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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<html>
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<head>
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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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</head>
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<body>
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2004-07-22 02:02:43 +08:00
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<div class="doc_title">System Library</div>
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2004-07-22 02:02:43 +08:00
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<div class="doc_warning">
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<p>Warning: This document is a work in progress.</p>
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</div>
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<ul>
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<li><a href="#abstract">Abstract</a></li>
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<li><a href="#requirements">System Library Requirements</a>
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<ol>
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<li><a href="#headers">Hide System Header Files</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="#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="#throw">Throw Only std::string</a></li>
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<li><a href="#throw_spec">No throw() Specifications</a></li>
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<li><a href="#nodupl">No Duplicate Implementations</a></li>
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</ol></li>
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<li><a href="#design">System Library Design</a>
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<ol>
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<li><a href="#nounused">No Unused Functionality</a></li>
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<li><a href="#highlev">High-Level Interface</a></li>
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<li><a href="#opaque">Use Opaque Classes</a></li>
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<li><a href="#common">Common Implementations</a></li>
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<li><a href="#multi_imps">Multiple Implementations</a></li>
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<li><a href="#lowlevel">Use Low Level Interfaces</a></li>
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<li><a href="#memalloc">No Memory Allocation</a></li>
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<li><a href="#virtuals">No Virtual Methods</a></li>
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</ol></li>
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<li><a href="#detail">System Library Details</a>
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<ol>
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<li><a href="#bug">Tracking Bugzilla Bug: 351</a></li>
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<li><a href="#refimpl">Reference Implementation</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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2004-07-22 02:04:27 +08:00
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<p>Written by <a href="rspencer@x10sys.com">Reid Spencer</a></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="abstract">Abstract</a></div>
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<div class="doc_text">
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<p>This document describes the requirements, design, and implementation
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details of LLVM's System Library. The library is composed of the header files
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in <tt>llvm/include/llvm/System</tt> and the source files in
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<tt>llvm/lib/System</tt>. The goal of this library is to completely shield
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LLVM from the variations in operating system interfaces. By centralizing
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LLVM's use of operating system interfaces, we make it possible for the LLVM
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tool chain and runtime libraries to be more easily ported to new platforms
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since (theoretically) only <tt>llvm/lib/System</tt> needs to be ported. This
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library also unclutters the rest of LLVM from #ifdef use and special
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cases for specific operating systems. Such uses are replaced with simple calls
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to the interfaces provided in <tt>llvm/include/llvm/System</tt>.</p> Note that
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lib/System is not intended to be a complete operating system wrapper (such as
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the Adaptive Communications Environment (ACE) or Apache Portable Runtime
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(APR)), but only to provide the functionality necessary 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 original work as part of the eXtensible Programming
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System (XPS).</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="requirements">System Library Requirements</a>
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</div>
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<div class="doc_text">
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<p>The System library's requirements are aimed at shielding LLVM from the
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variations in operating system interfaces. The following sections define the
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requirements needed to fulfill this objective. Of necessity, these requirements
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must be strictly followed in order to ensure the library's goal is reached.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="headers">Hide System Header Files</a></div>
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<div class="doc_text">
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<p>The library must shield LLVM from <em>all</em> system libraries. To obtain
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system level functionality, LLVM must <tt>#include "llvm/System/Thing.h"</tt>
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and nothing else. This means that <tt>Thing.h</tt> cannot expose any system
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header files. This protects LLVM from accidentally using system specific
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functionality except through the lib/System interface. Specifically this
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means that header files like "unistd.h", "windows.h", "stdio.h", and
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"string.h" are verbotten outside the implementation of lib/System.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="c_headers">Allow Standard C Headers</a>
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</div>
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<div class="doc_text">
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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 lib/System interface. These headers and the things
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they declare are considered to be platform agnostic. LLVM source files may
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include them or obtain their inclusion through lib/System interfaces.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="cpp_headers">Allow Standard C++ Headers</a>
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</div>
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<div class="doc_text">
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<p>The <em>standard</em> C++ headers from the standard C++ library and
