Could someone explain this? I cannot find any information online
I put definitions here awhile ago, and mentioned the Actually Portable Executable project as an example of taking the ABI philosophy to an extreme, and ignoring APIs (source code).
https://www.oilshell.org/cross-ref.html#API
Links to this thread:
https://news.ycombinator.com/item?id=12029321
It's unfortunately not explained well in many places, and even experienced C programmers don't understand the details. You won't learn about this in a C programming book, because it's not really part of the C language proper. It sits at an awkward spot between the language, the CPU, and the operating system.
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The relation to Linus Torvalds is that historically Unix kernels were developed in a single tree and there was no stable ABI. For example I believe OpenBSD and NetBSD are still like this. But Linux is different in that the kernel maintained a stable ABI, and you can run different user utilities on top, without necessarily recompiling them against headers.
Highly related blog post and discussion: https://old.reddit.com/r/ProgrammingLanguages/comments/tg55o... (including comments by me/oilshell explaining the Unix kernel interface issue)
All i/o (network, filesystem, ipc), memory allocation, process interaction, signals, etc go through syscalls. open(), read() and write() are all syscalls.
If the syscall interface exists then the only thing left to do is to execute the program -- which depends on the cpu executing the machine instructions. If the machine instructions are for the wrong cpu then emulation may come into play.