The help for erfl on my linux box says this :-

   Feature Test Macro Requirements for glibc (see feature_test_macros(
+7)):

       erf():
           _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE || _ISOC99_SOU
+RCE || _POSIX_C_SOURCE >= 200112L;
           or cc -std=c99
       erff(), erfl():
           _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISO
+C99_SOURCE || _POSIX_C_SOURCE >= 200112L;
           or cc -std=c99

[download]

so feature_test_macros is where it's at :)

In reply to Re: [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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