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standard template library are allowed to be exposed through the lib/System
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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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<div class="doc_subsection"><a name="nofunc">No Exposed Functions</a></div>
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<div class="doc_text">
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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. lib/System must not declare <tt>stat</tt>
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nor allow it to be declared. Instead it should provide its own interface to
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discovering information about files and directories. Those interfaces may be
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implemented in terms of <tt>stat</tt> but that is strictly an implementation
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detail.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="nodata">No Exposed Data</a></div>
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<div class="doc_text">
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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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<div class="doc_subsection"><a name="throw">Throw Only std::string</a></div>
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<div class="doc_text">
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<p>If an error occurs that lib/System cannot handle, the only action taken by
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lib/System is to throw an instance of std:string. The contents of the string
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must explain both what happened and the context in which it happened. The
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format of the string should be a (possibly empty) list of contexts each
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terminated with a : and a space, followed by the error message, optionally
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followed by a reason, and optionally followed by a suggestion.</p>
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<p>For example, failure to open a file named "foo" could result in a message
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like:</p>
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<ul><li>foo: Unable to open file because it doesn't exist."</li></ul>
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<p>The "foo:" part is the context. The "Unable to open file" part is the error
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message. The "because it doesn't exist." part is the reason. This message has
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no suggestion. Where possible, the implementation of lib/System should use
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operating system specific facilities for converting the error code returned by
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a system call into an error message. This will help to make the error message
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more familiar to users of that type of operating system.</p>
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<p>Note that this requirement precludes the throwing of any other exceptions.
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For example, various C++ standard library functions can cause exceptions to be
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thrown (e.g. out of memory situation). In all cases, if there is a possibility
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that non-string exceptions could be thrown, the lib/System library must ensure
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that the exceptions are translated to std::string form.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="throw_spec">No throw Specifications</a>
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</div>
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<div class="doc_text">
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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 the many call chains and as such can be frequently called. We
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need them to be as efficient as possible.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="nodupl">No Duplicate Implementations</a>
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</div>
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<div class="doc_text">
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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.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="design">System Library Design</a></div>
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<div class="doc_text">
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<p>In order to fulfill the requirements of the system library, strict design
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objectives must be maintained in the library as it evolves. The goal here
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is to provide interfaces to operating system concepts (files, memory maps,
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sockets, signals, locking, etc) efficiently and in such a way that the
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remainder of LLVM is completely operating system agnostic.</p>
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</div>
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2004-08-27 10:08:04 +08:00
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="nounused">No Unused Functionality</a></div>
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<div class="doc_text">
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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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<div class="doc_subsection"><a name="highlev">High Level Interface</a></div>
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<div class="doc_text">
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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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<div class="doc_subsection"><a name="highlev">Minimize Soft Errors</a></div>
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<div class="doc_text">
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<p>Operating system interfaces will generally provide errors 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". Well call the
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first group "soft" errors and the second group "hard" errors.<p>
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<p>lib/System must always attempt to minimize soft errors and always just
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throw a std::string on hard errors. 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 "can't" fail, except when faced with hard
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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>
|
|
|
|
|
<li>Handle internally the most common normal/good/soft error conditions
|
|
|
|
|
so the rest of LLVM doesn't have to.</li>
|
|
|
|
|
</ol>
|
|
|
|
|
|
|
|
|
|
<pre><tt>
|
|
|
|
|
Notes:
|
|
|
|
|
10. 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.
|
|
|
|
|
|
|
|
|
|
11. Implementations are separated first by the general class of operating system
|
|
|
|
|
as provided by the configure script's $build variable. This variable is used
|
|
|
|
|
to create a link from $BUILD_OBJ_ROOT/lib/System/platform to a directory in
|
|
|
|
|
$BUILD_SRC_ROOT/lib/System directory with the same name as the $build
|
|
|
|
|
variable. This provides a retargetable include mechanism. By using the link's
|
|
|
|
|
name (platform) we can actually include the operating specific
|
|
|
|
|
implementation. For example, support $build is "Darwin" for MacOS X. If we
|
|
|
|
|
place:
|
|
|
|
|
#include "platform/File.cpp"
|
|
|
|
|
into a a file in lib/System, it will actually include
|
|
|
|
|
lib/System/Darwin/File.cpp. What this does is quickly differentiate the basic
|
|
|
|
|
class of operating system that will provide the implementation.
