I'm fast getting to the opinion that upgrading the operating system is a bad idea, I previously had a working development system. iMAC (Retina 5K, 27-inch, Late 2015) Processor 4 GHz Quad-Core Intel Core i7 Memory 16 GB 1867 MHz DDR3 Graphics AMD Radeon R9 M395X 4 GB I've just tried to build a library that was previously ok, no changes to the source, now it will not build because unistd.h file not found. Other issues, unknown type name time_t My project is built using: Qt Creator 4.15.0-beta2 (4.14.83) Based on Qt 5.15.2 (Clang 11.0 (Apple), 64 bit) Built on Mar 24 2021 04:43:31 From revision 460d446c61 How can I fix this? Edit, I've just performed: brew update brew upgrade no different, same problem.