Why can't object (.obj) files be moved across platforms? - c++

Why can't we move .obj files from c compilation across OS platforms and use it to build the executable file at the end?
If we can do so can we call C a platform independent language like Java?

The C language is platform independent.
The files generated by the compiler, the object and executable files, are platform dependent. This is due to the fact the ultimate goal of a compiler is to generate an executable file for the target architecture only, not for every known architecture.
Java class files are platform independent because Sun was the only designer of Java, it actually made all the rules (from bytecode to file format and VM behavior) and everyone else had to adapt.
This didn't happen with native binary formats, every OS made its format, compiler made its object format and every CPU has its own ISA.

There is absolutely nothing in any specification that says this CAN'T be the case. (Note that the languages C and C++ are both platform independent, but the OBJECT files produced by C and C++ are what is NOT platform independent)
However, because C and C++ are both languages designed for performance, most compilers produce machine code for the target system. And you may then say "but my Linux machine runs on the same processor as my Windows machine", but of course, that's not the ONLY difference between object files or executable files on different OS architectures. And whilst it may be possible to convert object files containing machine code for the same processor from one format to another, it's fraught with problematic things like "what do with inlined system calls" (in other words, someone called gettimeofday via the std::chrono interface, and the compiler inlined this call, which is a call directly to the OS - well, Windows has no idea what gettimeofday is, it's called GetSystemTime or some such, and the method of calling the OS is completely different...)
If you want an OS independent system, then all object files must be "pure" - and of course, both systems need to support the same object file format (or support conversion of them).
One could make a C or C++ compiler that does what Java (and C#, etc) does, where the compiler doesn't produce machine-code for the target system, but produces a "intermediate form" - but that would be a little contrary to the ideas of C and C++, which is that the language is designed to be VERY efficient, and not have a lot of overhead. If portability is more important than performance, maybe you want to use Java? Or some other portable language...

Different platforms use different object file formats (ELF for Linux, COFF/PE for Windows), so an object file built on one platform may not be usable on another.
Remember that an object file is (usually) native machine code, just not in an executable form.

C is cross-platform in source code level.
Once it is compiled, the binary is subject to many factors.
In architecture level, it is possible to generate intermediate object code like LLVM, and do JIT on target machine so that the code fits for target architecture.
However, unless you are doing free-standing development, the platform dependency can comes to play a party preventing you to directly run the code. These dependency may include linking parameters, difference in implementation of standard libraries, platform-specific features, etc.
There is still exception, if the OS provide binary-level compatibility (like BSD ), you may indeed run code compiled for other platform directly.

Related

What affects generated machine code at each step of the compilation process?

