Index: docs/SystemLibrary.rst =================================================================== --- docs/SystemLibrary.rst +++ docs/SystemLibrary.rst @@ -2,245 +2,8 @@ System Library ============== -Abstract -======== - -This document provides some details on LLVM's System Library, located in the -source at ``lib/System`` and ``include/llvm/System``. The library's purpose is -to shield LLVM from the differences between operating systems for the few -services LLVM needs from the operating system. Much of LLVM is written using -portability features of standard C++. However, in a few areas, system dependent -facilities are needed and the System Library is the wrapper around those system -calls. - -By centralizing LLVM's use of operating system interfaces, we make it possible -for the LLVM tool chain and runtime libraries to be more easily ported to new -platforms since (theoretically) only ``lib/System`` needs to be ported. This -library also unclutters the rest of LLVM from #ifdef use and special cases for -specific operating systems. Such uses are replaced with simple calls to the -interfaces provided in ``include/llvm/System``. - -Note that the System Library is not intended to be a complete operating system -wrapper (such as the Adaptive Communications Environment (ACE) or Apache -Portable Runtime (APR)), but only provides the functionality necessary to -support LLVM. - -The System Library was written by Reid Spencer who formulated the design based -on similar work originating from the eXtensible Programming System (XPS). -Several people helped with the effort; especially, Jeff Cohen and Henrik Bach -on the Win32 port. - -Keeping LLVM Portable -===================== - -In order to keep LLVM portable, LLVM developers should adhere to a set of -portability rules associated with the System Library. Adherence to these rules -should help the System Library achieve its goal of shielding LLVM from the -variations in operating system interfaces and doing so efficiently. The -following sections define the rules needed to fulfill this objective. - -Don't Include System Headers ----------------------------- - -Except in ``lib/System``, no LLVM source code should directly ``#include`` a -system header. Care has been taken to remove all such ``#includes`` from LLVM -while ``lib/System`` was being developed. Specifically this means that header -files like "``unistd.h``", "``windows.h``", "``stdio.h``", and "``string.h``" -are forbidden to be included by LLVM source code outside the implementation of -``lib/System``. - -To obtain system-dependent functionality, existing interfaces to the system -found in ``include/llvm/System`` should be used. If an appropriate interface is -not available, it should be added to ``include/llvm/System`` and implemented in -``lib/System`` for all supported platforms. - -Don't Expose System Headers ---------------------------- - -The System Library must shield LLVM from **all** system headers. To obtain -system level functionality, LLVM source must ``#include "llvm/System/Thing.h"`` -and nothing else. This means that ``Thing.h`` cannot expose any system header -files. This protects LLVM from accidentally using system specific functionality -and only allows it via the ``lib/System`` interface. - -Use Standard C Headers ----------------------- - -The **standard** C headers (the ones beginning with "c") are allowed to be -exposed through the ``lib/System`` interface. These headers and the things they -declare are considered to be platform agnostic. LLVM source files may include -them directly or obtain their inclusion through ``lib/System`` interfaces. - -Use Standard C++ Headers ------------------------- - -The **standard** C++ headers from the standard C++ library and standard -template library may be exposed through the ``lib/System`` interface. These -headers and the things they declare are considered to be platform agnostic. -LLVM source files may include them or obtain their inclusion through -``lib/System`` interfaces. - -High Level Interface --------------------- - -The entry points specified in the interface of ``lib/System`` must be aimed at -completing some reasonably high level task needed by LLVM. We do not want to -simply wrap each operating system call. It would be preferable to wrap several -operating system calls that are always used in conjunction with one another by -LLVM. - -For example, consider what is needed to execute a program, wait for it to -complete, and return its result code. On Unix, this involves the following -operating system calls: ``getenv``, ``fork``, ``execve``, and ``wait``. The -correct thing for ``lib/System`` to provide is a function, say -``ExecuteProgramAndWait``, that implements the functionality completely. what -we don't want is wrappers for the operating system calls involved. - -There must **not** be a one-to-one relationship between operating system -calls and the System library's interface. Any such interface function will be -suspicious. - -No Unused Functionality ------------------------ - -There must be no functionality specified in the interface of ``lib/System`` -that isn't actually used by LLVM. We're not writing a general purpose operating -system wrapper here, just enough to satisfy LLVM's needs. And, LLVM doesn't -need much. This design goal aims to keep the ``lib/System`` interface small and -understandable which should foster its actual use and adoption. - -No Duplicate Implementations ----------------------------- - -The implementation of a function for a given platform must be written exactly -once. This implies that it must be possible to apply a function's -implementation to multiple operating systems if those operating systems can -share the same implementation. This rule applies to the set of operating -systems supported for a given class of operating system (e.g. Unix, Win32). - -No Virtual Methods ------------------- - -The System Library interfaces can be called quite frequently by LLVM. In order -to make those calls as efficient as possible, we discourage the use of virtual -methods. There is no need