|
|
|
|
|
|
|
|
|
|
12. Implementation files in lib/System need may only do two things: (1) define
|
|
|
|
|
functions and data that is *TRULY* generic (completely platform agnostic) and
|
|
|
|
|
(2) #include the platform specific implementation with:
|
|
|
|
|
|
|
|
|
|
#include "platform/Impl.cpp"
|
|
|
|
|
|
|
|
|
|
where Impl is the name of the implementation files.
|
|
|
|
|
|
|
|
|
|
13. Platform specific implementation files (platform/Impl.cpp) may only #include
|
|
|
|
|
other Impl.cpp files found in directories under lib/System. The order of
|
|
|
|
|
inclusion is very important (from most generic to most specific) so that we
|
|
|
|
|
don't inadvertently place an implementation in the wrong place. For example,
|
|
|
|
|
consider a fictitious implementation file named DoIt.cpp. Here's how the
|
|
|
|
|
#includes should work for a Linux platform
|
|
|
|
|
|
|
|
|
|
lib/System/DoIt.cpp
|
|
|
|
|
#include "platform/DoIt.cpp" // platform specific impl. of Doit
|
|
|
|
|
DoIt
|
|
|
|
|
|
|
|
|
|
lib/System/Linux/DoIt.cpp // impl that works on all Linux
|
|
|
|
|
#include "../Unix/DoIt.cpp" // generic Unix impl. of DoIt
|
|
|
|
|
#include "../Unix/SUS/DoIt.cpp // SUS specific impl. of DoIt
|
|
|
|
|
#include "../Unix/SUS/v3/DoIt.cpp // SUSv3 specific impl. of DoIt
|
|
|
|
|
|
|
|
|
|
Note that the #includes in lib/System/Linux/DoIt.cpp are all optional but
|
|
|
|
|
should be used where the implementation of some functionality can be shared
|
|
|
|
|
across some set of Unix variants. We don't want to duplicate code across
|
|
|
|
|
variants if their implementation could be shared.
|
|
|
|
|
</tt></pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
2004-07-17 18:04:49 +08:00
|
|
|
<!-- ======================================================================= -->
|
|
|
|
|
<div class="doc_subsection"><a name="opaque">Use Opaque Classes</a></div>
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>no public data</p>
|
|
|
|
|
<p>onlyprimitive typed private/protected data</p>
|
|
|
|
|
<p>data size is "right" for platform, not max of all platforms</p>
|
|
|
|
|
<p>each class corresponds to O/S concept</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
|
|
|
<div class="doc_subsection"><a name="common">Common Implementations</a></div>
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>To be written.</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
|
|
|
<div class="doc_subsection">
|
|
|
|
|
<a name="multi_imps">Multiple Implementations</a>
|
|
|
|
|
</div>
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>To be written.</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
|
|
|
<div class="doc_subsection"><a name="memalloc">No Memory Allocation</a></div>
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>To be written.</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
|
|
|
<div class="doc_subsection"><a name="virtuals">No Virtual Methods</a></div>
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>To be written.</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
<div class="doc_section"><a name="detail">System Library Details</a></div>
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>To be written.</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
2004-07-18 02:50:19 +08:00
|
|
|
<div class="doc_subsection"><a name="bug">Bug 351</a></div>
|
2004-07-17 18:04:49 +08:00
|
|
|
<div class="doc_text">
|
2004-07-18 02:50:19 +08:00
|
|
|
<p>See <a href="http://llvm.cs.uiuc.edu/PR351">bug 351</a>
|
2004-07-17 18:04:49 +08:00
|
|
|
for further details on the progress of this work</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
2004-08-27 02:52:52 +08:00
|
|
|
<div class="doc_subsection"><a name="bug">Rationale For #include Hierarchy</a>
|
|
|
|
|
</div>
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>In order to provide different implementations of the lib/System interface
|
|
|
|
|
for different platforms, it is necessary for the library to "sense" which
|
|
|
|
|
operating system is being compiled for and conditionally compile only the
|
2004-11-21 22:58:12 +08:00
|
|
|
applicable parts of the library. While several operating system wrapper
|