I am almost certain this question has been asked before, but I can not seem to find the right keywords to search for to get an answer. My apologies if this is a duplicate.
I am better trying to understand the compilation process of say a C++ file as it goes from the C++ syntax to the binary machine code. In addition I am trying to understand what influences the resulting machine code.
First, I am nearly certain that the following are the only factors (for most systems) that dictate the final machine code (please correct me if I am wrong here)
The tools used to compile, assemble, and link.
Things like gnu c compiler, clang, visual studio, nasm, ect.
The kernel of the system being used.
Whether its a specific version of the linux kernel, windows microkernel, or some other kernel like a mac os x one.
The operating system being used.
This one I am less clear about. I am unsure if machines running the same linux kernel, but different os, in this case let's say debian vs centos, will they produce different binaries.
Lastly the hardware architecture.
Different cpu architectures like arm 64, x86, power pc, ect. take different op codes so obviously the machine code should be different.
So with that being said here is my understanding of the compilation process and where each of these dependencies show up.
I write a C++ file and use code that my system can understand. A good example might be using <winsock.h> on windows and <sys/socket.h> on linux.
The preprocessor runs and executes any preprocessor macros.
Here I know that different preprocessors will define different macros but for now I will assume this is not too machine dependent. (This might be wrong to assume).
The compiler tools run to produce assembly file outputs.
Here the assembly produced depends on the compiler and what optimizations or choices it makes.
It also depends on the kernel because different kernels have different system calls and store files in different locations. This means the assembly might make changes such as different branching when calling functions specific to that kernel.
The operating system? Still unsure how the operating system fits in to this. If two machines have the same kernel, what does the operating system do to the binaries?
Finally the assembly code depends on the cpu architecture. I think that is a pretty obvious statement.
Once the compiler produces an assembly. We can then invoke the assembler to turn our assembly code into almost complete machine code. (I think machine code is identical to binary opcodes a cpu manual lists but this might be wrong).
The corresponding machine code files (often called object files I think) contain nearly all the instructions needed to run or reference other machine code files which will be linked in the next step.
This machine code usually has some format (I think ELF is a popular format for linux) and this format is dependent on the linker for sure.
I don't think the kernel, operating system, or hardware affect the layout/format of the object file but this is probably wrong. If they do please correct this.
The hardware will affect the actual machine code produced because again I think it is a 1 to 1 mapping of machine code instructions to opcodes for a cpu.
I am unsure if the kernel or operating system affect the linking process because I thought their changes were already incorporated in the compiling step.
Finally the linking step occurs.
I think this is as simple as the linker looking for all the referenced machine code and injecting it into one complete machine code file which can be executed.
I have no clue what affects this besides the linker tool itself.
So with all that, I need help identifying inaccuracies with the procedure I described above, and any dependencies I might have missed whether it be cpu, os, kernel, or tool ones.
Thank you and sorry for the long winded question. This probably should have been broken up into multiple questions but I am too far in. If this does not go well I may ask each part in individual questions.
EDIT:
Questions with more focus.
What components of a machine affect the machine code produced given a C++ file input?
Actually that is a lot of questions and usually you're question would be much too broad for SO (as you managed to recognize by yourself). But on the other hand you showed a deep interest (just by writing such a long and profound question) and also a lot of correct understanding of the process of compiling a program. The things you are missing or not understanding correctly (and you are probably the most interested in) are those things, that I myself found hard to learn. Thus I will provide you with some important points, that I think you are missing in the big picture.
Note that I am very much used to Linux, so I will mostly describe how things work on Linux. But I believe that most things also happen in a similar way on other operating systems.
Let's begin with the hardware. A modern computer has a CPU of some architecture. There are lots of different of CPU architectures. You mentioned some of them like arm, x86, etc. which are families of similar CPUs and can be divided into smaller groups by bit width and/or supported extensions. Ultimately your processor has a specified instruction set that defines which opcodes it supports and what those opcodes do. If a native (compiled) program runs, there are raw opcodes in the memory and the CPU directly executes them following its architecture specification.
Aside from the CPU there is a lot more hardware connected to your computer. Usually communicating with this hardware is complicated and not standardized. If a user program for example gets input keystrokes from the keyboard, in does not have to directly communicate with the keyboard, but rather does this via the operating system kernel. This works by a mechanism called syscall interrupt. The kernel installs an handler routine, that is called if a user program triggers such an interrupt with a special CPU instruction. You can think of it like a language agnostic function call from the program into the kernel. For example for Linux you can find a list of all syscalls at the syscall(2) man page. The syscalls form the kernel's Application Binary Interface (kernel ABI). Reading and writing from a terminal or using a filesystem are examples for syscall functionality.
As you can see, there are already very high level functions, that are implemented in the kernel. However the functionality is still quite limited for most typical applications. To encapsulate the syscalls and provide functions for memory management, utility functions, mathematical functions and many other things you probably use in your daily programs, there is usually another layer between the program and the kernel. This thing is called the C standard library, and it is a shared library (we will cover what exactly this is in a moment). On GNU/Linux it is the glibc which is the single most important library on a GNU/Linux system (and notably not part of the kernel 1). While it implements all the features that are required by the C standard (for example functions like malloc() or strcpy()), it also ships a lot of additional functions which are a superset of the ISO C standard library, the POSIX standard and some extensions. This interface is usually called the Application Programming Interface (API) of the operating system. While it is in principle possible to bypass the API and directly use the syscalls, almost all programs (even when written in other languages than C or C++) use the C library.
Now get yourself a coffee and a few minutes of rest. We now have enough background information to look at how a C++ program is transformed into a binary, and how exactly this binary is executed.
A C++ program consists of different compilation units (usually each different source file is a compilation unit). Each compilation unit undergoes the following steps
The preprocessor is run on the file. It includes header, expands macros and does some other stuff. As you wrote in your question this is rather platform independent. The preprocessor actions are standardized in the C++ standard.
