How kernel, compiler, and C library work together
Kernel (內核)

The kernel is the core of an operating system. In a traditional design, it is responsible for memory management, I/O, interrupt handling, and various other things. And even while some modern designs like?Microkernels?or?Exokernels?move several of these services into user space, this matters little in the scope of this document.

The kernel makes its services available through a set of system calls; how they are called and what they do exactly differs from kernel to kernel.

內核是操作系統(tǒng)的核心。在傳統(tǒng)的設計中，它是負責內存管理，I / O，中斷處理，和其他各種東西。即使在一些現(xiàn)代的設計，如微內核或Exokernels這些服務的幾個移動到用戶空間，這一點在本文的討論范圍的事項。
通過一組系統(tǒng)調用的內核使得其提供的服務，他們是如何完全不同。

C Library

One thing up front: When you begin working on your kernel, you do not have a C library available. You?must?not?#includeanything you did not write yourself. You will also have to port an existing C library. See?GCC Cross-Compiler, Future Steps / Standard Library.

第一件事：當你開始寫你的內核，如果你沒有一個C庫。你就不能＃include 任何不是你自己寫的東西。您可以移植現(xiàn)有的C庫。 GCC交叉編譯器，未來的步驟/標準庫。

The C library implements the standard C functions (i.e., the things declared in

C庫實現(xiàn)了標準的C函數(shù)（例如

In addition to standard C functions (as defined in the ISO standard), a C library might (and usually does) implement further functionality, which might or might not be defined by some standard. The standard C library says nothing about networking, for example. For Unix-like systems, the POSIX standard defines what is expected from a C library; other systems might differ fundamentally.

除了標準的C函數(shù)（如ISO標準中定義的），C庫（和通常一樣）可以實現(xiàn)更多的功能，這可能會超過一些標準。 在標準C庫裏，沒有關于網(wǎng)絡的function，例如。對于類Unix系統(tǒng)，POSIX標準定義C庫應該的期望是什么。

It should be noted that, in order to implement its functionality, the C library must call kernel functions. So, for your own OS, you can of course take a ready-made C library and just recompile it for your OS - but that requires that you tell the library how to call your kernel functions, and your kernel to actually provide those functions. The good news is that?relatively few of the library's functions do use some system call

應當指出的是，為了實現(xiàn)其功能，C庫必須調用內核函數(shù)。因此，對于自己的操作系統(tǒng)，你當然可以拿一個現(xiàn)成的C庫，只需重新編譯它適用于您的操作系統(tǒng) - 但是這需要你告訴圖書館如何打電話給你的內核函數(shù)，和你的內核，提供這些功能。好消息是，圖書館的功能比較少使用一些系統(tǒng)調用

Some implementations of the standard C library include:

GNU C library?(with info about?porting the glibc)newlib?(with info on the required OS functions detailed in the manual)uClibC?(although that is highly optimized to be used with an embedded?Linux).fdlibmdietlib

A more elaborate example is available in?Library Calls?or, you can roll your own?C Library.

Compiler / Assembler

An Assembler takes (plaintext) source code and turns it into (binary) machine code; more precisely, it turns the source into?object?code, which contains additional information like symbol names, relocation information etc.

一個assembler接受匯編源代碼為輸入，并把它轉化為機器代碼（二進制）;更確切地說，它原來的源代碼轉換成目標代碼，其中包含其他信息，如符號名稱(symbol)，重定位信息等。

A compiler takes higher-level language source code, and either directly turns it into object code, or (as is the case with GCC) turns it into Assembler source code and invokes an Assembler for the final step.

編譯器接受更高層次的語言源代碼為輸入，可以直接把它轉換成目標代碼，或像GCC那樣把它轉化為匯編程序源代碼，并調用匯編作為編譯的最后一步。

The resulting object code does?not?yet contain any code for standard functions called. If you?included e.g.??and usedprintf(), the object code will merely contain a?reference?stating that a function named?printf()?(and taking a?const char *?and a number of unnamed arguments as parameters) must be linked to the object code in order to receive a complete executable.

生成的目標代碼不包含任何代碼的標準函數(shù)調用。如果包括了如

Some compilers use standard library functions?internally, which might result in object files referencing e.g.?memset()?or?memcpy()?even though you did not include the header or used a function of this name. You will have to provide an implementation of these functions to the linker, or the linking will fail (see below).

一些編譯器內部使用標準庫函數(shù)，這可能會導致在目標文件中引用錯誤，如memset的（）的memcpy（），即使你沒有包含頭文件，或使用這個名字的函數(shù)。您必須提供一個實現(xiàn)這些功能的連接器，或否則會連接失?。ㄒ娤挛模?br />

Some advanced operations (e.g. 64-bits divisions on a 32-bits system) might involve?compiler-internal?functions. For?GCC, those functions are residing in libgcc.a. The content of this library is agnostic of what OS you use, and it won't taint your compiled kernel with licensing issues of whatever sort, so you are welcome to locate "libgcc.a" and link it with your kernel.

一些先進的操作（例如在32位系統(tǒng)的64位部門）可能涉及部編譯器內的功能。對于GCC，這些function是儲存在libgcc.a文件中。這個庫的內容是用家不需要知的，所以編譯器總是希望你把“l(fā)ibgcc.a的”和將它連接到你的內核編譯的內核Link在一起.

Linker

A linker takes the object code generated by the compiler / assembler, and?links?it against the C library (and / or libgcc.a or whatever link library you provide). This can be done in two ways: static, and dynamic.

