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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>System Library</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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<h1>System Library</h1>
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<ul>
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<li><a href="#abstract">Abstract</a></li>
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<li><a href="#requirements">Keeping LLVM Portable</a>
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<ol>
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<li><a href="#headers">Don't Include System Headers</a></li>
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<li><a href="#expose">Don't Expose System Headers</a></li>
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<li><a href="#c_headers">Allow Standard C Header Files</a></li>
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<li><a href="#cpp_headers">Allow Standard C++ Header Files</a></li>
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<li><a href="#highlev">High-Level Interface</a></li>
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<li><a href="#nofunc">No Exposed Functions</a></li>
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<li><a href="#nodata">No Exposed Data</a></li>
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<li><a href="#nodupl">No Duplicate Implementations</a></li>
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<li><a href="#nounused">No Unused Functionality</a></li>
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<li><a href="#virtuals">No Virtual Methods</a></li>
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<li><a href="#softerrors">Minimize Soft Errors</a></li>
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<li><a href="#throw_spec">No throw() Specifications</a></li>
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<li><a href="#organization">Code Organization</a></li>
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<li><a href="#semantics">Consistent Semantics</a></li>
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<li><a href="#bug">Tracking Bugzilla Bug: 351</a></li>
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</ol></li>
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</ul>
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<div class="doc_author">
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<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="abstract">Abstract</a></h2>
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<div>
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<p>This document provides some details on LLVM's System Library, located in
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the source at <tt>lib/System</tt> and <tt>include/llvm/System</tt>. The
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library's purpose is to shield LLVM from the differences between operating
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systems for the few services LLVM needs from the operating system. Much of
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LLVM is written using portability features of standard C++. However, in a few
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areas, system dependent facilities are needed and the System Library is the
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wrapper around those system calls.</p>
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<p>By centralizing LLVM's use of operating system interfaces, we make it
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possible for the LLVM tool chain and runtime libraries to be more easily
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ported to new platforms since (theoretically) only <tt>lib/System</tt> needs
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to be ported. This library also unclutters the rest of LLVM from #ifdef use
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and special cases for specific operating systems. Such uses are replaced
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with simple calls to the interfaces provided in <tt>include/llvm/System</tt>.
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</p>
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<p>Note that the System Library is not intended to be a complete operating
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system wrapper (such as the Adaptive Communications Environment (ACE) or
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Apache Portable Runtime (APR)), but only provides the functionality necessary
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to support LLVM.
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<p>The System Library was written by Reid Spencer who formulated the
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design based on similar work originating from the eXtensible Programming
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System (XPS). Several people helped with the effort; especially,
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Jeff Cohen and Henrik Bach on the Win32 port.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="requirements">Keeping LLVM Portable</a>
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</h2>
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<div>
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<p>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.</p>
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<!-- ======================================================================= -->
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<h3><a name="headers">Don't Include System Headers</a></h3>
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<div>
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<p>Except in <tt>lib/System</tt>, no LLVM source code should directly
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<tt>#include</tt> a system header. Care has been taken to remove all such
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<tt>#includes</tt> from LLVM while <tt>lib/System</tt> was being
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developed. Specifically this means that header files like "unistd.h",
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"windows.h", "stdio.h", and "string.h" are forbidden to be included by LLVM
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source code outside the implementation of <tt>lib/System</tt>.</p>
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<p>To obtain system-dependent functionality, existing interfaces to the system
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found in <tt>include/llvm/System</tt> should be used. If an appropriate
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interface is not available, it should be added to <tt>include/llvm/System</tt>
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and implemented in <tt>lib/System</tt> for all supported platforms.</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="expose">Don't Expose System Headers</a></h3>
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<div>
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<p>The System Library must shield LLVM from <em>all</em> system headers. To
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obtain system level functionality, LLVM source must
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<tt>#include "llvm/System/Thing.h"</tt> and nothing else. This means that
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<tt>Thing.h</tt> cannot expose any system header files. This protects LLVM
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from accidentally using system specific functionality and only allows it
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via the <tt>lib/System</tt> interface.</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="c_headers">Use Standard C Headers</a></h3>
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<div>
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<p>The <em>standard</em> C headers (the ones beginning with "c") are allowed
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to be exposed through the <tt>lib/System</tt> interface. These headers and
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the things they declare are considered to be platform agnostic. LLVM source
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files may include them directly or obtain their inclusion through
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<tt>lib/System</tt> interfaces.</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="cpp_headers">Use Standard C++ Headers</a></h3>
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<div>
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<p>The <em>standard</em> C++ headers from the standard C++ library and
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standard template library may be exposed through the <tt>lib/System</tt>
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interface. These headers and the things they declare are considered to be
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platform agnostic. LLVM source files may include them or obtain their
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inclusion through lib/System interfaces.</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="highlev">High Level Interface</a></h3>
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<div>
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<p>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.</p>
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<p>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: <tt>getenv, fork, execve,</tt> and <tt>wait</tt>. The
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correct thing for lib/System to provide is a function, say
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<tt>ExecuteProgramAndWait</tt>, that implements the functionality completely.