to use inheritance for implementation differences, it -just adds complexity. The ``#include`` mechanism works just fine. - -No Exposed Functions --------------------- - -Any functions defined by system libraries (i.e. not defined by ``lib/System``) -must not be exposed through the ``lib/System`` interface, even if the header -file for that function is not exposed. This prevents inadvertent use of system -specific functionality. - -For example, the ``stat`` system call is notorious for having variations in the -data it provides. ``lib/System`` must not declare ``stat`` nor allow it to be -declared. Instead it should provide its own interface to discovering -information about files and directories. Those interfaces may be implemented in -terms of ``stat`` but that is strictly an implementation detail. The interface -provided by the System Library must be implemented on all platforms (even those -without ``stat``). - -No Exposed Data ---------------- - -Any data defined by system libraries (i.e. not defined by ``lib/System``) must -not be exposed through the ``lib/System`` interface, even if the header file -for that function is not exposed. As with functions, this prevents inadvertent -use of data that might not exist on all platforms. - -Minimize Soft Errors --------------------- - -Operating system interfaces will generally provide error results for every -little thing that could go wrong. In almost all cases, you can divide these -error results into two groups: normal/good/soft and abnormal/bad/hard. That is, -some of the errors are simply information like "file not found", "insufficient -privileges", etc. while other errors are much harder like "out of space", "bad -disk sector", or "system call interrupted". We'll call the first group "*soft*" -errors and the second group "*hard*" errors. - -``lib/System`` must always attempt to minimize soft errors. This is a design -requirement because the minimization of soft errors can affect the granularity -and the nature of the interface. In general, if you find that you're wanting to -throw soft errors, you must review the granularity of the interface because it -is likely you're trying to implement something that is too low level. The rule -of thumb is to provide interface functions that **can't** fail, except when -faced with hard errors. - -For a trivial example, suppose we wanted to add an "``OpenFileForWriting``" -function. For many operating systems, if the file doesn't exist, attempting to -open the file will produce an error. However, ``lib/System`` should not simply -throw that error if it occurs because its a soft error. The problem is that the -interface function, ``OpenFileForWriting`` is too low level. It should be -``OpenOrCreateFileForWriting``. In the case of the soft "doesn't exist" error, -this function would just create it and then open it for writing. - -This design principle needs to be maintained in ``lib/System`` because it -avoids the propagation of soft error handling throughout the rest of LLVM. -Hard errors will generally just cause a termination for an LLVM tool so don't -be bashful about throwing them. - -Rules of thumb: - -#. Don't throw soft errors, only hard errors. - -#. If you're tempted to throw a soft error, re-think the interface. - -#. Handle internally the most common normal/good/soft error conditions - so the rest of LLVM doesn't have to. - -No throw Specifications ------------------------ - -None of the ``lib/System`` interface functions may be declared with C++ -``throw()`` specifications on them. This requirement makes sure that the -compiler does not insert additional exception handling code into the interface -functions. This is a performance consideration: ``lib/System`` functions are at -the bottom of many call chains and as such can be frequently called. We need -them to be as efficient as possible. However, no routines in the system -library should actually throw exceptions. - -Code Organization ------------------ - -Implementations of the System Library interface are separated by their general -class of operating system. Currently only Unix and Win32 classes are defined -but more could be added for other operating system classifications. To -distinguish which implementation to compile, the code in ``lib/System`` uses -the ``LLVM_ON_UNIX`` and ``_WIN32`` ``#defines``. Each source file in -``lib/System``, after implementing the generic (operating system independent) -functionality needs to include the correct implementation using a set of -``#if defined(LLVM_ON_XYZ)`` directives. For example, if we had -``lib/System/File.cpp``, we'd expect to see in that file: - -.. code-block:: c++ - - #if defined(LLVM_ON_UNIX) - #include "Unix/File.cpp" - #endif - #if defined(_WIN32) - #include "Win32/File.cpp" - #endif - -The implementation in ``lib/System/Unix/File.cpp`` should handle all Unix -variants. The implementation in ``lib/System/Win32/File.cpp`` should handle all -Win32 variants. What this does is quickly differentiate the basic class of -operating system that will provide the implementation. The specific details for -a given platform must still be determined through the use of ``#ifdef``. - -Consistent Semantics --------------------- - -The implementation of a ``lib/System`` interface can vary drastically between -platforms. That's okay as long as the end result of the interface function is -the same. For example, a function to create a directory is pretty straight -forward on all operating system. System V IPC on the other hand isn't even -supported on all platforms. Instead of "supporting" System V IPC, -``lib/System`` should provide an interface to the basic concept of -inter-process communications. The implementations might use System V IPC if -that was available or named pipes, or whatever gets the job done effectively -for a given operating system. In all cases, the interface and the -implementation must be semantically consistent. +Moved +===== +The System Library has been renamed to Support Library with documentation +available at :doc:`SupportLibrary`. Please, change your links to that page.