The resulting code is compiled. That means C++ code is translated into assembly code. Because assembly code directly reflects the CPU instructions, this step is dependent on the target CPU architecture, that the compiler was configured for (usually the host CPU). The compiler is allowed to optimize and translate the program in any way it wants, as long as it follows the as-if rule. Thus this step is also higly dependent on the compiler you are using.
Note: Symbols (especially functions) that are not defined, are left undefined. If you say call the malloc() function, this will not be compiled, but left unevaluated until later. Thus this step is also not much dependent on the operating system.
Assembling takes place. This is very straightforward. The assembly code usually can be converted directly into binary CPU instructions. Local symbols (such as goto labels etc.) are resolved and replaced by their corresponding addresses. Unknown external symbols such as the mentioned malloc() call still are left unevaluated and are stored in the object file's symbol table. Because most of the syscalls are wrapped in library functions, the assembly code will usually not directly contain syscall code. Thus this step is depended on the CPU architecture. It is however dependent on the ABI2, which in term is dependent on the compiler and the OS.
Linking takes place. The different compilation units are combined into a single executable binary in an OS-dependent format (e.g. GNU/Linux uses ELF). Here yet more symbols are resolved. For example if one compilation calls a function in another compilation unit, this call is resolved and the symbol is replaced by the function address. If you link to a library statically, this is just treated like another compilation unit and included into the executable with its symbols resolved.
Shared libraries are checked for the needed symbols, but not linked yet. For example in case of the malloc() call, the linker checks, that there is a malloc symbol in the glibc, but the symbol in the executable still remains unresolved.
At this point you have a executable binary. As you might noticed, there might still be unresolved symbols in that binary. Thus you cannot just load that binary into RAM and let the CPU execute it. A final step called dynamic linking is needed. On Linux the program that performs this step is called the dynamic linker/loader. Its task is to load the executable ELF file into memory, look up all the needed dynamic libraries, load them into memory as well (a list is stored in the ELF file) and resolve the remaining symbols. This last step happens each time the program is executed. Now finally the malloc() symbol is resolved with the address in the glibc shared library.
You have pure CPU instructions in memory, the CPU's program counter register (the one that tracks the next instruction) is set to the entry point, and the program can begin to run. Every now and then it is interrupted either because it makes a syscall, or because it is interrupted by the kernel scheduler to let another program run on that CPU core.
I hope I could answer some of your questions and satisfy your curiosity. I think the most important part you were missing, was how dynamic linking happens. This is a very interesting topic which is related to concepts like position independent code. I wish you could luck learning.
1 this is also one reason why some people insist on calling Linux based systems GNU/Linux. The glibc library (together with many other GNU programs) defines much of the operating system structure, interacts with supplementary programs and configuration files etc. There are however Linux based systems without glibc. One of them is Android, using Googles bionic libc.
2 The ABI is related to the calling convention. This is a mixture of operating system, programming language and compiler specification. It is one of the reasons (besides name mangling, see the comment of PeterCordes below) you need those extern "C" {...} scopes in C++ header files, that declare C functions in shared libraries. It basically is a convention on how to pass parameters and return values between functions.
Neither operating system nor kernel are directly involved in any of this.
Their limited involvement is in that if you want to build Linux 64 bit binaries for x86 using gnu tools then you need to in some way (download and install or build yourself) build the gnu tools themselves for that target processor and that operating system. As system calls are specific to the operating system and target, and also the binaries supported by that operating system. Not strictly just the elf file format, that is just a container, but the linking and possibly bootstrap is also specific to the operating systems loader. (or if building something for the kernel that would have other rules). For example, does the application loader initialize .bss and .data for you from specific information in the .elf file, or like on an mcu does the bootstrap code itself have to do this?
The builder for gnu tools for a target like linux and ideally a pre-built binary for your os and target, would have paths setup in some way. The c library would have a default linker script and its intimate partner the bootstrap.
After that point, it is just a dumb toolchain. Include files be they at the system level, compiler level, or programmer level are just includes in the C language. The default paths and gcc knows where it was executed from so it knows where in a normal build the gcc and other libraries live.
gcc itself is not a compiler actually it calls other programs like the preprocessor, the compiler itself, the assembler and linker.
The preprocessor is going to do the search and replace for includes and defines and end up with one great big cpp file, then pass that to the compiler.
The compiler front end (C++ language for gcc for example) turns that into an internal language, allocate an int with this name, and another add the two and blah. A pseudo code if you will. This gets a lot of the optimization work done on it then eventually the back end (which for gnu could be x86, mips, arm, etc independent to some extent of the front and middle). The LLVM tools, are at least capable of exposing that middle, internal, language to external files (external to the memory used by the compiler to do the compilation) and you can combine and optimize those bytecode files and then convert them to assembly or direct to object in the llvm world. I think this is an exception not a rule, others just use internal tables.
While I think it is wise and sane to use an assembly language step. Not all compilers do and do not assume that all compilers do. Some output objects.
Yes that assembly is naturally partial, external functions (labels) and variables (labels) cannot be resolved at the object level. The linker has to do that.
So the target (x86, arm, etc) does affect the construction of the elf file as
there are certain items, magic numbers specific to the target. As mentioned the operating system and or kernel do affect the elf in that there are rules for construction of the binary for that kernel or operating system. Remember that elf is just a container like tar or zip or mkv etc. Do not assume that the operating system can handle every possible choice you want to make with the contents that the linker will allow (the tools are dumb, do what they are told).
So your source.
All the relevant sources that go with it including system includes, compiler includes and your includes.
gcc/g++ is a wrapper program that manages the steps.
calls the pre-processor expands includes and defines into one file (no magic here)
call the compiler to parse that one file into internal tables, think pseudo code and data
many, many possible optimizers that operate on these structures
backend, including peephole optimizer, turns the tables into assembly language (for gnu at least)
assembler is called to turn the asm into an object
If all the objects are specified and gcc is told to link, then...
Linker combines all the objects for the binary, including the bootstrap, including already built libraries, stubs, etc, and command line or more likely a linker script (linker script and bootstrap have an intimate relationship they are not assumed to be separable and not part of the compiler they are part of a C library, etc).
Kernel module loader or operating system application loader fed the file and per the rules of that loader loads and runs the program.

