Windows, macOS, Linux, and BSD all (usually) run on Intel processors, so in theory it should be possible to write a program that runs on all three. However, despite ultimately using the same instruction sets, the four OSes use different formats to store metadata about the programs, and have different ways for programs to communicate with the operating system. Justine Tunney created two things:
1) Actually Portable Executables, a clever way of formatting a program so that all four OSes interpret it a valid program in their own format.
2) Cosmopolitan libc, a library for communicating with the OS that handles each OS's interface, allowing programs to work on all four.
1) Actually Portable Executables, a clever way of formatting a program so that all four OSes interpret it a valid program in their own format.
2) Cosmopolitan libc, a library for communicating with the OS that handles each OS's interface, allowing programs to work on all four.