Static Linking

When linking statically, the linker is invoked during the build process, just after the compiler / assembler run. It takes the object code, checks it for unresolved references, and checks if it can resolve these references from the available libraries. It then adds the binary code from these libraries to the executable; after this process, the executable is?complete, i.e. when running it does not require anything but the kernel to be present.

On the downside, the executable can become quite large, and code from the libraries is duplicated over and over, both on disk and in memory.

Dynamic Linking

When linking dynamically, the linker is invoked during the?loading?of an executable. The unresolved references in the object code are resolved against the libraries currently present in the system. This makes the on-disk executable much smaller, and allows for in-memory space-saving strategies such as?shared libraries?(see below).

On the downside, the executable becomes dependent on the presence of the libraries it references; if a system does not have those libraries, the executable cannot run.

Shared Libraries

A popular strategy is to?share?dynamically linked libraries across multiple executables. This means that, instead of attaching the binary of the library to the executable image, the references in the executable are tweaked, so that all executables refer to the same in-memory representation of the required library.

This requires some trickery. For one, the library must either not have any?state?(static or global data) at all, or it must provide a separatestate?for each executable. This gets even trickier with multi-threaded systems, where one executable might have more than one simultaneous control flow.

Second, in a virtual memory environment, it is usually impossible to provide a library to all executables in the system at the same virtual memory address. To access library code at an arbitrary virtual address requires the library code to be?position independent?(which can be achieved e.g. by setting the -PIC command line option for the?GCC?compiler). This requires support of the feature by the binary format (relocation tables), and can result in slightly less efficient code on some architectures.

ABI - Application Binary Interface

The ABI of a system defines how library function calls and kernel system calls are actually done. This includes e.g. whether parameters are passed on the stack or in registers, how function entry points are located in libraries etc.

When using static linkage, the resulting executable is depending on the executing kernel using the same ABI as the one it was built for; when using dynamic linkage, the executable is depending on the libraries' ABI staying the same.

Unresolved Symbols

The linker is the stage where you will find out about stuff that has been added without your knowledge, and which is not provided by your environment. This can include references to?alloca(),?memcpy(), or several others. This is usually a sign that either your toolchain or your command line options are not correctly set up for compiling your own OS kernel - or that you are using functionality that is not yet implemented in your C library / runtime environment!

It is?strongly?recommended to build a?GCC Cross-Compiler?to avoid this and similar problems right from the start.

Other symbols, such as _udiv* or __builtin_saveregs, are available in the OS-agnostic "libgcc.a". If you get errors about missing such symbols, try to link your kernel against that library too. See?this thread?for details.

__alloca, ___main

alloca()?is a "semi-standard" C function (from BSD?, but supported by most C implementations) that is used to allocate memory from the stack. On Windows this function is also used for stack probing. As such,?alloca()?is referenced in PE binaries, build e.g. by the Cygwin GCC. You can set?-mno-stack-arg-probe?to suppress those references.

Another "specialty" of PE binaries is that, if you define?int main(), a function?void __main()?is called first thing after entering?main(). You can either define that function, or omit?main()?from your kernel code, using a different function as entry point.

This explanation of?alloca()?comes from Chris Giese, posted to alt.os.dev:

>> I think _alloca() is for stack-probing, which is required by Windows.
> What is stack probing?
By default, Windows reserves 1 meg of virtual memory for the stack. No page of stack memory is actually allocated (committed) until the page is accessed. This is demand-allocation. The page beyond the top of the stack is the guard page. If this page is accessed, memory will be allocated for it, and the guard page moved downward by 4K (one page). Thus, the stack can grow beyond the initial 1 meg. Windows will not, however, let you grow the stack by accessing discontiguous pages of memory. Going beyond the guard page causes an exception. Stack-probing code prevents this.

Some more information about stack-probing:

http://groups.google.com/groups?&selm=702bki%24oki%241%40news.Eindhoven.NL.nethttp://groups.google.com/groups?&selm=3381B63D.6E39%40austin.finnigan.com

alloca()?is not just used for stack-probing, but also as a sort of?malloc()?-- for dynamically allocating memory for variables/buffers -- without the need to manually free the reserved memory (with?free()?as it should be done for?malloc()?allocated memory).

If you search the man pages for alloca on a UNIX OS you'll find something like this: "The alloca() function allocates space in the stack frame of the caller, and returns a pointer to the allocated block. This temporary space is automatically freed when the function from which alloca() is called returns."

On Windows: "_alloca allocates size bytes from the program stack. The allocated space is automatically freed when the calling function exits (not when the allocation merely passes out of scope). Therefore, do not pass the pointer value returned by _alloca as an argument to free."

Note that "a stack overflow exception is generated if the space cannot be allocated", so?alloca()?might cause the stack to grow.

Unfortunately, there are all kinds of caveats with using?alloca(). Notably, Turbo~C/C++ had the limitation that functions calling?alloca()needed to have at least one local variable, and Linux man-page warns the following:

The alloca function is machine and compiler dependent. On many systems its implementation is buggy. Its use is discouraged.

On many systems alloca cannot be used inside the list of arguments of a function call, because the stack space reserved by alloca would appear on the stack in the middle of the space for the function arguments.

memcpy

This function is used internally by?GCC. You should set the?--no-builtin?switch, and provide your own implementation of?memcpy()?if you want to be independent of?GCC?(OSD library?just has it?;).