ARM).
                     //
                     // Now, in practice this likely isn't needed even on CPUs
                     // where relaxed and consume mean different things. The
                     // symbols we're loading are probably present (or not) at
                     // init, and even if they aren't the runtime dynamic loader
                     // is extremely likely have sufficient barriers internally
                     // (possibly implicitly, for example the ones provided by
                     // invoking `mprotect`).
                     //
                     // That said, none of that's *guaranteed*, and so we fence.
                     atomic::fence(Ordering::Acquire);
                     Some(func)
                 }
             }
         }
     }

     // Cold because it should only happen during first-time initialization.
     #[cold]
     unsafe fn initialize(&self) -> Option<F> {
         assert_eq!(mem::size_of::<F>(), mem::size_of::<*mut libc::c_void>());

         let val = fetch(self.name);
         // This synchronizes with the acquire fence in `get`.
         self.func.store(val, Ordering::Release);

         if val.is_null() { None } else { Some(mem::transmute_copy::<*mut libc::c_void, F>(&val)) }
     }
 }

 unsafe fn fetch(name: &str) -> *mut libc::c_void {
     let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
         Ok(cstr) => cstr,
         Err(..) => return ptr::null_mut(),
     };
     libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr())
 }

 #[cfg(not(any(target_os = "linux", target_os = "android")))]
 pub(crate) macro syscall {
     (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
         unsafe fn $name($($arg_name: $t),*) -> $ret {
             weak! { fn $name($($t),*) -> $ret }

             if let Some(fun) = $name.get() {
                 fun($($arg_name),*)
             } else {
                 super::os::set_errno(libc::ENOSYS);
                 -1
             }
         }
     )
 }

 #[cfg(any(target_os = "linux", target_os = "android"))]
 pub(crate) macro syscall {
     (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
         unsafe fn $name($($arg_name:$t),*) -> $ret {
             weak! { fn $name($($t),*) -> $ret }

             // Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
             // interposition, but if it's not found just use a raw syscall.
             if let Some(fun) = $name.get() {
                 fun($($arg_name),*)
             } else {
                 // This looks like a hack, but concat_idents only accepts idents
                 // (not paths).
                 use libc::*;

                 syscall(
                     concat_idents!(SYS_, $name),
                     $($arg_name),*
                 ) as $ret
             }
         }
     )
 }

 #[cfg(any(target_os = "linux", target_os = "android"))]
 pub(crate) macro raw_syscall {
     (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
         unsafe fn $name($($arg_name:$t),*) -> $ret {
             // This looks like a hack, but concat_idents only accepts idents
             // (not paths).
             use libc::*;

             syscall(
                 concat_idents!(SYS_, $name),
                 $($arg_name),*
             ) as $ret
         }
     )
 }
	//! Support for "weak linkage" to symbols on Unix
	//!
	//! Some I/O operations we do in libstd require newer versions of OSes but we
	//! need to maintain binary compatibility with older releases for now. In order
	//! to use the new functionality when available we use this module for
	//! detection.
	//!
	//! One option to use here is weak linkage, but that is unfortunately only
	//! really workable with ELF. Otherwise, use dlsym to get the symbol value at
	//! runtime. This is also done for compatibility with older versions of glibc,
	//! and to avoid creating dependencies on GLIBC_PRIVATE symbols. It assumes that
	//! we've been dynamically linked to the library the symbol comes from, but that
	//! is currently always the case for things like libpthread/libc.
	//!
	//! A long time ago this used weak linkage for the __pthread_get_minstack
	//! symbol, but that caused Debian to detect an unnecessarily strict versioned
	//! dependency on libc6 (#23628) because it is GLIBC_PRIVATE. We now use `dlsym`
	//! for a runtime lookup of that symbol to avoid the ELF versioned dependency.

	// There are a variety of `#[cfg]`s controlling which targets are involved in
	// each instance of `weak!` and `syscall!`. Rather than trying to unify all of
	// that, we'll just allow that some unix targets don't use this module at all.
	#![allow(dead_code, unused_macros)]

	use crate::ffi::CStr;
	use crate::marker::PhantomData;
	use crate::mem;
	use crate::ptr;
	use crate::sync::atomic::{self, AtomicPtr, Ordering};

	// We can use true weak linkage on ELF targets.
	#[cfg(all(not(any(target_os = "macos", target_os = "ios")), not(bootstrap)))]
	pub(crate) macro weak {
	(fn $name:ident($($t:ty),*) -> $ret:ty) => (
	let ref $name: ExternWeak<unsafe extern "C" fn($($t),*) -> $ret> = {
	extern "C" {
	#[linkage = "extern_weak"]
	static $name: Option<unsafe extern "C" fn($($t),*) -> $ret>;
	}
	#[allow(unused_unsafe)]
	ExternWeak::new(unsafe { $name })
	};
	)
	}

	#[cfg(all(not(any(target_os = "macos", target_os = "ios")), bootstrap))]
	pub(crate) macro weak {
	(fn $name:ident($($t:ty),*) -> $ret:ty) => (
	let ref $name: ExternWeak<unsafe extern "C" fn($($t),*) -> $ret> = {
	extern "C" {
	#[linkage = "extern_weak"]
	static $name: *const libc::c_void;
	}
	#[allow(unused_unsafe)]
	ExternWeak::new(unsafe { $name })
	};
	)
	}

	// On non-ELF targets, use the dlsym approximation of weak linkage.
	#[cfg(any(target_os = "macos", target_os = "ios"))]
	pub(crate) use self::dlsym as weak;