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what we don't want is wrappers for the operating system calls involved.</p>
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<p>There must <em>not</em> be a one-to-one relationship between operating
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system calls and the System library's interface. Any such interface function
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will be suspicious.</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="nounused">No Unused Functionality</a></h3>
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<div>
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<p>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
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operating system wrapper here, just enough to satisfy LLVM's needs. And, LLVM
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doesn't need much. This design goal aims to keep the lib/System interface
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small and understandable which should foster its actual use and adoption.</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="nodupl">No Duplicate Implementations</a></h3>
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<div>
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<p>The implementation of a function for a given platform must be written
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exactly 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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</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="virtuals">No Virtual Methods</a></h3>
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<div>
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<p>The System Library interfaces can be called quite frequently by LLVM. In
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order to make those calls as efficient as possible, we discourage the use of
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virtual methods. There is no need to use inheritance for implementation
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differences, it just adds complexity. The <tt>#include</tt> mechanism works
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just fine.</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="nofunc">No Exposed Functions</a></h3>
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<div>
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<p>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 file
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for that function is not exposed. This prevents inadvertent use of system
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specific functionality.</p>
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<p>For example, the <tt>stat</tt> system call is notorious for having
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variations in the data it provides. <tt>lib/System</tt> must not declare
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<tt>stat</tt> nor allow it to be declared. Instead it should provide its own
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interface to discovering information about files and directories. Those
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interfaces may be implemented in terms of <tt>stat</tt> but that is strictly
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an implementation detail. The interface provided by the System Library must
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be implemented on all platforms (even those without <tt>stat</tt>).</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="nodata">No Exposed Data</a></h3>
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<div>
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<p>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 for
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that function is not exposed. As with functions, this prevents inadvertent use
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of data that might not exist on all platforms.</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="softerrors">Minimize Soft Errors</a></h3>
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<div>
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<p>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
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is, some of the errors are simply information like "file not found",
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"insufficient privileges", etc. while other errors are much harder like
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"out of space", "bad disk sector", or "system call interrupted". We'll call
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the first group "<i>soft</i>" errors and the second group "<i>hard</i>"
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errors.<p>
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<p>lib/System must always attempt to minimize soft errors.
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This is a design requirement because the
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minimization of soft errors can affect the granularity and the nature of the
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interface. In general, if you find that you're wanting to throw soft errors,
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you must review the granularity of the interface because it is likely you're
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trying to implement something that is too low level. The rule of thumb is to
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provide interface functions that <em>can't</em> fail, except when faced with
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hard errors.</p>
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<p>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
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to open the file will produce an error. However, lib/System should not
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simply throw that error if it occurs because its a soft error. The problem
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is that the interface function, OpenFileForWriting is too low level. It should
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be 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.</p>
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<p>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.</p>
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<p>Rules of thumb:</p>
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<ol>
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<li>Don't throw soft errors, only hard errors.</li>
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<li>If you're tempted to throw a soft error, re-think the interface.</li>
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<li>Handle internally the most common normal/good/soft error conditions
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so the rest of LLVM doesn't have to.</li>
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</ol>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="throw_spec">No throw Specifications</a></h3>
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<div>
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<p>None of the lib/System interface functions may be declared with C++
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<tt>throw()</tt> 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
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need them to be as efficient as possible. However, no routines in the
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system library should actually throw exceptions.</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="organization">Code Organization</a></h3>
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<div>
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<p>Implementations of the System Library interface are separated by their
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general class of operating system. Currently only Unix and Win32 classes are
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defined but more could be added for other operating system classifications.
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To 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 through the
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llvm/Config/config.h file. Each source file in lib/System, after implementing
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the generic (operating system independent) functionality needs to include the
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correct implementation using a set of <tt>#if defined(LLVM_ON_XYZ)</tt>
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directives. For example, if we had lib/System/File.cpp, we'd expect to see in
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that file:</p>
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<pre><tt>
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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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</tt></pre>
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<p>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
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for a given platform must still be determined through the use of
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<tt>#ifdef</tt>.</p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="semantics">Consistent Semantics</a></h3>
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<div>
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<p>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
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is 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, lib/System
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should provide an interface to the basic concept of inter-process
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communications. The implementations might use System V IPC if that was
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available or named pipes, or whatever gets the job done effectively for a
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given operating system. In all cases, the interface and the implementation
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must be semantically consistent. </p>
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</div>
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<!-- ======================================================================= -->
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<h3><a name="bug">Bug 351</a></h3>
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<div>
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<p>See <a href="http://llvm.org/PR351">bug 351</a>
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for further details on the progress of this work</p>
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</div>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
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<a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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Last modified: $Date$
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</address>
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</body>
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</html>
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