Will C++20 modules change the anatomy of static and shared libraries?

Traditionally, C++ libraries are made of a header file + the implementation compiled in a binary file (.a, .so, .dylib, .dll, ...). The header file is #included in the source code and the binary part is linked to the final executable.
Will modules from C++20 change such layout? If so, will operating systems have to upgrade the way they distribute their core libraries, e.g. the Standard Library in Linux or other core dlls in Windows?
Absolutely. Modules are a very different type of representing library code to users than conventional header/libraries. The main advantage of a module is to have it parsed to the level of the abstract syntax tree (AST) by the compiler. This only happens once per module -- in contrary to every time you include a particular header file. Thus, one possibility for speedup is to convert very frequent header files into modules and save a lot of compiler time in not re-compiling to AST many times, but just once. AST also works perfectly fine for templates ... it is a generic and complete description of the C++ language.
But this is currently also the main "drawback": ASTs are absolutely compiler dependent. There is no stability between vendors, systems, or even compiler versions. So distributing ASTs makes no sense, at least in the current toolchain environment.
Thus, in my understanding, modules are not easily a replacement for common header/libraries. They are a ideal replacement for lightweight (best header-only) and potentially highly templated code to be included many times in typical programs. Many libraries, foremost the standard libraries, are like this. But there are also many libraries of different design (small API+heavy binary backend). For those, I guess, we will have to continue to use include/libraries as before. But, C++ is backward compatible, so mixing modules with includes is no problem at all.
Also, modules can just wrap include files (e.g. with an API). This is probably not a very powerful usage of modules, but may lead to a more uniform programming pattern. In this case, you would still need to link your "so" or "dylib" or whatever library to the resulting code.
Thus, I believe the future at some point will distribute both, some C++ modules, but also conventional headers+libraries. Modules by themselves can be split into many files (only the "exported" symbols are visible to users). Modules will likely follow very different design guidelines than headers. It very likely will not be any advantage to have "one class per module", but rather "a whole bunch of logically connected code per module".