2004-08-27 02:52:52 +08:00
|
|
|
libraries (e.g. APR, ACE) choose to use #ifdef preprocessor statements in
|
|
|
|
|
combination with autoconf variable (HAVE_* family), lib/System chooses an
|
|
|
|
|
alternate strategy. <p>
|
|
|
|
|
<p>To put it succinctly, the lib/System strategy has traded "#ifdef hell" for
|
|
|
|
|
"#include hell". That is, a given implementation file defines one or more
|
|
|
|
|
functions for a particular operating system variant. The functions defined in
|
|
|
|
|
that file have no #ifdef's to disambiguate the platform since the file is only
|
|
|
|
|
compiled on one kind of platform. While this leads to the same function being
|
2004-11-21 22:58:12 +08:00
|
|
|
implemented differently in different files, it is our contention that this
|
2004-08-27 02:52:52 +08:00
|
|
|
leads to better maintenance and easier portability.</p>
|
|
|
|
|
<p>For example, consider a function having different implementations on a
|
|
|
|
|
variety of platforms. Many wrapper libraries choose to deal with the different
|
|
|
|
|
implementations by using #ifdef, like this:</p>
|
|
|
|
|
<pre><tt>
|
|
|
|
|
void SomeFunction(void) {
|
|
|
|
|
#if defined __LINUX
|
|
|
|
|
// .. Linux implementation
|
|
|
|
|
#elif defined __WIN32
|
|
|
|
|
// .. Win32 implementation
|
|
|
|
|
#elif defined __SunOS
|
|
|
|
|
// .. SunOS implementation
|
|
|
|
|
#else
|
|
|
|
|
#warning "Don't know how to implement SomeFunction on this platform"
|
|
|
|
|
#endif
|
|
|
|
|
}
|
|
|
|
|
</tt></pre>
|
|
|
|
|
<p>The problem with this is that its very messy to read, especially as the
|
|
|
|
|
number of operating systems and their variants grow. The above example is
|
|
|
|
|
actually tame compared to what can happen when the implementation depends on
|
|
|
|
|
specific flavors and versions of the operating system. In that case you end up
|
|
|
|
|
with multiple levels of nested #if statements. This is what we mean by "#ifdef
|
|
|
|
|
hell".</p>
|
2004-11-21 22:58:12 +08:00
|
|
|
<p>To avoid the situation above, we've chosen to locate all functions for a
|
2004-08-27 02:52:52 +08:00
|
|
|
given implementation file for a specific operating system into one place. This
|
|
|
|
|
has the following advantages:<p>
|
|
|
|
|
<ul>
|
|
|
|
|
<li>No "#ifdef hell"</li>
|
|
|
|
|
<li>When porting, the strategy is quite straight forward: copy the
|
|
|
|
|
implementation file from a similar operating system to a new directory and
|
|
|
|
|
re-implement them.<li>
|
|
|
|
|
<li>Correctness is helped during porting because the new operating system's
|
|
|
|
|
implementation is wholly contained in a separate directory. There's no
|
|
|
|
|
chance to make an error in the #if statements and affect some other
|
|
|
|
|
operating system's implementation.</li>
|
|
|
|
|
</ul>
|
|
|
|
|
<p>So, given that we have decided to use #include instead of #if to provide
|
|
|
|
|
platform specific implementations, there are actually three ways we can go
|
|
|
|
|
about doing this. None of them are perfect, but we believe we've chosen the
|
|
|
|
|
lesser of the three evils. Given that there is a variable named $OS which
|
|
|
|
|
names the platform for which we must build, here's a summary of the three
|
|
|
|
|
approaches we could use to determine the correct directory:</p>
|
|
|
|
|
<ol>
|
|
|
|
|
<li>Provide the compiler with a -I$(OS) on the command line. This could be
|
|
|
|
|
provided in only the lib/System makefile.</li>
|
|
|
|
|
<li>Use autoconf to transform #include statements in the implementation
|
|
|
|
|
files by using substitutions of @OS@. For example, if we had a file,
|
|
|
|
|
File.cpp.in, that contained "#include <@OS@/File.cpp>" this would get
|
|
|
|
|
transformed to "#include <actual/File.cpp>" where "actual" is the
|
|
|
|
|
actual name of the operating system</li>
|
|
|
|
|
<li>Create a link from $OBJ_DIR/platform to $SRC_DIR/$OS. This allows us to
|
|
|
|
|
use a generic directory name to get the correct platform, as in #include
|
|
|
|
|
<platform/File.cpp></li>
|
|
|
|
|
</ol>
|
|
|
|
|
<p>Let's look at the pitfalls of each approach.</p>
|
|
|
|
|
<p>In approach #1, we end up with some confusion as to what gets included.
