CMake CheckCSourceCompiles传递预定义给armlink及thread_local测试失败求助

I ran into exactly this issue with armcc and CMake's CHECK_C_SOURCE_COMPILES macro recently, so I know how frustrating it is when the code compiles fine manually but fails in CMake's test. Let's break down what's happening and how to fix it.

The Root Cause #

When you use CHECK_C_SOURCE_COMPILES, CMake automatically passes the test's preprocessor definition (in your case, -DGCC_THREAD_LOCAL_AVAILABLE) to both the compiler (armcc) and the linker (armlink). The problem is that armlink doesn't recognize the -D flag—it's a compiler-only option. Even though your code compiles without errors, the link step fails because armlink chokes on the unexpected -D argument, causing the entire test to report failure.

The Solution: Use try_compile Instead #

The CHECK_C_SOURCE_COMPILES macro is convenient but inflexible here. Instead, use CMake's try_compile command, which lets you explicitly control which flags go to the compiler vs. the linker, avoiding the accidental -D pass-through to armlink.

Here's a working example tailored to your use case:

# Test code to verify __thread support
set(THREAD_LOCAL_TEST_SNIPPET "int main(){ static __thread int num; return 0;}")

# Run the compile test with explicit control over flags
try_compile(
    GCC_THREAD_LOCAL_AVAILABLE
    ${CMAKE_BINARY_DIR}/thread_local_test_build_dir  # Temp build dir for the test
    SOURCES <STRING>${THREAD_LOCAL_TEST_SNIPPET}     # Inline test code
    CMAKE_FLAGS 
        "-DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}"     # Use your armcc compiler
        "-DCMAKE_C_FLAGS=${CMAKE_C_FLAGS}"           # Reuse your existing C flags
    OUTPUT_VARIABLE TEST_COMPILE_OUTPUT              # Capture output for debugging
)

# Report the result
if(GCC_THREAD_LOCAL_AVAILABLE)
    message(STATUS "✅ armcc supports __thread (thread_local)")
else()
    message(STATUS "❌ armcc does NOT support __thread. Debug output:\n${TEST_COMPILE_OUTPUT}")
endif()

Why This Works #

• try_compile doesn't automatically inject the test result's preprocessor definition into the link command. It only uses the flags you explicitly specify.
• By using <STRING> for the source, you avoid creating temporary files (though you could also use a separate .c file if preferred).
• The OUTPUT_VARIABLE lets you debug exactly what went wrong if the test fails, which is helpful for tweaking flags further if needed.

Alternative: Patch CHECK_C_SOURCE_COMPILES Behavior (Less Recommended) #

If you really want to stick with CHECK_C_SOURCE_COMPILES, you can override how CMake handles link flags for armcc. Add this before calling the macro:

# Tell CMake not to pass compiler defines to armlink
set(CMAKE_REQUIRED_LINK_OPTIONS "")
set(CMAKE_C_LINK_FLAGS "")

But this can have side effects on other tests, so try_compile is the cleaner, more reliable approach.

内容的提问来源于stack exchange，提问作者g-217