I keep seeing this come up and every time I look it up I never get a good explanation of what it is or what it means to me.
What is POSIX compliance? How does assuming my program will only be run on POSIX-compliant machines simplify things for me as a programmer? Does it even?
POSIX defines a set of C headers, System Interfaces, a Shell Command Language and Utilities, that a conforming system must implement.
As a developer, you can rely on these standard interfaces being available on every POSIX system. If your program uses the standard interfaces, it can operate on all POSIX systems.
Porting across non-standard systems is more work, as the system interfaces e.g. for multithreading or networking are different.
Related
I was reading "The Art of Unix Programming" book and I found a quote saying :
In the Unix world, libraries which are delivered as libraries should come with exerciser programs.
So what exactly is a library exerciser?
A library exerciser is simply a program, or a collection of programs, that is testing that library (by calling some, or most, and ideally all public functions or methods of that library). Read also about unit testing.
BTW, this advice to make library exercisers is IMHO not specific to the Unix world (it should also hold for GNU Hurd, POSIX, VMS and even Windows systems), but generally useful for any software library. I guess it is related to modules and names in linkers. In some exotic, but interesting, programming environments (think of Lisp or Smalltalk machines, or persistent academic OSes like Grasshopper) the very notion of library does not exist, or is so far from Linux-like libraries (written in C or C++) that exercisers might not mean the same thing... ....
Notice that some languages (Ocaml, Go, D, ... but not C11 or C++14) might know some notion of modules and have a module-aware notion of libraries
Which programming languages provide the best support for self-modifying code?
In particular, since the program will need to make extensive use of self-modifying code, I am looking forward at the ability to remove from memory some parts of code, after they are no longer needed, thus freeing that memory. Also, it would be a plus if there was the ability to identify and index the routines (procedures, functions, etc) with some sort of serial number, so that they could be easily managed in the memory (deleted, cloned etc) at runtime.
Operating systems need to have some more-or-less "self-modifying code" in order to load programs and dynamic link libraries from storage into RAM and later free up that RAM for other things, do relocation fix-ups, etc.
My understanding is that currently the C programming language is by far the most popular language for writing an operating systems.
The OSDev.org wiki has many tips of writing a new custom operating system, including a brief discussion of languages suitable for writing an operating system -- C, Assembly language, Lisp, Forth, C++, C#, PL/1, etc.
Just-in-time (JIT) compilers also need to have some more-or-less "self-modifying code" to compile source text into native instructions and run them, then later free up that memory for the next hot-spot.
Perhaps you could find some OS project or JIT project and use their code with relatively little modification.
A few people, when they say they want "self-modifying code", really want a language that supports homoiconicity such Scheme or some other dialect of Lisp, Prolog, TCL, Curl, etc.
My Professor is going to correct our Operating Systems final project in a Mac OS and in a Linux. So we should use only Unix standard system calls. I searched for a list with all standard system calls, but I'm not sure if any of the sites I found is truly reliable. Does anybody knows a good site for Unix standard system calls?
The only system call I used in my project that I'm not sure if it is Unix standard, is waitpid(). Is it standard?
The waitpid() function is specified by the POSIX standard, which is about close as we can get to a standard for UNIX.
References:
POSIX
waitpid()
Is there a standard that defines the syntax and semantics of assembly language? Similarly as language C has ISO standard and language C# has ECMA standard? Is there only one standard, or are there more of them?
I'm asking because I noticed that assembly language code looked different on Windows and Linux environment. I hoped that assembly language is not dependent on OS, that it's only language with some defined standard and via assembler (compiler of assembly language) is translated into machine instructions for particular processor.
thank you for answer
Yes, there is a standard.
People that built assemblers even up til the 1980s chose an incredible variety of syntax schemes.
The IEEE community reacted with a standard to try to avoid that problem:
694-1985 - IEEE Standard for Microprocessor Assembly Language
As with many things in the software world, it was and continues to be largely ignored.
