Re: pg_preadv() and pg_pwritev()

>> The symptoms sound consistent with using bleeding-edge Xcode on a
>> Catalina machine ... please report exact OS and Xcode versions.
> 
> macOS 10.15.7 (19H2)
> Xcode 12.3 (12C33)
> macOS SDK 11.1 (20C63)
> 

Everything is fine if I run "configure" with
PG_SYSROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk

I noticed that "cc" invoked from command line uses:
-isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk

But "xcodebuild -version -sdk macosx Path" invoked by "configure" when 
PG_SYSROOT is not provided gives:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk

Now I'm confused about different SDK versions and locations used by 
Xcode and CommandLineTools :)