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<h2><a name="abstract">Abstract</a></h2>
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<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
<h2>
<a name="requirements">Keeping LLVM Portable</a>
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<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>
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<h3><a name="headers">Don't Include System Headers</a></h3>
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<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
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<h3><a name="expose">Don't Expose System Headers</a></h3>
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<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
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<h3><a name="c_headers">Use Standard C Headers</a></h3>
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<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
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<h3><a name="cpp_headers">Use Standard C++ Headers</a></h3>
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<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
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<h3><a name="highlev">High Level Interface</a></h3>
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<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
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<h3><a name="nounused">No Unused Functionality</a></h3>
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<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
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<h3><a name="nodupl">No Duplicate Implementations</a></h3>
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<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
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<h3><a name="virtuals">No Virtual Methods</a></h3>
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<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
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<h3><a name="nofunc">No Exposed Functions</a></h3>
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<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
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<h3><a name="nodata">No Exposed Data</a></h3>
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<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
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<h3><a name="softerrors">Minimize Soft Errors</a></h3>
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<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
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<h3><a name="throw_spec">No throw Specifications</a></h3>
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<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
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<h3><a name="organization">Code Organization</a></h3>
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<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.
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<h3><a name="semantics">Consistent Semantics</a></h3>
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<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
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<h3><a name="bug">Bug 351</a></h3>
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<p>See <a href="http://llvm.org/PR351">bug 351</a>
for further details on the progress of this work</p>
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