fix(android/aarch64): use `libc::ucontext_t` from Android NDK by StackOverflowExcept1on · Pull Request #5189 · rust-lang/libc

StackOverflowExcept1on · 2026-06-21T10:53:41Z

Resolves #5159

https://android.googlesource.com/platform/bionic/+/refs/tags/ndk-r29/libc/include/sys/ucontext.h#102

It would be good to backport this to libc-0.2

r? @tgross35

rustbot · 2026-06-21T10:53:44Z

Some changes occurred in an Android module

cc @maurer

teor2345 · 2026-06-25T12:41:58Z

+        pub uc_sigmask__c_anonymous_union: __c_anonymous_uc_sigmask,
+        /* The kernel adds extra padding after uc_sigmask to match
+         * glibc sigset_t on ARM64. */
+        __padding: Padding<[c_char; 128 - size_of::<crate::sigset_t>()]>,


Why is this subtracting sigset_t (32 bit?), but the comment refers to ARM64?

This calculation seems weird for either 32-bit or 64-bit, and might need further comments:

the union is the size of sigset64_t (assuming it's bigger) but we subtract sigset_t (the 32-bit size) from the padding

View changes since the review

https://android.googlesource.com/platform/bionic/+/refs/tags/ndk-r29/libc/include/sys/ucontext.h#102

teor2345

I'm not an expert here, but I have coded a lot of C, and some parts of this seem weird without further context

View changes since this review

teor2345 · 2026-06-25T12:44:44Z

+    if #[cfg(feature = "extra_traits")] {
+        impl PartialEq for __c_anonymous_uc_sigmask {
+            fn eq(&self, other: &__c_anonymous_uc_sigmask) -> bool {
+                unsafe { self.uc_sigmask == other.uc_sigmask }


Is this sound and correct if the 64-bit sigmask was initialised in the union?

if the 64-bit sigmask is larger and has no padding, then the equality check misses some bytes

if the 32-bit sigmask is larger (unlikely), then we're comparing uninitialised bytes

if there's padding, then does it need to be ignored?

teor2345 · 2026-06-25T12:45:06Z

+        impl Eq for __c_anonymous_uc_sigmask {}
+        impl hash::Hash for __c_anonymous_uc_sigmask {
+            fn hash<H: hash::Hasher>(&self, state: &mut H) {
+                unsafe { self.uc_sigmask.hash(state) }


Same here, not sure we can do this with unions, without some guarantee they are the same size.

This code was simply borrowed from the ARM module:

libc/src/unix/linux_like/android/b32/arm.rs

Lines 59 to 73 in a8e2958

cfg_if! {

if #[cfg(feature = "extra_traits")] {

impl PartialEq for __c_anonymous_uc_sigmask {

fn eq(&self, other: &__c_anonymous_uc_sigmask) -> bool {

unsafe { self.uc_sigmask == other.uc_sigmask }

}

}

impl Eq for __c_anonymous_uc_sigmask {}

impl hash::Hash for __c_anonymous_uc_sigmask {

fn hash<H: hash::Hasher>(&self, state: &mut H) {

unsafe { self.uc_sigmask.hash(state) }

}

}

}

}

x86_64 has same layout:

libc/src/unix/linux_like/android/b64/x86_64/mod.rs

Lines 170 to 184 in a8e2958

cfg_if! {

if #[cfg(feature = "extra_traits")] {

impl PartialEq for __c_anonymous_uc_sigmask {

fn eq(&self, other: &__c_anonymous_uc_sigmask) -> bool {

unsafe { self.uc_sigmask == other.uc_sigmask }

}

}

impl Eq for __c_anonymous_uc_sigmask {}

impl hash::Hash for __c_anonymous_uc_sigmask {

fn hash<H: hash::Hasher>(&self, state: &mut H) {

unsafe { self.uc_sigmask.hash(state) }

}

}

}

}

StackOverflowExcept1on · 2026-06-25T13:08:26Z

@teor2345 I created test repository demonstrating that the offsets are correct (Android NDK C++). It also has passed CI.

