Question 26808 in CentOS Question posted in
The official documentation can be found here.

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

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?

Tags:

2

Answers


  1. 0

    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

    1. 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.
    2. 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.
    3. 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.
    4. 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.

    Login or Signup to reply.
  2. 0

    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.

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search