The closest thing to a standard is that the vendor that created the processor/instruction set will have a document describing that language and often that vendor will provide some sort of an assembler (program). Some vendors are more detail and standard oriented than others so you get what you get. Then things like this intel/at&t happen to mess things up. Add to that gnu assembler loves to mess up the assembly language for the chips it supports as well so in general you have chaos.
If there were an assembly language whose use were comparable to C or C++ then you would expect an organization to try to come up with a standard. Part of the problem would still be that with things like the C language there is an interpretation before it hits the hardware, with assembler there is none to very little so a chip vendor is going to make whatever they want to make due to market factors and the standard would have to be dragged along to match the hardware, instead of the other way around where a standard drives the vendors.
The opencore processor might be one that could be standards driven since it is not vendor specific, perhaps it is already.
With assembly assume that each version of each assembler program/software/tool has its own syntax rules within the same instruction set as well as across different instruction sets. (which is actually what you get with C/C++ but that is another topic) either choose your favorite tool and only know it, or try to memorize all the variations across all the tools, or my preference is to try to avoid as many tool specific syntax and nuances, and try to find the middle ground that works or at least has a chance to work or port across tools.
No, there is no standard.
There are even two different types of syntax: the intel-syntax which is predominant on Windows plattforms and the AT&T-sytanx which is dominant in the *nix-world.
Regarding the differently looking code in the wikipedia: the windows example uses the Win32API and the linux example uses a system call of the 0x80 interrupt.
Assembly languages differ from processor to processor so no, there is no standard.
In general, the "standard" assembly language for a particular family of processor is whatever the processor designers say it is. For example, the "standard" syntax for x86 is whatever Intel says it is. However, that doesn't prevent other people from creating a variant of the assembly language that targets the processor with slightly different syntax or additional features (Nasm is one example).
Well, I'm not sure if you are asking about syntax for x86 processors (I suppose yes, because you're mentioning NASM).
But there are two common standards:
Intel syntax that was originally used for documentation of the x86 platform
AT&T syntax which is common in Linux/Unix worlds.
NASM you have mentioned prefers the Intel syntax.
You can find some examples of the syntax differences in this article: http://www.ibm.com/developerworks/linux/library/l-gas-nasm/index.html.
There's none because there are many different CPUs with different instructions and other peculiarities and it's entirely up to their designer what syntax to use and how to name things. And there's little need to standardize that because assembly code is inherently unportable and needs to be rewritten for a different CPU anyway.
Assembly language is not OS-specific per se, it's CPU-specific, but for an assembly routine to access things that appear standard to you (e.g. some subroutine to print text in the console) OS-specific code is needed. For MSDOS you'd use BIOS and DOS interrupt service routines (invokable on the x86 CPU through int 13h, int 10h, int 21h, int 33h, etc instructions), for Windows you'd use Windows' (available through int 2eh and sysenter/syscall instructions), for Linux you'd use Linux' (e.g. int 80h). All of them are implemented differently in different OSes and expect different number and kinds of parameters and in different places (registers or memory). You can't standardize this part. The only thing you can do about it is build a compatibility/abstraction layer on top of the OS functionality so it looks the same from your assembly routines' point of view.
Assembly syntax / language depends on CPU rather then OS. For the x86 CPU family there are however two syntax's AT&T (used by Unix like operating systems by default) and Intel (used by Windows and DOS etc.)
However the two assembly examples on the wiki are both doing different things. The windows example uses the WIN32 API and to show a message box, so all function arguments are pushed onto the stack in reversed order and then calls the function MessageBox() which on his turn creates the messagebox.
The linux example uses the write syscall to write a string to stdout. Here all 'arguments' are stored in the registers and then the int 0x80 creates an 'interrupt' now the OS is entering kernel land and the kernel prints the string to stdout.
