[libc] Fix warning on 'extern "C" int main' in test suite

[jhuber6]

Summary:
According to the C++ standard, The main function shall not be declared
with a linkage-specification. after some changes in
#101853 this started emitting
warnings when building / testing the C library. This source file is
shared with the overlay tests as well as the full build tests. The full
build tests are compiled with -ffreestanding, as are all the startup /
integration files. The standard says freestanding environment are all
implementation defined, so this is valid in those cases. This patch
simply prevents adding the linkage when we are compiling unit tests,
which are hosted. This is a continuation on
#102825.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Joseph Huber (jhuber6)

Changes

Summary:
According to the C++ standard, The main function shall not be declared
with a linkage-specification. after some changes in
#101853 this started emitting
warnings when building / testing the C library. This source file is
shared with the overlay tests as well as the full build tests. The full
build tests are compiled with -ffreestanding, as are all the startup /
integration files. The standard says freestanding environment are all
implementation defined, so this is valid in those cases. This patch
simply prevents adding the linkage when we are compiling unit tests,
which are hosted. This is a continuation on
#102825.

---

Full diff: https://github.com/llvm/llvm-project/pull/102973.diff

2 Files Affected:

• (modified) libc/test/UnitTest/CMakeLists.txt (+1)
• (modified) libc/test/UnitTest/LibcTestMain.cpp (+9-1)

diff --git a/libc/test/UnitTest/CMakeLists.txt b/libc/test/UnitTest/CMakeLists.txt
index 6427b861580777..9e9fc8153769cf 100644
--- a/libc/test/UnitTest/CMakeLists.txt
+++ b/libc/test/UnitTest/CMakeLists.txt
@@ -32,6 +32,7 @@ function(add_unittest_framework_library name)
   _get_hermetic_test_compile_options(compile_options -nostdinc++)
   target_include_directories(${name}.hermetic PRIVATE ${LIBC_BUILD_DIR}/include)
   target_compile_options(${name}.hermetic PRIVATE ${compile_options})
+  target_compile_definitions(${name}.hermetic PRIVATE LIBC_HERMETIC_TEST)
 
   if(TEST_LIB_DEPENDS)
     foreach(dep IN ITEMS ${TEST_LIB_DEPENDS})
diff --git a/libc/test/UnitTest/LibcTestMain.cpp b/libc/test/UnitTest/LibcTestMain.cpp
index 94536e97164686..ece5639769d215 100644
--- a/libc/test/UnitTest/LibcTestMain.cpp
+++ b/libc/test/UnitTest/LibcTestMain.cpp
@@ -43,7 +43,15 @@ TestOptions parseOptions(int argc, char **argv) {
 
 } // anonymous namespace
 
-extern "C" int main(int argc, char **argv, char **envp) {
+// The C++ standard forbids declaring the main function with a linkage specifier
+// outisde of 'freestanding' mode, only define the linkage for hermetic tests.
+#ifdef LIBC_HERMETIC_TEST
+#define TEST_MAIN extern "C" int main
+#else
+#define TEST_MAIN int main
+#endif
+
+TEST_MAIN(int argc, char **argv, char **envp) {
   LIBC_NAMESPACE::testing::argc = argc;
   LIBC_NAMESPACE::testing::argv = argv;
   LIBC_NAMESPACE::testing::envp = envp;

[SchrodingerZhu]

I think clang should fix this instead of libc.

[jhuber6]

I think clang should fix this instead of libc.

clang is doing its job, according to the standard extern "C" int main is not allowed. During -ffreestanding mode it is implementation defined and thus allowed, that's why it's not a warning in that case. This requires special handling because we compile LibCTestMain in both.

[lntue]

Do you need to explicitly check if it's not 0? Look like it's always defined.

$ diff <(clang++ -dM -E - < /dev/null) <(clang++ -dM -E -ffreestanding - < /dev/null)
86d85
< #define __GCC_HAVE_DWARF2_CFI_ASM 1
243c242
< #define __STDC_HOSTED__ 1
---
> #define __STDC_HOSTED__ 0

[jhuber6]

You're right, just noticed once I got undefined references in the GPU tests. Just checking the value is how the clang headers do it.