|
|
|
|
|
Suppose we have lib/System/File.cpp that includes just File.cpp to get the
|
|
|
|
|
platform specific part of the implementation. In this case, the include
|
|
|
|
|
directive with the <> syntax will include the right file but the include
|
|
|
|
|
directive with the "" syntax will recursively include the same file,
|
|
|
|
|
lib/System/File.cpp. In the case of #include <File.cpp>, the -I options
|
|
|
|
|
to the compiler are searched first so it works. But in the #include "File.cpp"
|
|
|
|
|
case, the current directory is searched first. Furthermore, in both cases,
|
|
|
|
|
neither include directive documents which File.cpp is getting included.</p>
|
|
|
|
|
<p>In approach #2, we have the problem of needing to reconfigure repeatedly.
|
|
|
|
|
Developer's generally hate that and we don't want lib/System to be a thorn in
|
|
|
|
|
everyone's side because it will constantly need updating as operating systems
|
|
|
|
|
change and as new operating systems are added. The problem occurs when a new
|
|
|
|
|
implementation file is added to the library. First of all, you have to add a
|
|
|
|
|
file with the .in suffix, then you have to add that file name to the list of
|
|
|
|
|
configurable files in the autoconf/configure.ac file, then you have to run
|
|
|
|
|
AutoRegen.sh to rebuild the configure script, then you have to run the
|
|
|
|
|
configure script. This is deemed to be a pretty large hassle.</p>
|
|
|
|
|
<p>In approach #3, we have the problem that not all platforms support links.
|
|
|
|
|
Fortunately the autoconf macro used to create the link can compensate for
|
|
|
|
|
this. If a link can't be made, the configure script will copy the correct
|
|
|
|
|
directory from $BUILD_SRC_DIR to $BUILD_OBJ_DIR under the new name. The only
|
|
|
|
|
problem with this is that if a copy is made, the copy doesn't get updated if
|
|
|
|
|
the programmer adds or modifies files in the $BUILD_SRC_DIR. A reconfigure or
|
|
|
|
|
manual copying is needed to get things to compile.<p>
|
|
|
|
|
<p>The approach we have taken in lib/System is #3. Here's why:<p>
|
|
|
|
|
<ul>
|
|
|
|
|
<li>Approach #1 is rejected because it doesn't document what's actually
|
|
|
|
|
getting included and the potential for mistakes with alternate include
|
|
|
|
|
directive forms is high.</li>
|
|
|
|
|
<li>Approach #2 are both viable and only really impact development when new
|
|
|
|
|
files are added to the library.</li>
|
|
|
|
|
<li>However, approach #2 impacts every new file on every platform all the
|
|
|
|
|
time. With approach #3, only those platforms not supporting links will be
|
|
|
|
|
affected. The number of platforms not supporting links is very small and
|
|
|
|
|
they are generally archaic.</li>
|
|
|
|
|
<li>Given the above, approach #3 seems to have the least impact.</li>
|
|
|
|
|
</ul>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
2004-07-17 18:04:49 +08:00
|
|
|
<div class="doc_subsection">
|
|
|
|
|
<a name="refimpl">Reference Implementation</a>
|
|
|
|
|
</div>
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>The <tt>linux</tt> implementation of the system library will always be the
|
|
|
|
|
reference implementation. This means that (a) the concepts defined by the
|
|
|
|
|
linux must be identically replicated in the other implementations and (b) the
|
|
|
|
|
linux implementation must always be complete (provide implementations for all
|
|
|
|
|
concepts).</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<hr>
|
|
|
|
|
<address>
|
|
|
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
|
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
|
|
|
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
|
|
|
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
|
|
|
|
|
|
|
|
|
|
<a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
|
|
|
|
|
<a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
|
|
|
|
|
Last modified: $Date$
|
|
|
|
|
</address>
|
|
|
|
|
</body>
|
|
|
|
|
</html>
|
