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https://en.cppreference.com/w/cpp/utility/feature_test.html
Library feature-test macros (since C++20) - cppreference.com
feature test macros20 cppreferencelibrarysince
https://lwn.net/Articles/590381/
Glibc feature test macros [LWN.net]
As evidenced by the frequency of misreported bugs to the Linux man-pages project, feature test [...]
feature test macrosglibclwn
https://man7.org/linux/man-pages/man7/feature_test_macros.7.html
feature_test_macros(7) - Linux manual page
feature test macros7 linux manual