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==============
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System Library
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==============
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Abstract
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========
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This document provides some details on LLVM's System Library, located in the
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source at ``lib/System`` and ``include/llvm/System``. The library's purpose is
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to shield LLVM from the differences between operating systems for the few
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services LLVM needs from the operating system. Much of LLVM is written using
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portability features of standard C++. However, in a few areas, system dependent
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facilities are needed and the System Library is the wrapper around those system
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calls.
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By centralizing LLVM's use of operating system interfaces, we make it possible
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for the LLVM tool chain and runtime libraries to be more easily ported to new
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platforms since (theoretically) only ``lib/System`` needs to be ported. This
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library also unclutters the rest of LLVM from #ifdef use and special cases for
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specific operating systems. Such uses are replaced with simple calls to the
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interfaces provided in ``include/llvm/System``.
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Note that the System Library is not intended to be a complete operating system
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wrapper (such as the Adaptive Communications Environment (ACE) or Apache
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Portable Runtime (APR)), but only provides the functionality necessary to
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support LLVM.
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The System Library was written by Reid Spencer who formulated the design based
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on similar work originating from the eXtensible Programming System (XPS).
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Several people helped with the effort; especially, Jeff Cohen and Henrik Bach
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on the Win32 port.
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Keeping LLVM Portable
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=====================
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In order to keep LLVM portable, LLVM developers should adhere to a set of
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portability rules associated with the System Library. Adherence to these rules
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should help the System Library achieve its goal of shielding LLVM from the
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variations in operating system interfaces and doing so efficiently. The
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following sections define the rules needed to fulfill this objective.
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Don't Include System Headers
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----------------------------
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Except in ``lib/System``, no LLVM source code should directly ``#include`` a
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system header. Care has been taken to remove all such ``#includes`` from LLVM
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while ``lib/System`` was being developed. Specifically this means that header
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files like "``unistd.h``", "``windows.h``", "``stdio.h``", and "``string.h``"
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are forbidden to be included by LLVM source code outside the implementation of
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``lib/System``.
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To obtain system-dependent functionality, existing interfaces to the system
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found in ``include/llvm/System`` should be used. If an appropriate interface is
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not available, it should be added to ``include/llvm/System`` and implemented in
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``lib/System`` for all supported platforms.
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Don't Expose System Headers
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---------------------------
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The System Library must shield LLVM from **all** system headers. To obtain
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system level functionality, LLVM source must ``#include "llvm/System/Thing.h"``
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and nothing else. This means that ``Thing.h`` cannot expose any system header
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files. This protects LLVM from accidentally using system specific functionality
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and only allows it via the ``lib/System`` interface.
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Use Standard C Headers
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----------------------
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The **standard** C headers (the ones beginning with "c") are allowed to be
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exposed through the ``lib/System`` interface. These headers and the things they
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declare are considered to be platform agnostic. LLVM source files may include
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them directly or obtain their inclusion through ``lib/System`` interfaces.
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Use Standard C++ Headers
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------------------------
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The **standard** C++ headers from the standard C++ library and standard
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template library may be exposed through the ``lib/System`` interface. These
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headers and the things they declare are considered to be platform agnostic.
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LLVM source files may include them or obtain their inclusion through
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``lib/System`` interfaces.
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High Level Interface
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--------------------
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The entry points specified in the interface of ``lib/System`` must be aimed at
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completing some reasonably high level task needed by LLVM. We do not want to
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simply wrap each operating system call. It would be preferable to wrap several
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operating system calls that are always used in conjunction with one another by
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LLVM.
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For example, consider what is needed to execute a program, wait for it to
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complete, and return its result code. On Unix, this involves the following
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operating system calls: ``getenv``, ``fork``, ``execve``, and ``wait``. The
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correct thing for ``lib/System`` to provide is a function, say
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``ExecuteProgramAndWait``, that implements the functionality completely. what
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we don't want is wrappers for the operating system calls involved.
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There must **not** be a one-to-one relationship between operating system
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calls and the System library's interface. Any such interface function will be
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suspicious.
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No Unused Functionality
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-----------------------
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There must be no functionality specified in the interface of ``lib/System``
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that isn't actually used by LLVM. We're not writing a general purpose operating
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system wrapper here, just enough to satisfy LLVM's needs. And, LLVM doesn't
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need much. This design goal aims to keep the ``lib/System`` interface small and
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understandable which should foster its actual use and adoption.
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No Duplicate Implementations
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----------------------------
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The implementation of a function for a given platform must be written exactly
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once. This implies that it must be possible to apply a function's
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implementation to multiple operating systems if those operating systems can
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share the same implementation. This rule applies to the set of operating
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systems supported for a given class of operating system (e.g. Unix, Win32).
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No Virtual Methods
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------------------
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The System Library interfaces can be called quite frequently by LLVM. In order
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to make those calls as efficient as possible, we discourage the use of virtual
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methods. There is no need to use inheritance for implementation differences, it
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just adds complexity. The ``#include`` mechanism works just fine.
