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//! 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 on Linux. Hence, 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).
// 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;
use crate::mem;
use crate::sync::atomic::{AtomicUsize, Ordering};
macro_rules! weak {
(fn $name:ident($($t:ty),*) -> $ret:ty) => (
static $name: crate::sys::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
crate::sys::weak::Weak::new(concat!(stringify!($name), '\0'));
)
}
pub struct Weak<F> {
name: &'static str,
addr: AtomicUsize,
_marker: marker::PhantomData<F>,
}
impl<F> Weak<F> {
pub const fn new(name: &'static str) -> Weak<F> {
Weak { name, addr: AtomicUsize::new(1), _marker: marker::PhantomData }
}
pub fn get(&self) -> Option<F> {
assert_eq!(mem::size_of::<F>(), mem::size_of::<usize>());
unsafe {
if self.addr.load(Ordering::SeqCst) == 1 {
self.addr.store(fetch(self.name), Ordering::SeqCst);
}
match self.addr.load(Ordering::SeqCst) {
0 => None,
addr => Some(mem::transmute_copy::<usize, F>(&addr)),
}
}
}
}
unsafe fn fetch(name: &str) -> usize {
let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
Ok(cstr) => cstr,
Err(..) => return 0,
};
libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr()) as usize
}
#[cfg(not(target_os = "linux"))]
macro_rules! syscall {
(fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
unsafe fn $name($($arg_name: $t),*) -> $ret {
use super::os;
weak! { fn $name($($t),*) -> $ret }
if let Some(fun) = $name.get() {
fun($($arg_name),*)
} else {
os::set_errno(libc::ENOSYS);
-1
}
}
)
}
#[cfg(target_os = "linux")]
macro_rules! 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 c_long),*
) as $ret
}
)
}
|
