An oft repeated fable is that open source software is portable, all thanks to C and Unix. The reality is that open source lives in an environment that is evolving to become a monoculture that does not require portability, this is being driven by the law of the jungle.
First some background and history. Portability requires that source code have the same behavior on different platforms, or rather than programs built from that code have the same behavior, this requires that:
- all compilers assign the same semantics to a given piece of code,
- all operating systems include support the same set of libraries in the same way.
If you want portability across lots of compilers then Fortran has stood head and shoulders above the competition for decades. Now some C folk may point out that they have been compiling some large code base for decades, with few changes necessitated by compiler differences; yes this has been possible in certain niche markets where there is a dominant supplier who has a vested interest in not breaking customer code. C has a long history of widespread large variation in behavior of across compilers.
What about Cobol you ask? Cobol is all about data manipulation and unless you have data in the format expected by a Cobol program you have no need for that program. Nobody cares about portable Cobol programs unless they are also interested in portable data.
If you want portability across lots of operating systems the solution has always been to minimise the dependency on system/third-party library calls (to the extent of including source code for functionality often supported by an OS). The reason for minimising OS dependency is the huge variation in support for different libraries and a wide range of behaviors for supposedly the same functionality. But you say, Unix is an OS that did/does provide a common set of libraries that have the same behavior; no, this is history seen through rose tinted glasses as anyone who knows about the Unix wars will tell you.
In the last century, to experience ‘portability’ Unix developers had to live in a monoculture of either PDP-11s or Sun workstations.
Open source, as it existed in the 1970s, 80s and into the 90’s was Fortran code that ran on a surprisingly wide range of OS/cpu/compilers, along with a smattering of other languages. Back then there were not many software applications and when they did exist many were written in Fortran (Oracle being an early, lots of Fortran, example), this created a strong incentive for vendors to support a Fortran compiler that did things the same way as everybody else (which did not prevent them adding proprietary goodies to try and lure customers towards lock-in).
How did we get to today’s dominance of C and Unix? Easy, evolution at a rate that caused competitors to die out until there was a last man standing. That last man standing was gcc and Linux. The portability problem has been solved by removing the need to port code; it is compiled by the same compiler to run on the same cpu (Intel x86 family) to run under the same OS (Linux).
Of course some of today’s open source C is compiled using non-gcc compilers, but the percentage is small and specialised (a lot of the older code is portable because it used to exist in a multi-compiler/cpu/OS world and had to evolve into being portable). The gcc competitor, llvm, is working long and hard to ensure compatibility and somehow has to differentiate itself while being compatible, a tough fight for developer hearts and minds.
Differences in CPU characteristics are a big headache to any compiler writer wanting to support identical behavior across platforms; having a single cpu family as the market leader more or less solves this problem. ARM has become a major player in the CPU world, but it shares many developer visible characteristics with Intel x86 (e.g., 32-bit int, 64-bit long, pointer and ints are the same size and IEEE floating-point) and options are available for handling some of the other potential differences (e.g., right shift of signed integers).
The Unix wars have not gone away, they have moved to more far flung battlefields leaving behind some hard fought over common ground. Anybody who wants to see the scares left by these war only needs to look at the
#ifs in system headers or the parameters selected inside
Having everybody use the same compiler/cpu/OS saves having to make a huge time/money investment in making software portable, at least until the invention of photonic computers or the arrival of aliens (whose computers are unlikely to contain a CPU that shares Intel/ARM characteristics or have the same libraries as Linux).