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<h1>System Library</h1>
<ul>
<li><a href="#abstract">Abstract</a></li>
<li><a href="#requirements">Keeping LLVM Portable</a>
<ol>
<li><a href="#headers">Don't Include System Headers</a></li>
<li><a href="#expose">Don't Expose System Headers</a></li>
<li><a href="#c_headers">Allow Standard C Header Files</a></li>
<li><a href="#cpp_headers">Allow Standard C++ Header Files</a></li>
<li><a href="#highlev">High-Level Interface</a></li>
<li><a href="#nofunc">No Exposed Functions</a></li>
<li><a href="#nodata">No Exposed Data</a></li>
<li><a href="#nodupl">No Duplicate Implementations</a></li>
<li><a href="#nounused">No Unused Functionality</a></li>
<li><a href="#virtuals">No Virtual Methods</a></li>
<li><a href="#softerrors">Minimize Soft Errors</a></li>
<li><a href="#throw_spec">No throw() Specifications</a></li>
<li><a href="#organization">Code Organization</a></li>
<li><a href="#semantics">Consistent Semantics</a></li>
<li><a href="#bug">Tracking Bugzilla Bug: 351</a></li>
</ol></li>
</ul>
<div class="doc_author">
<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p>
</div>
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<h2><a name="abstract">Abstract</a></h2>
<div>
<p>This document provides some details on LLVM's System Library, located in
the source at <tt>lib/System</tt> and <tt>include/llvm/System</tt>. 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.</p>
<p>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 <tt>lib/System</tt> 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 <tt>include/llvm/System</tt>.
</p>
<p>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.
<p>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.</p>
</div>
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<h2>
<a name="requirements">Keeping LLVM Portable</a>
</h2>
<div>
<p>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.</p>
<!-- ======================================================================= -->
<h3><a name="headers">Don't Include System Headers</a></h3>
<div>
<p>Except in <tt>lib/System</tt>, no LLVM source code should directly
<tt>#include</tt> a system header. Care has been taken to remove all such
<tt>#includes</tt> from LLVM while <tt>lib/System</tt> 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 <tt>lib/System</tt>.</p>
<p>To obtain system-dependent functionality, existing interfaces to the system
found in <tt>include/llvm/System</tt> should be used. If an appropriate
interface is not available, it should be added to <tt>include/llvm/System</tt>
and implemented in <tt>lib/System</tt> for all supported platforms.</p>
</div>
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<h3><a name="expose">Don't Expose System Headers</a></h3>
<div>
<p>The System Library must shield LLVM from <em>all</em> system headers. To
obtain system level functionality, LLVM source must
<tt>#include "llvm/System/Thing.h"</tt> and nothing else. This means that
<tt>Thing.h</tt> cannot expose any system header files. This protects LLVM
from accidentally using system specific functionality and only allows it
via the <tt>lib/System</tt> interface.</p>
</div>
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<h3><a name="c_headers">Use Standard C Headers</a></h3>
<div>
<p>The <em>standard</em> C headers (the ones beginning with "c") are allowed
to be exposed through the <tt>lib/System</tt> 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
<tt>lib/System</tt> interfaces.</p>
</div>
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<h3><a name="cpp_headers">Use Standard C++ Headers</a></h3>
<div>
<p>The <em>standard</em> C++ headers from the standard C++ library and
standard template library may be exposed through the <tt>lib/System</tt>
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.</p>
</div>
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<h3><a name="highlev">High Level Interface</a></h3>
<div>
<p>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.</p>
<p>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: <tt>getenv, fork, execve,</tt> and <tt>wait</tt>. The
correct thing for lib/System to provide is a function, say
<tt>ExecuteProgramAndWait</tt>, that implements the functionality completely.
what we don't want is wrappers for the operating system calls involved.</p>
<p>There must <em>not</em> be a one-to-one relationship between operating
system calls and the System library's interface. Any such interface function
will be suspicious.</p>
</div>
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<h3><a name="nounused">No Unused Functionality</a></h3>
<div>
<p>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.</p>
</div>
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<h3><a name="nodupl">No Duplicate Implementations</a></h3>
<div>
<p>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).
</p>
</div>
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<h3><a name="virtuals">No Virtual Methods</a></h3>
<div>
<p>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 <tt>#include</tt> mechanism works
just fine.</p>
</div>
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<h3><a name="nofunc">No Exposed Functions</a></h3>
<div>
<p>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.</p>
<p>For example, the <tt>stat</tt> system call is notorious for having
variations in the data it provides. <tt>lib/System</tt> must not declare
<tt>stat</tt> 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 <tt>stat</tt> but that is strictly
an implementation detail. The interface provided by the System Library must
be implemented on all platforms (even those without <tt>stat</tt>).</p>
</div>
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<h3><a name="nodata">No Exposed Data</a></h3>
<div>
<p>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.</p>
</div>
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<h3><a name="softerrors">Minimize Soft Errors</a></h3>
<div>
<p>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 "<i>soft</i>" errors and the second group "<i>hard</i>"
errors.<p>
<p>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 <em>can't</em> fail, except when faced with
hard errors.</p>
<p>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.</p>
<p>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.</p>
<p>Rules of thumb:</p>
<ol>
<li>Don't throw soft errors, only hard errors.</li>
<li>If you're tempted to throw a soft error, re-think the interface.</li>
<li>Handle internally the most common normal/good/soft error conditions
so the rest of LLVM doesn't have to.</li>
</ol>
</div>
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<h3><a name="throw_spec">No throw Specifications</a></h3>
<div>
<p>None of the lib/System interface functions may be declared with C++
<tt>throw()</tt> 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.</p>
</div>
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<h3><a name="organization">Code Organization</a></h3>
<div>
<p>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 <tt>#if defined(LLVM_ON_XYZ)</tt>
directives. For example, if we had lib/System/File.cpp, we'd expect to see in
that file:</p>
<pre><tt>
#if defined(LLVM_ON_UNIX)
#include "Unix/File.cpp"
#endif
#if defined(LLVM_ON_WIN32)
#include "Win32/File.cpp"
#endif
</tt></pre>
<p>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
<tt>#ifdef</tt>.</p>
</div>
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<h3><a name="semantics">Consistent Semantics</a></h3>
<div>
<p>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. </p>
</div>
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<h3><a name="bug">Bug 351</a></h3>
<div>
<p>See <a href="http://llvm.org/PR351">bug 351</a>
for further details on the progress of this work</p>
</div>
</div>
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<a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
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Last modified: $Date: 2011-04-22 20:30:22 -0400 (Fri, 22 Apr 2011) $
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