The man page for feature_test_macros has lots of details on how to use these, and the info page has even more.

But the easiest way is to set the compiler command line option -std=c99 to ensure that everything (compiler/libc) complies with the C99 standard.

In reply to Re^3: [C Question] Determine if gcc provides erfl() function by RichardK
in thread [C Question] Determine if gcc provides erfl() function by syphilis

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