CMake has a note at check_function_exists help page.
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Prefer using CheckSymbolExists instead of this module, for the following reasons:
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check_function_exists()can’t detect functions that are inlined in headers or specified as a macro. -
check_function_exists()can’t detect anything in the 32-bit versions of the Win32 API, because of a mismatch in calling conventions. -
check_function_exists()only verifies linking, it does not verify that the function is declared in system headers.
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futimens neither inlined nor a macros nor a part of Win32 API.
What I’m worried about is a last point.
What if a function isn’t declared but it exists in previous versions of macOS?
It makes sense why check_function_exists confirms existence of futimens.