The linux assemly could be rewritten like:
section .data
msg: db "Hello, world!", 10
.len: equ $ - msg
section .text
extern write
extern exit
global _start
_start:
push msg.len
push msg
push dword 1
call write
push dword 0
call exit
The above assembly must be linked against libc and then this will call write in libc which on his turn executes exactly the same code as the example on the wiki.
Another thing to note, is that Windows and Unix like operating system use different file formats in there libraries and applications.
Unix like systems use ELF http://en.wikipedia.org/wiki/Executable_and_Linkable_Format and windows uses PE http://en.wikipedia.org/wiki/Portable_Executable
This is why you see different sections in the assemblies on the wiki page.
I would want to compile existing software into presentation that can later be run on different architectures (and OS).
For that I need a (byte)code that can be easily run/emulated on another arch/OS (LLVM IR? Some RISC assemby?)
Some random ideas:
Compiling into JVM bytecode and running with java. Too restricting? C-compilers available?
MS CIL. C-Compilers available?
LLVM? Can Intermediate representation be run later?
Compiling into RISC arch such as MMIX. What about system calls?
Then there is the system call mapping thing, but e.g. BSD have system call translation layers.
Are there any already working systems that compile C/C++ into something that can later be run with an interpreter on another architecture?
Edit
Could I compile existing unix software into not-so-lowlevel binary, which could be "emulated" more easily than running full x86 emulator? Something more like JVM than XEN HVM.
There are several C to JVM compilers listed on Wikipedia's JVM page. I've never tried any of them, but they sound like an interesting exercise to build.
Because of its close association with the Java language, the JVM performs the strict runtime checks mandated by the Java specification. That requires C to bytecode compilers to provide their own "lax machine abstraction", for instance producing compiled code that uses a Java array to represent main memory (so pointers can be compiled to integers), and linking the C library to a centralized Java class that emulates system calls. Most or all of the compilers listed below use a similar approach.
C compiled to LLVM bit code is not platform independent. Have a look at Google portable native client, they are trying to address that.
Adobe has alchemy which will let you compile C to flash.
There are C to Java or even JavaScript compilers. However, due to differences in memory management, they aren't very usable.
Web Assembly is trying to address that now by creating a standard bytecode format for the web, but unlike the JVM bytecode, Web Assembly is more low level, working at the abstraction level of C/C++, and not Java, so it's more like what's typically called an "assembly language", which is what C/C++ code is normally compiled to.
LLVM is not a good solution for this problem. As beautiful as LLVM IR is, it is by no means machine independent, nor was it intended to be. It is very easy, and indeed necessary in some languages, to generate target dependent LLVM IR: sizeof(void*), for example, will be 4 or 8 or whatever when compiled into IR.
LLVM also does nothing to provide OS independence.
One interesting possibility might be QEMU. You could compile a program for a particular architecture and then use QEMU user space emulation to run it on different architectures. Unfortunately, this might solve the target machine problem, but doesn't solve the OS problem: QEMU Linux user mode emulation only works on Linux systems.
JVM is probably your best bet for both target and OS independence if you want to distribute binaries.
As Ankur mentions, C++/CLI may be a solution. You can use Mono to run it on Linux, as long as it has no native bits. But unless you already have a code base you are trying to port at minimal cost, maybe using it would be counter productive. If it makes sense in your situation, you should go with Java or C#.
Most people who go with C++ do it for performance reasons, but unless you play with very low level stuff, you'll be done coding earlier in a higher level language. This in turn gives you the time to optimize so that by the time you would have been done in C++, you'll have an even faster version in whatever higher level language you choose to use.
The real problem is that C and C++ are not architecture independent languages. You can write things that are reasonably portable in them, but the compiler also hardcodes aspects of the machine via your code. Think about, for example, sizeof(long). Also, as Richard mentions, there's no OS independence. So unless the libraries you use happen to have the same conventions and exist on multiple platforms then it you wouldn't be able to run the application.
Your best bet would be to write your code in a more portable language, or provide binaries for the platforms you care about.