Hopefully this is a simple problem... This is a brand new install of the Xcode 12 command line tools (17 Sept 2020) after an upgrade to Catalina (10.15.6). I already tried several other fixes which people have posted, I get 190 warnings on a simple C compile of #include <stdio.h> #include <stdlib.h> int main(int argc, char **argv) {     printf("FOOOO!\n");     exit(0); } ... /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/stdlib.h:356:13: note: insert '_Nonnull' if the pointer should never       be null extern char *suboptarg;         /* getsubopt(3) external variable */             ^               _Nonnull  190 warnings generated.