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No Exposed Functions
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--------------------
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Any functions defined by system libraries (i.e. not defined by ``lib/System``)
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must not be exposed through the ``lib/System`` interface, even if the header
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file for that function is not exposed. This prevents inadvertent use of system
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specific functionality.
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For example, the ``stat`` system call is notorious for having variations in the
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data it provides. ``lib/System`` must not declare ``stat`` nor allow it to be
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declared. Instead it should provide its own interface to discovering
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information about files and directories. Those interfaces may be implemented in
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terms of ``stat`` but that is strictly an implementation detail. The interface
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provided by the System Library must be implemented on all platforms (even those
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without ``stat``).
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No Exposed Data
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---------------
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Any data defined by system libraries (i.e. not defined by ``lib/System``) must
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not be exposed through the ``lib/System`` interface, even if the header file
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for that function is not exposed. As with functions, this prevents inadvertent
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use of data that might not exist on all platforms.
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Minimize Soft Errors
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--------------------
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Operating system interfaces will generally provide error results for every
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little thing that could go wrong. In almost all cases, you can divide these
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error results into two groups: normal/good/soft and abnormal/bad/hard. That is,
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some of the errors are simply information like "file not found", "insufficient
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privileges", etc. while other errors are much harder like "out of space", "bad
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disk sector", or "system call interrupted". We'll call the first group "*soft*"
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errors and the second group "*hard*" errors.
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``lib/System`` must always attempt to minimize soft errors. This is a design
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requirement because the minimization of soft errors can affect the granularity
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and the nature of the interface. In general, if you find that you're wanting to
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throw soft errors, you must review the granularity of the interface because it
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is likely you're trying to implement something that is too low level. The rule
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of thumb is to provide interface functions that **can't** fail, except when
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faced with hard errors.
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For a trivial example, suppose we wanted to add an "``OpenFileForWriting``"
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function. For many operating systems, if the file doesn't exist, attempting to
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open the file will produce an error. However, ``lib/System`` should not simply
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throw that error if it occurs because its a soft error. The problem is that the
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interface function, ``OpenFileForWriting`` is too low level. It should be
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``OpenOrCreateFileForWriting``. In the case of the soft "doesn't exist" error,
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this function would just create it and then open it for writing.
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This design principle needs to be maintained in ``lib/System`` because it
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avoids the propagation of soft error handling throughout the rest of LLVM.
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Hard errors will generally just cause a termination for an LLVM tool so don't
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be bashful about throwing them.
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Rules of thumb:
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#. Don't throw soft errors, only hard errors.
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#. If you're tempted to throw a soft error, re-think the interface.
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#. Handle internally the most common normal/good/soft error conditions
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so the rest of LLVM doesn't have to.
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No throw Specifications
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-----------------------
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None of the ``lib/System`` interface functions may be declared with C++
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``throw()`` specifications on them. This requirement makes sure that the
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compiler does not insert additional exception handling code into the interface
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functions. This is a performance consideration: ``lib/System`` functions are at
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the bottom of many call chains and as such can be frequently called. We need
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them to be as efficient as possible. However, no routines in the system
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library should actually throw exceptions.
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Code Organization
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-----------------
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Implementations of the System Library interface are separated by their general
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class of operating system. Currently only Unix and Win32 classes are defined
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but more could be added for other operating system classifications. To
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distinguish which implementation to compile, the code in ``lib/System`` uses
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the ``LLVM_ON_UNIX`` and ``LLVM_ON_WIN32`` ``#defines`` provided via configure
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through the ``llvm/Config/config.h`` file. Each source file in ``lib/System``,
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after implementing the generic (operating system independent) functionality
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needs to include the correct implementation using a set of
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``#if defined(LLVM_ON_XYZ)`` directives. For example, if we had
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``lib/System/File.cpp``, we'd expect to see in that file:
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.. code-block:: c++
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#if defined(LLVM_ON_UNIX)
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#include "Unix/File.cpp"
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#endif
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#if defined(LLVM_ON_WIN32)
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#include "Win32/File.cpp"
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#endif
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The implementation in ``lib/System/Unix/File.cpp`` should handle all Unix
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variants. The implementation in ``lib/System/Win32/File.cpp`` should handle all
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Win32 variants. What this does is quickly differentiate the basic class of
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operating system that will provide the implementation. The specific details for
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a given platform must still be determined through the use of ``#ifdef``.
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Consistent Semantics
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--------------------
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The implementation of a ``lib/System`` interface can vary drastically between
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platforms. That's okay as long as the end result of the interface function is
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the same. For example, a function to create a directory is pretty straight
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forward on all operating system. System V IPC on the other hand isn't even
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supported on all platforms. Instead of "supporting" System V IPC,
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``lib/System`` should provide an interface to the basic concept of
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inter-process communications. The implementations might use System V IPC if
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that was available or named pipes, or whatever gets the job done effectively
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for a given operating system. In all cases, the interface and the
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implementation must be semantically consistent.
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