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+==============
+System Library
+==============
+
+.. sectionauthor:: Reid Spencer <rspencer@x10sys.com>
+
+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 ``LLVM_ON_WIN32`` ``#defines`` provided via configure
+through the ``llvm/Config/config.h`` file. 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(LLVM_ON_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.
+
+Bug 351
+-------
+See `bug 351 <http://llvm.org/PR351>`_ for further details on the progress of
+this work.
+