How does the C++ compiler know which CPU architecture is being used

with reference to : http://www.cplusplus.com/articles/2v07M4Gy/
During the compilation phase,
This phase translates the program into a low level assembly level code. The compiler takes the preprocessed file ( without any directives) and generates an object file containing assembly level code. Now, the object file created is in the binary form. In the object file created, each line describes one low level machine level instruction.
Now, if I am correct then different CPU architectures works on different assembly languages/syntax.
My question is how does the compiler comes to know to which assembly language syntax the source code has to be changed? In other words, how does the C++ compiler know which CPU architecture is there in the machine it is working on ?
Is there any mapping used by assembler w.r.t the CPU architecture for generating assembly code for different CPU architectures?
N.S : I am beginner !!
Each compiler needs to be "ported" to the given system. For each system supported, a "compiler port" needs to be programmed by someone who knows the system in-depth.
WARNING : This is extremely simplified
In short, there are three main parts to a compiler :
"Front-end" : This part reads the language (in this case c++) and converts it to a sort of pseudo-code specific to the compiler. (An Abstract Syntactic Tree, or AST)
"Optimizer/Middle-end" : This part takes the AST and make a non-architecture-dependant optimized one.
"Back-end" : This part takes the AST, and converts it to binary executable code, specific to the architecture you want to compile your language on.
When you download a c++ compiler for your platform, you, in fact, download the c++ frontend with the linux-amd64 backend, for example.
This coding architecture is extremely helpful, because it allows to port the compiler for another architecture without rewriting the whole parsing/optimizing thing. It also allows someone to create another optimizer, or even another frontend supporting a whole different language, and, as long as it outputs a correct AST, it will be compatible with every single backend ever written for this compiler.
Simply put, the knowledge of the target system is coded into the compiler.
So you might have a C compiler that generates SPARC binaries, and a C compiler that generates VAX binaries. They both accept the same input language (as defined in the C standard), but produce different programs from it.
Often we just refer to "the compiler", meaning the one that will generate binaries for our current environment.
In modern times, the distinction has become less obvious with compiler collections such as GCC. Now the "different compilers" are often the same compiler program, just set up with different configurations (these are the "target description files").
Just to complete the answers given here:
The target architecture is indeed coded into the specific compiler instance you're using. This is important also for a process called "cross-compiling" - The process of compiling on a certain system an executable that would operate on another system/architecture.
Consider working on an embedded system-on-chip that uses a completely different instruction set than your own - You're working on an x86/64 Linux system, but need to compile a mobile app running on an ARM micro-processor, or some other type of assembly architecture.
It would be unreasonable to compile your code on the target system, which might be so limited in CPU and memory that it can't feasibly run a compiler - and so you can use a GCC (or any other compiler) port for that target architecture on your favorite system.
It's also quite critical to remember that the entire tool-chain is often compatible to the target system, for instance when shared libraries such as libc are getting in play - as the target OS could be a different release of Linux and would have different versions of common functions - In which case it's common to use tool-chains that contain all the necessary libraries and use something like chroot or mock to compile in the "target environment" from within your system.

c++ compiles to portable assembly?

