Link libatomic when required for 64-bit atomics

.github/workflows/unittest_linux_32bit.yml

@@ -0,0 +1,19 @@
+name: linux_32bit
+on:
+  push:
+    paths-ignore:
+    - 'docs/**'
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  linux_32bit:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - uses: seanmiddleditch/gha-setup-ninja@master
+    - name: install 32-bit toolchain
+      run: sudo apt-get update && sudo apt-get install -y g++-multilib pkg-config
+    - name: make test
+      run: make -f makefile.dev test_atomic_32

CMake/CheckAtomic.cmake

@@ -0,0 +1,30 @@
+# Some 32-bit targets need libatomic for the library's 64-bit counters.
+function(ixwebsocket_check_atomic)
+  include(CheckCXXSourceCompiles)
+  include(CMakePushCheckState)
+  cmake_push_check_state(RESET)
+  # A static-library try_compile would hide unresolved atomic runtime symbols.
+  set(CMAKE_TRY_COMPILE_TARGET_TYPE EXECUTABLE)
+  set(atomic_test_source "
+    #include <atomic>
+    #include <cstdint>
+    int main(int argc, char**) {
+      std::atomic<std::uint64_t> counter(0);
+      counter.fetch_add(static_cast<std::uint64_t>(argc));
+      return static_cast<int>(counter.load());
+    }
+  ")
+  check_cxx_source_compiles("${atomic_test_source}" IXWEBSOCKET_HAVE_BUILTIN_ATOMICS)
+  if (NOT IXWEBSOCKET_HAVE_BUILTIN_ATOMICS)
+    set(CMAKE_REQUIRED_LIBRARIES atomic)
+    check_cxx_source_compiles("${atomic_test_source}" IXWEBSOCKET_HAVE_LIBATOMIC)
+    if (NOT IXWEBSOCKET_HAVE_LIBATOMIC)
+      message(FATAL_ERROR "Unable to link 64-bit C++ atomics, even with libatomic. Install libatomic for the target toolchain.")
+    endif()
+    target_link_libraries(ixwebsocket PRIVATE atomic)
+    set(IXWEBSOCKET_PRIVATE_LIBS "-latomic" PARENT_SCOPE)
+  endif()
+  cmake_pop_check_state()
+endfunction()
+
+ixwebsocket_check_atomic()

CMakeLists.txt

@@ -196,6 +196,9 @@ else()
     )
 endif()
 
+set(IXWEBSOCKET_PRIVATE_LIBS "")
+include(CheckAtomic)
+
 if (USE_TLS)
     target_compile_definitions(ixwebsocket PUBLIC IXWEBSOCKET_USE_TLS)
     if (USE_NETWORK_FRAMEWORK)

docs/build.md

@@ -41,6 +41,12 @@ make install # will install to /usr/local on Unix, on macOS it is a good idea to
 Headers and a static library will be installed to the target dir.
 There is a unittest which can be executed by typing `make test`.
 
+CMake checks whether 64-bit atomics need `libatomic` and links it when required,
+including for consumers of the installed static library. Cross-compilation uses
+a compile-and-link check without running target executables. On Linux with GCC
+multilib installed, `make -f makefile.dev test_atomic_32` builds and runs the
+32-bit installed-consumer checks.
+
 Options for building:
 
 * `-DBUILD_SHARED_LIBS=ON` will build the unittest as a shared libary instead of a static library, which is the default

ixwebsocket.pc.in

@@ -7,5 +7,6 @@ Name: ixwebsocket
 Description: websocket and http client and server library, with TLS support and very few dependencies
 Version: @CMAKE_PROJECT_VERSION@
 Libs: -L${libdir} -lixwebsocket
+Libs.private: @IXWEBSOCKET_PRIVATE_LIBS@
 Cflags: -I${includedir}
 Requires: @requires@

makefile.dev

@@ -134,6 +134,17 @@ test:
 	(cd build ; ninja)
 	(cd build ; ninja -v test)
 
+.PHONY: test_atomic_32
+test_atomic_32:
+	cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug -DCMAKE_UNITY_BUILD=ON -DCMAKE_C_FLAGS="-m32 -march=i486" -DCMAKE_CXX_FLAGS="-m32 -march=i486" -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY -DUSE_TLS=OFF -DUSE_NETWORK_FRAMEWORK=OFF -DUSE_ZLIB=OFF -DUSE_TEST=OFF -DUSE_WS=OFF -DCMAKE_INSTALL_PREFIX="$(CURDIR)/build/atomic-install" -DCMAKE_INSTALL_LIBDIR=lib
+	cmake --build build --parallel
+	cmake --install build
+	cmake -S test/cmake/atomic -B build/atomic-consumer -DCMAKE_CXX_FLAGS="-m32 -march=i486" -DCMAKE_PREFIX_PATH="$(CURDIR)/build/atomic-install"
+	cmake --build build/atomic-consumer --parallel
+	./build/atomic-consumer/atomic_consumer
+	$(CXX) -std=c++11 -m32 -march=i486 test/cmake/atomic/main.cpp -o build/atomic-pkg-consumer $$(PKG_CONFIG_PATH="$(CURDIR)/build/atomic-install/lib/pkgconfig" pkg-config --cflags --libs --static ixwebsocket)
+	./build/atomic-pkg-consumer
+
 test_asan:
 	mkdir -p build && (cd build ; cmake -GNinja -DCMAKE_INSTALL_MESSAGE=LAZY -DCMAKE_UNITY_BUILD=ON -DCMAKE_BUILD_TYPE=Debug -DUSE_TLS=1 -DUSE_TEST=1 .. -DCMAKE_C_FLAGS="-fsanitize=address -fno-omit-frame-pointer" -DCMAKE_CXX_FLAGS="-fsanitize=address -fno-omit-frame-pointer")
 	(cd build ; ninja)

test/cmake/atomic/CMakeLists.txt

@@ -0,0 +1,12 @@
+cmake_minimum_required(VERSION 3.4.1...3.17.2)
+project(ixwebsocket_atomic_consumer LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
+# The installed target also carries the library's existing thread dependency.
+find_package(Threads REQUIRED)
+find_package(ixwebsocket CONFIG REQUIRED)
+add_executable(atomic_consumer main.cpp)
+target_link_libraries(atomic_consumer PRIVATE ixwebsocket::ixwebsocket)

test/cmake/atomic/main.cpp

@@ -0,0 +1,14 @@
+/*
+ *  Check that installed consumers can link the library's 64-bit atomic users.
+ */
+#include <ixwebsocket/IXConnectionState.h>
+#include <ixwebsocket/IXWebSocketTransport.h>
+
+int main()
+{
+    ix::ConnectionState first;
+    ix::ConnectionState second;
+    ix::WebSocketTransport transport;
+    return first.getId() == second.getId() ||
+           transport.getReadyState() != ix::WebSocketTransport::ReadyState::CLOSED;
+}