	#[cfg(not(bootstrap))]
	pub(crate) struct ExternWeak<F: Copy> {
	weak_ptr: Option<F>,
	}

	#[cfg(not(bootstrap))]
	impl<F: Copy> ExternWeak<F> {
	#[inline]
	pub(crate) fn new(weak_ptr: Option<F>) -> Self {
	ExternWeak { weak_ptr }
	}

	#[inline]
	pub(crate) fn get(&self) -> Option<F> {
	self.weak_ptr
	}
	}

	#[cfg(bootstrap)]
	pub(crate) struct ExternWeak<F> {
	weak_ptr: *const libc::c_void,
	_marker: PhantomData<F>,
	}

	#[cfg(bootstrap)]
	impl<F> ExternWeak<F> {
	#[inline]
	pub(crate) fn new(weak_ptr: *const libc::c_void) -> Self {
	ExternWeak { weak_ptr, _marker: PhantomData }
	}
	}

	#[cfg(bootstrap)]
	impl<F> ExternWeak<F> {
	#[inline]
	pub(crate) fn get(&self) -> Option<F> {
	unsafe {
	if self.weak_ptr.is_null() {
	None
	} else {
	Some(mem::transmute_copy::<*const libc::c_void, F>(&self.weak_ptr))
	}
	}
	}
	}

	pub(crate) macro dlsym {
	(fn $name:ident($($t:ty),*) -> $ret:ty) => (
	dlsym!(fn $name($($t),*) -> $ret, stringify!($name));
	),
	(fn $name:ident($($t:ty),*) -> $ret:ty, $sym:expr) => (
	static DLSYM: DlsymWeak<unsafe extern "C" fn($($t),*) -> $ret> =
	DlsymWeak::new(concat!($sym, '\0'));
	let $name = &DLSYM;
	)
	}
	pub(crate) struct DlsymWeak<F> {
	name: &'static str,
	func: AtomicPtr<libc::c_void>,
	_marker: PhantomData<F>,
	}

	impl<F> DlsymWeak<F> {
	pub(crate) const fn new(name: &'static str) -> Self {
	DlsymWeak { name, func: AtomicPtr::new(ptr::invalid_mut(1)), _marker: PhantomData }
	}

	#[inline]
	pub(crate) fn get(&self) -> Option<F> {
	unsafe {
	// Relaxed is fine here because we fence before reading through the
	// pointer (see the comment below).
	match self.func.load(Ordering::Relaxed) {
	func if func.addr() == 1 => self.initialize(),
	func if func.is_null() => None,
	func => {
	let func = mem::transmute_copy::<*mut libc::c_void, F>(&func);
	// The caller is presumably going to read through this value
	// (by calling the function we've dlsymed). This means we'd
	// need to have loaded it with at least C11's consume
	// ordering in order to be guaranteed that the data we read
	// from the pointer isn't from before the pointer was
	// stored. Rust has no equivalent to memory_order_consume,
	// so we use an acquire fence (sorry, ARM).
	//
	// Now, in practice this likely isn't needed even on CPUs
	// where relaxed and consume mean different things. The
	// symbols we're loading are probably present (or not) at
	// init, and even if they aren't the runtime dynamic loader
	// is extremely likely have sufficient barriers internally
	// (possibly implicitly, for example the ones provided by
	// invoking `mprotect`).
	//
	// That said, none of that's guaranteed, and so we fence.
	atomic::fence(Ordering::Acquire);
	Some(func)
	}
	}
	}
	}

	// Cold because it should only happen during first-time initialization.
	#[cold]
	unsafe fn initialize(&self) -> Option<F> {
	assert_eq!(mem::size_of::<F>(), mem::size_of::<*mut libc::c_void>());

	let val = fetch(self.name);
	// This synchronizes with the acquire fence in `get`.
	self.func.store(val, Ordering::Release);

	if val.is_null() { None } else { Some(mem::transmute_copy::<*mut libc::c_void, F>(&val)) }
	}
	}

	unsafe fn fetch(name: &str) -> *mut libc::c_void {
	let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
	Ok(cstr) => cstr,
	Err(..) => return ptr::null_mut(),
	};
	libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr())
	}

	#[cfg(not(any(target_os = "linux", target_os = "android")))]
	pub(crate) macro syscall {
	(fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
	unsafe fn $name($($arg_name: $t),*) -> $ret {
	weak! { fn $name($($t),*) -> $ret }

	if let Some(fun) = $name.get() {
	fun($($arg_name),*)
	} else {
	super::os::set_errno(libc::ENOSYS);
	-1
	}
	}
	)
	}

	#[cfg(any(target_os = "linux", target_os = "android"))]
	pub(crate) macro syscall {
	(fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
	unsafe fn $name($($arg_name:$t),*) -> $ret {
	weak! { fn $name($($t),*) -> $ret }

	// Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
	// interposition, but if it's not found just use a raw syscall.
	if let Some(fun) = $name.get() {
	fun($($arg_name),*)
	} else {
	// This looks like a hack, but concat_idents only accepts idents
	// (not paths).
	use libc::*;

	syscall(
	concat_idents!(SYS_, $name),
	$($arg_name),*
	) as $ret
	}
	}
	)
	}

	#[cfg(any(target_os = "linux", target_os = "android"))]
	pub(crate) macro raw_syscall {
	(fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
	unsafe fn $name($($arg_name:$t),*) -> $ret {
	// This looks like a hack, but concat_idents only accepts idents
	// (not paths).
	use libc::*;

	syscall(
	concat_idents!(SYS_, $name),
	$($arg_name),*
	) as $ret
	}
	)
	}