#include <ucontext.h>

static_assert(offsetof(ucontext_t, uc_flags) == 0);
static_assert(offsetof(ucontext_t, uc_link) == 8);
static_assert(offsetof(ucontext_t, uc_stack) == 16);
static_assert(offsetof(ucontext_t, uc_sigmask) == 40);
static_assert(offsetof(ucontext_t, uc_mcontext) == 176);

static_assert(sizeof(ucontext_t) == 4560);

git clone https://github.com/StackOverflowExcept1on/library-android.git
cd library-android
PATH_TO_NDK=~/Android/Sdk/ndk/27.2.12479018
./build_android_arm64_v8a.sh $PATH_TO_NDK

+ '[' -z /home/.../Android/Sdk/ndk/27.2.12479018 ']'
+ CMAKE_BUILD_TYPE=Release
+ PATH_TO_NDK=/home/.../Android/Sdk/ndk/27.2.12479018
+ mkdir -p build
+ cd build
+ cmake -DCMAKE_TOOLCHAIN_FILE=/home/.../Android/Sdk/ndk/27.2.12479018/build/cmake/android.toolchain.cmake -DANDROID_ABI=arm64-v8a -DANDROID_PLATFORM=35 -DANDROID_STL=c++_shared -DANDROID_USE_LEGACY_TOOLCHAIN_FILE=OFF -DCMAKE_BUILD_TYPE=Release ..
-- Android: Targeting API '35' with architecture 'arm64', ABI 'arm64-v8a', and processor 'aarch64'
-- Android: Selected unified Clang toolchain
-- The C compiler identification is Clang 18.0.3
-- The CXX compiler identification is Clang 18.0.3
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /home/.../Android/Sdk/ndk/27.2.12479018/toolchains/llvm/prebuilt/linux-x86_64/bin/clang - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /home/.../Android/Sdk/ndk/27.2.12479018/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done (0.9s)
-- Generating done (0.0s)
-- Build files have been written to: /mnt/tmpfs/library-android/build
+ cmake --build .
[ 50%] Building CXX object CMakeFiles/library.dir/library.cpp.o
[100%] Linking CXX shared library liblibrary.so
[100%] Built target library
+ cmake --install .
-- Install configuration: "Release"
-- Installing: /mnt/tmpfs/library-android/build/bin/liblibrary.so

I also cloned my branch with the patch in rust-lang/libc and built it on my phone in Termux.

cargo new app
cd app
cargo add libc --git https://github.com/StackOverflowExcept1on/libc.git --branch fix-android

src/main.rs

fn main() {
    const _: () = {
        assert!(std::mem::offset_of!(libc::ucontext_t, uc_flags) == 0);
        assert!(std::mem::offset_of!(libc::ucontext_t, uc_link) == 8);
        assert!(std::mem::offset_of!(libc::ucontext_t, uc_stack) == 16);
        assert!(std::mem::offset_of!(libc::ucontext_t, uc_sigmask__c_anonymous_union) == 40);
        assert!(std::mem::offset_of!(libc::ucontext_t, uc_mcontext) == 176);

        assert!(std::mem::size_of::<libc::ucontext_t>() == 4560);
    };
}

rustc --version --verbose
rustc 1.96.0 (ac68faa20 2026-05-25) (built from a source tarball)
binary: rustc
commit-hash: ac68faa20c58cbccd01ee7208bf3b6e93a7d7f96
commit-date: 2026-05-25
host: aarch64-linux-android
release: 1.96.0
LLVM version: 21.1.8
cargo build --release
   Compiling libc v1.0.0-alpha.3 (https://github.com/StackOverflowExcept1on/libc.git?branch=fix-android#f674b086)
   Compiling app v0.1.0 (/data/data/com.termux/files/home/app)
    Finished `release` profile [optimized] target(s) in 2.41s

rustbot added the S-waiting-on-review label Jun 21, 2026

rustbot assigned tgross35 Jun 21, 2026

fix(android/aarch64): use libc::ucontext_t from Android NDK

f674b08

StackOverflowExcept1on force-pushed the fix-android branch from 43ad692 to f674b08 Compare June 21, 2026 11:17

StackOverflowExcept1on mentioned this pull request Jun 23, 2026

refactor(protocol/lazy-pages): improve #[cfg(...)] attributes, add support Android x86_64 gear-tech/gear#5599

Merged

teor2345 reviewed Jun 25, 2026

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fix(android/aarch64): use `libc::ucontext_t` from Android NDK#5189

fix(android/aarch64): use `libc::ucontext_t` from Android NDK#5189
StackOverflowExcept1on wants to merge 1 commit into
rust-lang:mainfrom
StackOverflowExcept1on:fix-android

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	cfg_if! {
	if #[cfg(feature = "extra_traits")] {
	impl PartialEq for __c_anonymous_uc_sigmask {
	fn eq(&self, other: &__c_anonymous_uc_sigmask) -> bool {
	unsafe { self.uc_sigmask == other.uc_sigmask }
	}
	}
	impl Eq for __c_anonymous_uc_sigmask {}
	impl hash::Hash for __c_anonymous_uc_sigmask {
	fn hash<H: hash::Hasher>(&self, state: &mut H) {
	unsafe { self.uc_sigmask.hash(state) }
	}
	}
	}
	}

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