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Why would it matter what system programming languages are there? Unix can't run other languages?

koprulusector 5y ago
The kernel is written in C. Ok, why does that matter? Because its interfaces and architecture are tightly intertwined with C. Remember that C was partially developed to use in building Unix. So, kernel was written in C and therefore uses C data structures, pointers, and follows C conventions for using them (for example, kernel might return a struct of pointers). This applies to interfaces and structures, making syscalls (depending on which syscall and what is needed), checking return values or error codes. You don’t NEED this stuff exactly to run on Unix. What and how much you need, or even just convenience, are how it boils down. If you want to use eBPF to trace things from raw network packets, syscalls, file systems, you can do some in Python but you’ll need some degree of C. You want to run a Django or flask app in just Python? Good to go. [This article on Linux ABI](https://en.wikipedia.org/wiki/Application_binary_interface https://en.wikipedia.org/wiki/Application_binary_interface) might help explain better/in more detail. Sorry if this isn’t the best explanation, I’m on mobile and up too late.
- GoblinSlayer 5y ago
  Design of unix was influenced more by PDP-7 than by C. What unix does, it does PDP-7 stuff, not C stuff. C itself isn't limited to POSIX.
johannes1234321 5y ago
Unix can run other languages. But for answering your question deeper, we have to find an understanding what boundary we're talking about. The kernel syscall interface? Things like Linux's vdso? POSIX library (libc)? The more we go on in that sequence the more C we get into the interfaces and many of those APIs are essential for writing well-behaving non-trivial programs. The closer we stay to the kernel's syscall interface the less C you get, but still the syscall interface and C are related. Strings are often NULL-terminated. Arguemtns are passed (via registers) from left to right, not like in Pascal convention right to left. (First arg always in EBX, second in EXC etc.) A different language will need some interoperability with C conventions for being a good citizen on Unix. A Pascal operating system for instance would use Pascal strings (historically what tis now called ShortString) where length is encoded in the first byte.
- Joker_vD 5y ago
  > A Pascal operating system for instance would use Pascal strings Only if the designers would make that decision, and they are not in any way obliged to make such decision! Nobody stops them from using e.g. a moral equivalent of "RECORD size: CARDINAL; str_ptr: ^CHAR END" for passing strings in their system calls (which would probably coincide with using "size_t size, char *str_ptr" in the C prototype at the binary level), or something else entirely. Is the idea of an OS not exposing its implementation language in its public API that foreign so I have to express it three times in this thread and every time be replied with "No, what are you talking about, of course the OS would use reuse the structures of the runtime of the language that it is written in in its public syscall API"? Gee, I wonder how the syscall calling convention would look if I wrote an OS entirely in assembly.
  - johannes1234321 5y ago
    It's not what the implementation uses, but "which conventions does the call interface assume" you can use Pascal to create an operating system with a system interface that benefits Fortran. (I know too little about Fortran to describe what that would look like) With C and Unix the issue of course is that both were developed around the same time by the same people for working together.
- GoblinSlayer 5y ago
  Or it could be designed for interoperability like ODBC which was designed to be compatible with languages that have ISO standards: Ada, Fortran, C, COBOL, Pascal without tightly locking to either.
  - johannes1234321 5y ago
    What is cause, what is effect? "All" languages (which survived) can call into C.
    - GoblinSlayer 5y ago
      C can call into Pascal too. Does it mean libraries should use pascal strings in their interface?

The kernel is written in C. Ok, why does that matter? Because its interfaces and architecture are tightly intertwined with C. Remember that C was partially developed to use in building Unix. So, kernel was written in C and therefore uses C data structures, pointers, and follows C conventions for using them (for example, kernel might return a struct of pointers). This applies to interfaces and structures, making syscalls (depending on which syscall and what is needed), checking return values or error codes. You don’t NEED this stuff exactly to run on Unix. What and how much you need, or even just convenience, are how it boils down. If you want to use eBPF to trace things from raw network packets, syscalls, file systems, you can do some in Python but you’ll need some degree of C. You want to run a Django or flask app in just Python? Good to go. [This article on Linux ABI](https://en.wikipedia.org/wiki/Application_binary_interface https://en.wikipedia.org/wiki/Application_binary_interface) might help explain better/in more detail. Sorry if this isn’t the best explanation, I’m on mobile and up too late.

GoblinSlayer 5y ago
Design of unix was influenced more by PDP-7 than by C. What unix does, it does PDP-7 stuff, not C stuff. C itself isn't limited to POSIX.

johannes1234321 5y ago

Unix can run other languages. But for answering your question deeper, we have to find an understanding what boundary we're talking about. The kernel syscall interface? Things like Linux's vdso? POSIX library (libc)? The more we go on in that sequence the more C we get into the interfaces and many of those APIs are essential for writing well-behaving non-trivial programs. The closer we stay to the kernel's syscall interface the less C you get, but still the syscall interface and C are related. Strings are often NULL-terminated. Arguemtns are passed (via registers) from left to right, not like in Pascal convention right to left. (First arg always in EBX, second in EXC etc.) A different language will need some interoperability with C conventions for being a good citizen on Unix. A Pascal operating system for instance would use Pascal strings (historically what tis now called ShortString) where length is encoded in the first byte.

Joker_vD 5y ago
> A Pascal operating system for instance would use Pascal strings Only if the designers would make that decision, and they are not in any way obliged to make such decision! Nobody stops them from using e.g. a moral equivalent of "RECORD size: CARDINAL; str_ptr: ^CHAR END" for passing strings in their system calls (which would probably coincide with using "size_t size, char *str_ptr" in the C prototype at the binary level), or something else entirely. Is the idea of an OS not exposing its implementation language in its public API that foreign so I have to express it three times in this thread and every time be replied with "No, what are you talking about, of course the OS would use reuse the structures of the runtime of the language that it is written in in its public syscall API"? Gee, I wonder how the syscall calling convention would look if I wrote an OS entirely in assembly.
- johannes1234321 5y ago
  It's not what the implementation uses, but "which conventions does the call interface assume" you can use Pascal to create an operating system with a system interface that benefits Fortran. (I know too little about Fortran to describe what that would look like) With C and Unix the issue of course is that both were developed around the same time by the same people for working together.
GoblinSlayer 5y ago
Or it could be designed for interoperability like ODBC which was designed to be compatible with languages that have ISO standards: Ada, Fortran, C, COBOL, Pascal without tightly locking to either.
- johannes1234321 5y ago
  What is cause, what is effect? "All" languages (which survived) can call into C.
  - GoblinSlayer 5y ago
    C can call into Pascal too. Does it mean libraries should use pascal strings in their interface?