All the generated binaries seem to be only OS dependent, and not hardware dependent.
I thought the assembly for each cpu was different, which would mean you have to compile for each diffrent cpu type.
So then why is there compatibility?
Your question appears completely unclear: (cross-)complied binaries are of course OS/Machine dependent.
"so then why is there compatibility?"
At the portable language level (plain standard c++ functions and classes), you can compile your code to run on various OS/machine architectures.
This doesn't mean you just can copy artifacts compiled for a particular OS/machine environment to another one without recompiling from source there (or using a cross-compiler).

What do terms like static compiler and runtime compilers practically mean?

I'm working to learn C++ more and trying to know basics about different compilers and their technologies. I googled this a lot but every time I stepped through I happened to meet new terms which need more explanation. So, what do these terms such as static compiling, dynamic linking and so on which reside in this topic mean in action?
Some languages like C++ compile all of a program to "native machine code" understood by the CPU before it starts running (i.e. actually being used). That's "static compilation".
Other languages (e.g. Java) use "Just In Time" compilers to produce CPU-native code from some other "byte-code" representation of the program, but do so only once they start running. That's "runtime" compilation.
Many other languages (e.g. common python, perl, ruby, Java implementations) use an "interpreter", which means they have native code that keeps consulting some manner of "byte-code" to work out what to do next. (Some very basic inter-company or specialised interpreters even keep consulting the source code without ever generating a more compact byte-code representation thereof, but no popular language does that - it's awfully slow and clumsy).
A single language may potentially use any combination of these approaches, but in general it's either static compilation, or an interpreter that might add a just-in-time compiler to speed up execution.
Sometimes, a single language has implementations that use different approaches, for example: there are limited C++ interpreters (like http://root.cern.ch/drupal/content/cint, but I've never heard of it being used "in anger"), and systems that compile python to native code.
For "dynamic linking": say you have a function "void f();" that does something wonderful. If you put that function in a library for use by many applications, you could "statically link" the function into a specific application to take a "snapshot" of f()'s functionality at the specific point in time your program's executable is being created. Then if f() changes later you have to re-link and redistribute your application to incorporate the changes to f(). Alternatively, you could put f() into a dynamic linked library, which means a separate library file containing f() is distributed alongside - or independently from - your program. Each time your program starts running, if looks to the dynamic library file for the code to use for f(). So, if you distribute an updated dynamic library you can update f() without redistributing all the application programs that call f(). Sometimes, that's just a better model for distributing updated software to your users, and avoiding getting each individual application program involved in the distribution of updates to f(). (Occasionally it's a disaster because a dynamic version of f() hasn't actually been tested with the application, and does something subtly differently that breaks the application).
About static or dynamic linking, read also Levine's Linker & Loader book.
About shared or static libraries, read Program Library HowTo
For shared objects (or libraries) on Linux, read Drupper's How To Write Shared Libraries paper
You could load plugins with dlopen(3) but then read C++ dlopen mini-howto.
Compilation is usually static, since it is ahead of time (e.g. when compiling with GCC). Sometimes, a just in time compilation is done (e.g. by most JVMs).
If coding in C++ on Linux, you'll want to compile with g++ -Wall -g (and later use -O2 to ask GCC to optimize when your program is debugged). See this and that hints.
Also, learn C++11 and use the latest GCC 4.8.2 compiler (GCC 4.9 might be released in a few weeks, e.g. in march or april 2014).